1. JAMES PATRICK FOLSOM
Heat Inactivation in Apple Juice ofEscherichia coli O157:H7 Exposed to Chlorine
(Under the direction of JOSEPH F. FRANK)
The effect of sub-lethal chlorine treatment on thermal inactivation ofEscherichia
coli O157:H7 was determined. D58values were calculated for stationary phase cells
exposed to 0.6 ppm total available chlorine and unchlorinated cells in commercial shelf
stable apple juice (pH=3.6). D58values (min) for unchlorinated and chlorine exposed
cells in buffer were 5.45 and 1.65, respectively (p<0.01). Death curves of chlorine
exposed and unchlorinated cells in applejuice are not linear. Unchlorinated cells heated
in apple juice exhibit a three minute delay before onset of linear inactivation. Chlorine
treatment eliminated this hump. This resulted in an overall loss of thermotolerance. D58
values (min) taken from the linear portions of the death curves are 0.77 for unexposed
cells and 1.19 for chlorine exposed cells (p=0.05). These D-values represent a small
portion of the population and indicate that chlorine treatment causes approximately one
to five percent of the total population to exhibit marginal induced thermotolerance. In
addition, time (min) to kill the initial 90% of the cell population was calculated to be
3.14 for unchlorinated versus 0.3 for chlorine exposed cells (p<.001). These
observations taken together indicate that the effect of chlorine treatment can be
neglected for the purposes of heat inactivation of E. coli O157:H7 in applejuice. In
addition this implies that chlorinated wash water could be used to reduce the heat
treatment normally used for apple juice.
INDEX WORDS: Escherichia coli O157:H7, Hypochlorous acid, Chlorine, heat
resistance, biocide inactivation

2. HEAT INACTIVATION IN APPLE JUICE OF ESCHERICHIA COLI O157:H7
EXPOSED TO CHLORINE
by
JAMES PATRICK FOLSOM
B. S. A., The University of Georgia, GA, 1996
A Thesis Submitted to the Graduate Faculty
of The University of Georgia in Partial Fulfillment
of the
Requirement for the Degree
MASTER OF SCIENCE
ATHENS, GEORGIA
1999

3. HEAT INACTIVATION IN APPLE JUICE OF ESCHERICHIA COLI O157:H7
EXPOSED TO CHLORINE
by
JAMES PATRICK FOLSOM

4. TABLE OF CONTENTS
CHAPTER
IN T R O D U C T IO N ...................................................................................... 1
LITERATURE R E V I E W .......................................................................... 3
GENERAL INFORMATION.....................................................................3
THE CHEMISTRY OF HYPOCHLOROUS ACID . . .4
THE EFFECTS OF HYPOCHLOROUS ACID ON CELLS . . 9
RESISTANCE OF MICROORGANISMS TO HOC1 . .12
THE soxRS REGULON.........................................................................12
EXTRA-CYTOPLASMIC S T R E S S .................................................13
MATERIALS AND METHODS.........................................................................16
R E S U L T S .................................................................................................19
D IS C U S S IO N ................................................................................................ 21
F I G U R E S ................................................................................................ 24
B IB L IO G R A P H Y .................................................................................... 28

5. INTRODUCTION
Escherichia coli O157:H7 can be found in the feces of deer, dairy and beef cattle,
and wild birds (73, 100, 107). If E. coli O157:H7 is deposited on the skin of an apple that
is later damaged, (e.g. wind fallen apples) E. coli O157:H7 can grow in the damaged area
and be spread to other damaged apples by fruit flies (47). Surveys of small apple cider
producers have found most producers used wind fallen apples (10, 87). Thus, E. coli
O157:H7 may be present in high concentrations in apples and released into the juice
during pressing.
Fresh unpasteurized apple juice (cider) has a pH range reported to be 2.92 to 6.54
with most researchers reporting a range of three to four (31, 47, 58, 87, 89, 106). Since
most juice qualifies as a high acid food, heat treatment has not been required and E. coli
may be able to survive until time of sale (44, 85). In fact, an outbreak due to Salmonella
in 1974 associated with apple cider was the first indication that pathogens could survive
and infect consumers (31). Since then, outbreaks due to Escherichia coli O157:H7
associated with apple cider occurred in 1991 and 1996 (6,10). Steele et al. (90) also
reported a 1980 outbreak of hemolytic uremic syndrome of undetermined cause
associated with apple cider. These outbreaks illustrate that E. coli O157:H7 can
contaminate cider, survive and cause illness.
On July 8th, 1998 the FDA published requirements for the labeling of cider. This
label warns consumers of products that have not been treated to destroy pathogens. This
label is required on all cider not produced under a system validated to reduce the
pathogen of concern by five logs (4). This label is a temporary measure until a HAACP
system can be developed and mandated. The proposal for a HAACP program was
1

6. 2
published in the Federal register on April 24th, 1998. The HAACP plan is to include a
validated process that reduces E. coli O157:H7 by five logs (5).
It has been reported that 0.2 mg/l to 1.0 mg/l of hypochlorous acid induces the
production of heat shock proteins in E. coli O157:H7 (25). Hypochlorous acid is the
bacteriocidal agent found in chlorinated water used to wash apples prior to pressing.
These concentrations are small fractions of the amount actually used in wash water.
However, these levels of exposure might be realized during the chlorination of apples if
cells are protected by attachment, or sheltered by damaged apple tissue, especially when
chlorine concentration isn’t properly maintained. Thus E. coli O157:H7 may be exposed
to levels of chlorine that may improve its ability to survive a subsequent heat treatment.
The goal of this investigation is to determine if exposure of log or stationary phase E. coli
O157:H7 to chlorine will affect thermal inactivation. If low concentrations of chlorine
cause a measurable increase in thermotolerance, then this effect needs to be accounted for
when determining heat treatment to be employed.

7. LITERATURE REVIEW
GENERAL INFORMATION
Escherichia coli O157:H7 was first isolated from a patient suffering from
hemorrhagic colitis in 1975, but the causative agent was not associated with the disease
until 1982 when the same serotype ofE. coli caused a major outbreak of hemorrhagic
colitis in Oregon and Michigan (74,101). It was found that this serotype ofE. coli was
different from other pathogenic E. coli. In particular, the O157:H7 serotype was negative
for invasiveness (sereny test), elaborated no colonization factors (CFA/I or CFA/II), did
not produce heat stable or heat labile toxins and was non-hemolytic. In addition, E. coli
O157:H7 is sorbitol negative where 93% of all E. coli ferment sorbitol (101). E. coli
O157:H7 also lacks the ability to hydrolyze 4-methylumbelliferyl-ß-D-glucuronide
(MUG) (24) and does not grow at 45°C in the presence of 0.15% bile salts. Because of
the latter characteristic this serotype cannot be isolated by using standard fecal coliform
methods that include incubation at 45°C (24, 92).
Genetic analysis by Whittam et al (102) found that O157:H7 serotypes of E. coli
are closely related and descend from a common ancestor. It was also concluded that
different members of the group had diverged in plasmid content more than chromosomal
content, and that O157:H7 serotypes were no more related to other verotoxin producing
strains than they were to any other randomly chosen E. coli serotype. Consistent with
these findings, Whittam et al (102) concluded that E. coli O157:H7 and O55:H7 were
most closely related and diverged from a common ancestor that only possessed the ability
to form attaching and effacing lesions. Whittam et al also concludes that O157:H7
serotypes arose as a result of horizontal gene transfer of virulence factors.
3

8. Among these virulence factors are a periplasmic catalase and shiga-like toxins.
Shiga-like toxins are iron regulated toxins that catalytically inactivate 60s ribosomal
subunits of eukaryotic cells blocking mRNA translation and causing cell death (72).
Shiga-like toxins are functionally identical to toxins produced by virulent Shigella species
(12). Strains of E. coli that express shiga-like toxins gained this ability due to infection
with a prophage containing the structural coding for the toxins. Non-producing strains
become infected and produce shiga-like toxins after incubation with strains that produce
shiga-like toxins (65, 91). The periplasmic catalase is encoded on the pO157 plasmid and
is believed to be involved in virulence by providing additional oxidative protection while
in the host (11).
MacConkey media with sorbitol substituted for lactose (SMAC) was the first
medium recommended for clinical screening for E. coli O157:H7 (101). March and
Ratnam (57) conducted a study to determine the usefulness of SMAC for screening stool
samples for E. coli O157:H7. They concluded that SMAC is a useful tool for the
detection of E. coli O157:H7 for several reasons. They sampled 240 stools and found
that only 15% of the normal flora were unable to ferment sorbitol. They found that
growth on SMAC culture plates from stools containing E. coli O157:H7 mostly contained
sorbitol negative colonies, where the same culture media inoculated from normal stools
containing other non-sorbitol fermenters yielded colorless colonies that were heavily
crowded by sorbitol fermenting colonies. All bloody stools yielded SMAC culture plates
heavily crowed with colorless colonies of E. coli O157:H7.
THE CHEMISTRY OF HYPOCHLOROUS ACID
Aqueous solutions of chlorine and hypochlorite compounds (household bleach)
have been widely used as a biocide for surfaces and drinking water. In 1825, calcium
hypochlorite was used in the sanitation of morgues, sewers, stables, hospitals, ships
prisons and surgical dressings (81). Other biocides have taken its place for some
applications but hypochlorous acid is still widely used today. The reasons for this include
potency, range of effectiveness, ease of application, and lack of toxic effects (64).
4

9. 5
Hypochlorous acid (HOCl) is a potent antimicrobial agent that is generated by solvation
of chlorine gas, calcium hypochlorite, lithium hypochlorite or sodium hypochlorite
(NaOCl) in water (27). Hypochlorous acid (HOCl) is the most biocidal ion species that
can be found in a chlorine solution. The pKa of hypochlorous acid at 25°C is 7.497 (63)
and thus effectiveness of a chlorine solution improves with decreasing pH.
Liquefied chlorine gas forms hypochlorous acid by formula one (29). Calcium
hypochlorite and sodium hypochlorite are
available as solids or in aqueous solution.
Lithium hypochlorite is available as a powder
(27). Sodium hypochlorite, which is probably
the most common hypochlorite, forms
hypochlorous acid by dissociation and release
Formulas numbered from top to of hypochlorite ions (OC1-) as shown by
bottom: One through three formula two. The hypochlorite ion then
equilibrates to form hypochlorous acid as
shown by formula three. The other hypochlorites dissociate similarly (52). Both
hypochlorite solutions and aqueous chlorine gas reach the same ion concentrations when
dissolved in water (29). The hypochlorite ion alone is reported to have 1% to 1.25% as
much biocidal effect as HOCl (29, 64).
HOCl is also generated in activated neutrophils by myeloperoxidase mediated
peroxidation of chloride ions (35). Thus, scientists have tried to explain how HOCl kills
microorganisms from both an industrial and a medical perspective. Work studying the
effects of HOCl on microorganisms has used both sodium hypochlorite and
myeloperoxidase for the generation of HOCl. There are two ways to produce HOCl with
myeloperoxidase. One method uses hydrogen peroxide (H2O2), myeloperoxidase and
sodium chloride, and the alternative is to use glucose oxidase and glucose to generate
H2O2. Research has also shown that one mole of hydrogen peroxide added to a
myeloperoxidase system exhibits the same lethal effects as one mole ofNaOCl (93), and

10. the results of Winterbourne (103) support this. Thus research using hypochlorous acid
produced by myeloperoxidase is equivalent to research using NaOCl. Hypochlorous acid
can react with a wide variety of organic molecules including DNA, RNA, (3, 21, 45, 69)
fatty acid groups, cholesterol, (7, 16, 23, 37, 39, 97, 98, 104) and proteins (2, 9, 37, 46,
48, 99, 103).
Sulfhydryl (SH) groups react with HOCl to yield disulfide crosslinks (67). One
SH containing amino acid can scavenge up to four molecules of HOCl (103). Consistent
with this it has been proposed that sulfhydryl groups of sulfur containing amino acids can
be oxidized a total of three times by three HOCl molecules, with the fourth reacting with
the α-amino group. The first reaction yields a thiol (R-SOH) then a sulfenic acid (R-
S02H) and finally a sulfinic acid (R-S03H). Each of those intermediates can also
condense with another sulfhydryl group causing cross linking and aggregation of proteins.
Sulfenic and sulfinic acid derivatives are only produced at high molar excesses of HOCl,
and disulfides are primarily formed at bacteriocidal levels (69). Disulfide bonds can also
be oxidized by HOCl to sulfinic acid (67). The oxidation of sulfhydryls and disulfides
evolves HCl (69) and thus neutralizes the oxidative potential of HOCl. Knox et al. (48)
first noted that HOCl was a sulfhydryl inhibitor that in sufficient quantity could
completely inactivate proteins containing sulfhydryl groups. NH2Cl, which will be
discussed later, has been found to oxidize SH groups by a similar mechanism as HOCl
(46). Levels of NH2Cl much higher than the amount needed to kill E. coli are required to
eliminate all sulfhydryl groups in vivo. Sulfhydryl inhibition can be bacteriostatic
because once the residual chlorine is dissipated some Sulfhydryl function can be restored
(46). Also, as has been previously mentioned, disulfides can be oxidized by HOCl. It
follows that disulfides can only form after the HOCl has been dissipated, and therefore
injured microorganisms could conceivably repair some damage if the dose of chlorine
was not sufficient to maintain a chlorine residual.
Hypochlorous acid reacts readily with amino acids that have amino group side
chains. The chlorine from HOCl displaces a hydrogen resulting in an organic chloramine
6

11. (27). Chlorinated amino acids rapidly decompose but protein chloramines are longer
lived and still have oxidative capacity (103). This is consistent with reports by Thomas
(93) that most HOCl quickly forms chlorinated nitrogen derivatives that dissipate over
time and are capable of causing further toxicity. They also observed that chlorinated
peptide fragments appeared in the media and concluded that these were the result of
fragmented proteins. Thomas et al (93) concluded from their results that most
chloramines decayed by internal rearrangement and that fewer available NH2groups
promoted attack on the peptide bond resulting in cleavage of the protein. McKenna and
Davies (60) found that 10 mM or greater HOCl was necessary to fragment proteins in
vivo. Consistent with these results it was later found that the chloramine undergoes a
molecular rearrangement releasing HCl and ammonia to form an amide (38). The amide
group can further react with another amino group to form a Schiffbase causing cross
linking and aggregation of proteins (37).
It has been proposed that a Cl-and a proton can react with a chloramine to
regenerate the amino group and release Cl2(39). These reactions also result in the
dissipation of hypochlorous acid. The presence of amino acids and poly-lysine have been
shown to reduce the effectiveness of HOCl; (94) and this is corroborated by McKenna
and Davies (60). Consistent with these observations, is the observation that cells starved
for nitrogen are 3-5 fold more sensitive to HOCl inactivation (62). Other observed
reaction products of HOCl are 3-chlorotyrosine (22) and tyrosine dimers (99). All of
these reactions can lead to the inactivation of proteins.
When HOCl reacts with ammonia the product is inorganic chloramine (NH2Cl)
(29). Thomas (94) reports that NH2Cl improves the effectiveness of HOCl. Thomas also
reports that the release of peptides into the media that is caused by HOCl is blocked by
NH2Cl. This implies the inhibition of protein fragmentation. NH2Cl has been reported to
oxidize sulfhydryls and disulfides yielding the same products as HOCl (46). Shih and
Lederberg (86) also found that Bacillus subtilis exposed to NH2Cl had more chain breaks,
nuclease sensitive DNA and alkali sensitive DNA when compared to unexposed cells.
7

12. Dukan and Touati found that E. coli deficient in the ability to fix DNA chain breaks was
hypersensitive to hypochlorous acid (26). Taken together these observations may mean
that the NH2Cl diffuses through the cell membrane. These observations may also be a
consequence of DNA binding to the cytoplasmic membrane during cell division (78). It
is important to note that NH2Cl alone has only 0.4% the biocidal effect of HOCl (64). It
is also important to remember that ammonia is released during the decay of organic
chloramines and could react with HOCl to form NH2Cl.
HOCl reacts with DNA and all nucleotides in vitro (21, 68). HOCl reacts with the
bases of nucleotides much the same way it reacts with proteins. HOCl reacts with both
the heterocyclic NH group and the amino group of guanine. Thus guanine is the most
reactive and is followed by adenosine and cytidine which only have a reactive amino
group. Similarly thymine only has a heterocyclic NH group that is reactive with HOCl
(68). Uracil has been reported to be reactive only at a very slow rate (3, 21).
Heterocyclic NH groups are more reactive than amino groups and the secondary
chloramines are able to donate the chlorine (69). These reactions would interfere with
DNA base pairing and consistent with this, Prü tz (68) has reported a decrease in viscosity
of DNA exposed to HOCl that is similar to that seen with heat denaturation. The sugar
moieties are unreactive and the DNA backbone is not broken (68). NADH can react with
chlorinated TMP and UMP as well as HOCl. This reaction can regenerate UMP and
TMP and results in the 5-hydroxy derivative of NADH. The reaction with TMP or UMP
is slowly reversible to regenerate HOCl. A second slower reaction that results in
cleavage of the pyridine ring occurs when excess HOCl is present. NAD+is inert to
HOCl (68, 69).
HOCl can also react with unsaturated bonds in lipids, though saturated bonds are
non-reactive, and the OCl- ion does not participate in this reaction. This reaction occurs
by a hydrolysis of the double bond by adding chlorine to one of the carbons and a
hydroxyl to the other resulting in a chlorhydrin (7). The polar chlorine disrupts the lipid
bilayer and could increase permeability (16). Chlorhydrin formation can occur in lipid
8

13. bi-layers of red blood cells and results in increased permeability, and disruption could
occur if enough chlorhydrin is formed (7, 98). The addition of preformed chlorhydrins to
red blood cells can affect permeability as well (23). Phosphatidylserine and
phosphatidylethanolamine have a primary amino group and can also take part in
chloramine reactions (16), but it is still unclear how this would affect membrane stability.
Cholesterol chlorhydrins have also been observed (16, 39), but do not greatly affect
permeability, and it is believed that Cl2is responsible for this reaction (39).
Other toxic mechanisms of HOCl have been proposed. The iron released by
HOCl may react with HOCl in a Fenton like reaction to generate hydroxyl radicals (14,
15). Also the singlet oxygen that is generated by the reaction of H2O2with HOCl could
react with additional HOCl in a Haber-Weiss type reaction to generate hydroxyl radicals
(14, 15). However, recent data indicates that singlet oxygen is not generated from the
H0C1/H2O2reaction at physiological conditions (82).
THE EFFECTS OF HYPOCHLOROUS ACID ON CELLS
E. coli exposed to hypochlorous acid lose viability in less than 100 ms due to
inactivation of many vital systems (2, 70, 71, 76, 79). Hypochlorous acid has a reported
LD500.0104 ppm - 0.156 ppm (17) and McKenna and Davies (60) reports that 2.6 ppm
caused 100% growth inhibition in 5 minutes. However it should be noted the
concentration required for bactericidal activity is also highly correlated with bacterial
concentration (48). In this section, I will try to bring together both the available
observations of in vivo effects and the known chemistry already discussed to clarify
factors causing cellular inactivation. In doing so, I will adhere to the premise of Knox et
al (48), that only effects that occur at or below bacteriocidal levels of chlorine are
important for cellular inactivation.
In 1947, Knox et al (48) proposed the idea that inhibition of glucose oxidation
was a major factor in the toxicity of chlorine solutions. He proposed that the active agent
or agents diffused across the cytoplasmic membrane to inactivate key Sulfhydryl
containing enzymes in the glycolytic pathway. This group was also the first to note that
9

14. 10
chlorine solutions (HOCl) inhibited sulfhydryl enzymes. Later studies have shown that at
bactericidal levels the cytosol components do not react with HOCl (1). In agreement
with this, McFeters and Camper (59) found that aldolase, an enzyme that Knox proposes
would be inactivated, was unaffected by HOCl in vivo. It has been further shown that
loss of sulfhydryls do not correlate with inactivation (46). That leaves the question what
causes inhibition of glucose oxidation. The discovery that HOCl blocks induction of ß-
galactosidase by added lactose (8) led to a possible answer to this question. The uptake
of radiolabeled substrates by both ATP hydrolysis and proton co-transport may be
blocked by exposure to HOCl preceding loss of viability (1). From this observation it has
been concluded that HOCl blocks uptake of nutrients by neutralizing the transport
proteins (1,9, 13, 59). It is important to remember that many of the reactions previously
discussed do not permanently inactivate proteins and thus it is not clear if this is the
penultimate mechanism of inactivation or merely the result of inactivation.
The questions of loss of glucose oxidation has been further explored in terms of
loss of respiration. Venkobachar et al (96) found that succinic dehydrogenase was
inhibited in vitro by HOCl and this led to the investigation of the possibility that electron
transport loss could be the cause of bacterial inactivation. Albrich et al (3) subsequently
found that HOCl destroys cytochromes and iron sulfur clusters. This group further
observed that oxygen uptake is abolished by HOCl and adenine nucleotides are lost.
They also observed that irreversible oxidation of cytochromes paralleled loss of
respiratory activity.
One way of addressing the loss of oxygen uptake was by studying the effects of
HOCl on succinate dependant electron transport (43). Rosen et al (79) found levels of
reductable cytochromes in HOCl treated cells were normal and these cells were unable to
reduce them. This group subsequently found that succinate dehydrogenase was inhibited
by HOCl, stopping the flow of electrons to oxygen. Later studies showed a more precise
mechanism (71). Ubiquinol oxidase activity ceases first, and the still active cytochromes
reduce the remaining quinone. The cytochromes then pass the electrons to oxygen. This

15. explains why the cytochromes cannot be reoxidized as observed by Rosen et al. Later
Albrich et al (2) found that cellular inactivation precedes loss of respiration by using a
flow mixing system that allowed evaluation of viability on a much smaller time scale.
This group found that cells capable of respiring could not divide after exposure to HOCl.
This observation eliminates the possibility of inhibition of respiration as the mechanism
by which HOCl inhibits microorganisms. Having eliminated loss of respiration this
group proposes that the cause of death may be due to metabolic dysfunction caused by
depletion of adenine nucleotides.
Barrette et al (8) studied the loss of adenine nucleotides by studying the energy
charge of HOCl exposed cells. They found that cells exposed to HOCl were unable to
step up their energy charge after addition of nutrients. It was concluded that exposed
cells have lost the ability to regulate their adenylate pool. This conclusion was based on
the fact that metabolite uptake was only 45% deficient after exposure to HOCl and the
observation that HOCl causes intracellular ATP hydrolysis. This conclusion is also based
on their confirmation that at bacteriocidal levels of HOCl cytosolic components are
unaffected. Thus they propose that modification of some membrane bound protein
results in extensive ATP hydrolysis, and this, coupled with the cells inability to remove
AMP from the cytosol depresses metabolic function. One protein involved in loss of
ability to regenerate ATP has been found to be ATP synthetase (9). Much of the
research on respiration reconfirms the observation that relevant reactions take place at the
cell membrane (8, 9, 75).
The most recent explanation for cellular inactivation by HOCl is that of inhibition
DNA replication. McKenna and Davies (60) first noted a precipitous decline in DNA
synthesis in HOCl exposed cells. This group also found that DNA inhibition occurred
much sooner than inhibition of protein synthesis, and that DNA inhibition closely
paralleled loss of viability. Rosen et al (78) confirms all these observations. They also
observed that HOCl treatment decreases the affinity of extracted membranes for oriC, and
showed that this decreased affinity paralleled loss of viability. Rosen’s group proposed
11

16. that inactivation of membrane proteins involved in DNA replication are the target of
HOCl. A later study by Rosen et al (77) compares the rate of inhibition of DNA
replication ofplasmids with different replication origins and found that certain plasmids
exhibited a delay in inhibition when compared to plasmids containing oriC. The
conclusion is that depletion of high energy nucleotides is not the cause of inhibition of
genome replication.
RESISTANCE OF MICROORGANISMS TO HOCl
Very little literature exists on the nature of resistance ofE. coli to HOCl. Here is
a brief summary of what is known. Dukan et al found that HOCl at concentrations below
0.3 ppm can induce H2O2resistance and that it is not oxyR dependant. They also found
that HOCl resistance is σsdependant in stationary but not in exponential phase (26).
Glutathione deficient E. coli were significantly less resistant to HOCl (17). Acid
adaptation is reported to sensitize Salmonella typhimurium to HOCl and this is attributed
to changes cell membrane composition (54). Exposure of Enterococcusfeacalis to HOCl
does not induce resistance to higher doses of HOCl, in fact, it sensitized the cells to
HOCl. Also acquisition resistance upon entrance to stationary phase was independent of
de novo protein synthesis (51). Starvation has been reported to increase resistance (56)
but growth on minimal media has been reported to decrease sensitivity of E. coli (62).
THE soxRS REGULON
The soxRS regulon is part of the defense that was found to be responsible for
protection from redox cycling agents that divert electrons from NADPH to oxygen to
produce singlet oxygen. These compounds include methyl viologen or paraquat and
napthoquinone menadione (32, 36). The soxRS regulon is also important because it is
activated by hypochlorous acid (25). The protein that is transcribed by σ70from soxR is a
positive transcriptional regulator of the soxRS regulon (33,40, 41, 95). The regulon is
activated when there is a decrease in the NADPH/NADP+ratio which indicates a loss of
reductive potential in the cytoplasm of Escherichia coli. This regulon functions to
maintain the reductive potential inside the cell during times of oxidative challenge (55).
12

17. 13
The soxR protein is an iron sulfur protein that is easily oxidized and in its oxidized form
activates the transcription ofsoxS. SoxS then activates the nine proteins of the regulon by
binding their promoters (40, 41, 55). This regulon functions to maintain reductive
potential in many ways.
The soxRS regulon maintains reductive potential by stimulating
glucose-6-phosphate dehydrogenase which replenishes NADPH. In addition it minimizes
NADPH depletion by activating transcription of a manganese containing superoxide
dismutase that supplements the iron containing superoxide dismutase that is more
sensitive to oxidation. The constitutive fumarase is also unstable in a less reductive
environment and is supplemented with a more stable fumarase C. SoxRS also enhances
DNA repair by producing exonuclease III and endonuclease IV (20, 55).
EXTRA-CYTOPLASMIC STRESS
The σEregulon responds to extra-cytoplasmic stress that would denature proteins
and responds to ethanol and misfolded outer membrane proteins (61, 80). σEis the
protein product of rpoE and is an alternate sigma factor that is known to transcribe at
least 10 proteins including four that have been identified rpoH, rpoE, degP, andfkpA.
The rpoli gene encodes σ32, which is the heat shock alternate sigma factor. The protein
DegP (HtrA) is a periplasmic endopeptidase. FkpA is a periplasmic peptidyl prolyl
isomerase. It is believed that this regulon helps maintain cell envelope integrity under
conditions that cause proteins to become unfolded (19, 42, 50, 80). The exact
mechanisms of how σE is activated are still unclear but the operon containing rpoE also
contains three other open reading frames designated rseABC. It is believed that the stress
signal is transduced by RseA which is an integral inner membrane protein that binds σEon
the cytoplasmic side and RseB on the periplasmic side. RseC still doesn’t have a clear
function (66).
DegP, a periplasmic endopeptidase, is regulated by σE, cpx, and a third unknown
regulator. It is essential for cell envelope integrity at all temperatures and over
expression can restore some function in rpoE deletion mutants. It is believed to function

18. in proper protein folding and turnover in the periplasm. Additionally rpoE deletion
mutants are sensitive to crystal violet and detergents in addition to heat and ethanol (19).
This sigma factor is not required for induction of thermotolerance but rpoE
deletion mutants are more sensitive to killing at 50°C than wild type, though they
maintain the ability to step up thermotolerance (80). However in wild type E. coli
incubated at 50°C almost all of the transcription of rpoH is by the σEpromoter and thus it
contributes to survival at lethal temperatures (28). The importance of this is that proteins
that are denatured at the outer membrane by HOCl may cause induction of σE and
subsequently activate rpoH causing an increase in thermotolerance. HOCl may induce
heat shock via this pathway.
The cpxRA two component regulon also regulates extra-cytoplasmic stress and
over expression partially substitutes for σE in rpoE deletion mutants. This regulatory
complex is composed of the CpxA phosphokinase with CpxR as the signal transducer and
activates a disulfide bond isomerase (dsbA) and a peptidyl-prolyl-isomerase (ppi) as well
as degP. Over expression of nlpE which is a serine protease inhibitor activates cpx and
can restore function in rpoE deficient cells. Thus it is theorized that nlpE monitors
protease availability in the periplasm and activates cpx when protease activity is
deficient. This is similar to the way rpoH is regulated (19). It is unclear if HOCl could
activate cpx but it seems likely.
The alternate sigma factor, σ32, responds to denatured protein in the cytoplasm and
is most commonly associated with thermal stress. The rpoH gene is the only gene in its
transcriptional unit (28) and has at least 4 promoters. Three of these promoters are
transcribed by σ70(rpoD) and one is transcribed by σE. The gene is constitutively
expressed but it is repressed in several different ways. The mRNA product of rpoH can
base pair in such a way as to block initiation of translation. In addition certain products
of the σ32regulon, DnaK, DnaJ and GrpE, bind to and promote proteolysis of σ32. Those
proteins that bind to σ32are chaperonins that are necessary in increased quantity as
growth temperature is elevated. As more of these proteins are required to correct
14

19. 15
denatured proteins, σ32accumulates and produce more heat shock proteins until sufficient
levels are present to again bind free σ32. The function of this regulon is prevent protein
inactivation, re-fold inactivated proteins, and destroy unrepairable proteins (105). It is
unlikely that this regulon is activated directly by HOCl, but it is discussed because of its
interrelation with rpoE.

20. MATERIALS AND METHODS
PREPARATION OF CULTURE
Seattle FDA isolate 59-SEA13B88 of Escherichia coli O157:H7 (Center for Food
Safety and Quality Enhancement, Griffin, GA.) was used in all experiments. The culture
was stored on cryogenic beads (Microbank®, Pro-Lab, Ontario, Canada) at ~80°C. Beads
were transferred to 6 ml tryptic soy broth [TSB] (Difco, Detroit, MI). After 24 hours, 40
µLwas transferred to 6 mLof TSB. Experiments used cultures transferred three times.
Cells were washed twice in 0.05 M KH2PO4[phosphate buffer] (J. T. Baker, Phillipsburg,
NJ) and suspended to OD600=1.6.
To obtain log phase cells, five ml of culture were transferred to 100 ml of TSB
and incubated at 37°C on a rotary shaker at 200 rpm (Orbit Shaker, Lab Line, Melrose
Park, 1L) for two hours. These cells were rapidly increasing in numbers at the time of
harvest. Stationary phase cells were obtained by transferring 600 µLof culture into 25 mL
of nutrient broth (Difco, Detroit MI) buffered at pH 7.00 with 0.05 M KH2PO4and
supplemented with 0.1% glucose (Fisher Scientific, Fair lawn, MI). Cells were then
incubated for 3.5 days at 37°C on an orbital shaker at 200 rpm. These cells were
observed to be spherical at time of harvest.
CHLORINE TREATMENT
All chlorine solutions were prepared in phosphate buffer made with water filtered
to a resistivity of 16 million ohms/cm [HQ water] (Modulab®, US Filter, Lowell MA) and
buffered to pH 7.000 (Model 720A, Orion Research Inc., Boston, MA). Chlorine
concentrations are reported as total available chlorine. All glassware were rendered free
of chlorine reactive compounds by soaking in 2% sodium hypochlorite solution or
sulfochromic acid followed by rinsing with HQ water. Several dilutions of chlorine were
16

21. 17
prepared to achieve exposure concentrations. First a 200 mg/Lchlorine solution was
prepared from reagent grade sodium hypochlorite solution (Fisher Scientific, Fair Lawn,
MI). The concentration of this solution was confirmed by UV spectroscopy (Ultraspec
4050, LKB Biochrom, Cambridge, England) using the following molar absorption
constants: OCl-; 99.6 M-1cm-1at 235 nm and 26.9 M-1cm-1at 290 nm, HOCl; 7.8 M-1
cm-1at 235 nm and 350.4 M-1cm-1at 290 nm (63). This solution was further diluted to
two mg/Land the concentration of this solution and exposure solutions prepared from it
were confirmed by the method of Chesney et al. (18). Cells were diluted 1:5 in chlorine
or phosphate buffer to achieve chlorinated and unchlorinated cells. After 20 minutes,
chlorine was neutralized with the addition of 1mLsodium thiosulfate (J. T. Baker,
Phillipsburg, NJ) to a concentration of 0.05 mM.
HEATING AND ENUMERATION OF CELLS
Heating of E. coli O157:H7 was done in 50 µLcapillary tubes (Corning, Corning,
NY) with a circulating water bath (Lauda M20, Lauda Dr. R. Wobser GMBH &
Co.,Lauda-Königshofen, Germany). The capillary tubes were sealed with an oxygen/gas
cutting torch. Capillary tubes were sterilized in 70% ethanol, rinsed in sterile phosphate
buffer (pH 7.00) and broken in 5 ml of peptone water. Surviving and initial cell
populations were enumerated by spiral plating (Autoplate® 4000, Exotech, Inc.,
Gaithersburg, MD) on plate count agar (Difco, Detroit, MI) supplemented with 0.1%
sodium pyruvate (Sigma®, St. Louis, MO) (83).
INITIAL CHLORINE TREATMENT STUDIES
Escherichia coli O157:H7 in log or stationary phase were treated in duplicate with
total chlorine solutions of 0 to 1 mg/l in increments of 0.2 mg/l. After chlorine
neutralization, one duplicate received 4 ml of 0.05M KH2PO4buffer. The other duplicate
received 4 ml of nutrient broth (final dilution of original culture was 1:10) (25). Two 50
µLcapillary tubes were filled with each treatment; one was heated one hour after
neutralizing the chlorine and the other was unheated. Log phase cells were heated for 1.5
minutes at 58°C and stationary phase cells were heated at 60°C for 3 minutes. All work

22. 18
was done in a bio-hazard hood (SterilGard® II, The Baker Company, Sanford, ME). Data
were analyzed by three-way mixed ANOVAusing SAS® (SAS Institute, Cary, NC) where
the level of total chlorine was treated as a random variable. The factors of broth addition
and heat treatment were treated as fixed variables.
EFFECT OF CHLORINATION ON D58VALUES
Escherichia coli 0157:H7 in stationary phase were prepared as previously
described. The spent media pH was 7.26 (SD=0.067). D58-values were determined for E.
coli treated with 0.6 mg/l total available chlorine and unchlorinated E. coli suspended in
phosphate buffer or apple juice. Apple juice was shelf stable, contained no added sugar
or preservatives, and was purchased from a local grocery store. The pH (3.6), titratable
acidity (0.4% as malic acid) and °Brix (12°) were measured at the time of purchase. After
chlorine was neutralized, chlorinated and unchlorinated cells were harvested by
centrifugation and suspended in applejuice or phosphate buffer at a final dilution of 1:10
of the original prepared culture. Unchlorinated cells were heated after 30 minutes of
exposure to the suspension medium and chlorinated cells were heated after 20 minutes of
exposure. Bacterial suspensions were heated and sampled over a time sufficient to
achieve a three log reduction and D58values were determined (84). Death curves
determined in buffer and apple juice were from four replications. Death curves of cells
treated with 0.6 mg/Ltotal available chlorine were determined in apple juice and buffer
and were composed of three replications. Least squares linear regression was used to
calculate D-values for each replication.

23. RESULTS
INITIAL CHLORINE TREATMENT STUDIES
Stationary phase and log phase Escherichia coli 0157:H7 heated in nutrient broth
were no more thermotolerant than those heated in buffer (p>0.05). There was no
interaction between the effects of heat treatment and chlorine treatment in log phase cells.
This allows the calculation of the portion killed by heating which was 2.24 (SD=0.08) log
cycles. Reduction in cell numbers due to chlorine treatment range from 0.05 (SD=0.03)
log cycles at 0.2 mg/L total available chlorine to 0.43 (SD=0.07) log cycles at 1mg/L total
available chlorine.
There was interaction between the effects of heat and chlorine treatment using
stationary phase E. coli 0157:H7 (p<0.01). Log reduction of cell population due to
chlorine treatment range from 0.04 (SD=0.014) at 0.2 mg/L total available chlorine to 1.6
(SD=0.19) at 1 mg/L total available chlorine. Due to interaction between the effects of
heat and chlorine treatments the log reduction due to heat increases from 1.00
(SD=0.054) to 2.88 (SD=0.46) with increasing concentration of total available chlorine.
Figure 2 shows that 0.6 mg/L total available chlorine caused the greatest reduction in heat
resistance without significant kill [fig 1].
EFFECTS CHLORINATION ON D58VALUES
The D58for inactivation of unchlorinated stationary phase Escherichia coli
0157:H7 was 5.45 (SD=0.89) and 1.59 (SD=0.34) in buffer and apple juice, respectively.
Stationary phase E. coli cells treated with 0.6 mg/L total available chlorine had D58values
of 1.65 (SD=0.3) and 0.80 (SD=0.055) in buffer and apple juice, respectively. These
values show that chlorine treatments decrease the heat resistance of E. coli 0157:H7 in
buffer (p<0.01) and apple juice (p<0.05). Death curves in buffer were linear [fig 2.] and
19

24. death curves in apple juice were not [fig. 3]. The average R-square for D-values were
0.935 for cells heated in buffer, 0.975 for chlorinated cells heated in buffer, 0.825 for
cells heated in apple juice and 0.9 for chlorinated cells heated in apple juice.
Since death curves in apple juice used to calculate D-values were nonlinear [fig.
3] it was necessary to consider more detailed analysis. Numbers of unchlorinated cells
heated in apple juice are reduced by 83% (SD=15.3) upon exposure to heat in the first
minute followed by little kill for up to three minutes. After three minutes, 19% of the
original population remains and declines with a D58of 0.77 (SD=0.25). Numbers of
chlorinated cells decline by 95 to 99% in 30 seconds. The remaining population
represents about 1% to 5% of the original population and declines with a D58of 1.19,
without delay [fig. 3]. Solid lines in figure three show linear regression of the linear
portions of the death curves in apple juice. The p-value for difference between the means
is a marginal 0.05. Time (min) to kill 90% of the population was also calculated for
chlorine treated and untreated cells in apple juice and found to be 3.14 (SD=0.11) and 0.3
(SD=0.05), respectively.

25. DISCUSSION
This work determined the effects of chlorine treatment on stationary and log phase
Escherichia coli O157:H7 grown at neutral pH. Conditions of cell growth necessary to
maintain a neutral pH were used because acid adaption results in decreased resistance to
hypochlorous acid (54). Cells were washed twice to remove organic material present in
the inoculum as well as to ensure that reactions take place at the cell membrane.
Phosphate buffer was used to reduce reaction of chlorine with organic material.
The initial chlorine treatment experiments determined if chlorine treatment of E.
coli O157:H7 could induce heat resistance and what chlorine concentration had the
greatest effect on thermal inactivation of stationary and log phase cells. Broth and buffer
were used in these experiments because Dukan et al (25) reported that broth addition is
required to allow production of heat shock proteins after chlorine exposure. Thus
interaction between the effects of broth, heat and chlorine would indicate induction of
heat resistance. Since there was no such interaction for either log or stationary phase E.
coli O157:H7, we concluded that induced heat resistance was not detectable by these
experiments. These experiments also indicate that chlorine treatment had no apparent
effect on thermal inactivation of log phase cells. Since 0.6 mg/L total available chlorine
had the greatest effect on thermal inactivation of stationary phase E. coli O157:H7, only
stationary phase cells were selected for D58measurements.
The D58of untreated E. coli O157:H7 in applejuice, calculated using all data
points, of 1.59 minutes is similar to the 1minute value reported by Splittstoesser et al.
(89). This overall D-value has limited practical use since it is heavily influenced by the
initial hump on the death curve. However, this overall D-value as well as the time to kill
90% of the population indicates that chlorine treatment sensitizes most of the cell
21

26. 22
population to thermal inactivation by eliminating the initial hump of the inactivation
curve.
The death curve of chlorine treated E. coli O157:H7 in apple juice differs in shape
from that of untreated cells [fig. 1]. The absence of a hump in the curve for treated cells
is because hypochlorous acid increases the permeability of the E. coli cell membrane to
small molecules (88). The untreated cells lose cell envelope integrity as they are heated,
which may produce an effect similar to the increased permeability caused by
hypochlorous acid. Thus it is possible that treated cells lack the hump because their cell
envelopes were rendered permeable by the chlorine.
1% to 5% of chlorinated E. coli O157:H7 heated in apple juice are marginally
more heat resistant than unchlorinated cells remaining after heating for three minutes in
apple juice. This increased heat resistance may be the result of exposure to chlorine, and
was not observed with treated cells heated in buffer. These observations are consistent
with reports by Dukan et al (25) that heat shock proteins are produced by E. coli
O157:H7 if broth is added after sublethal exposure. The induced heat resistance would
not have been detected by the initial chlorine exposure experiments either because those
experiments used a single heat treatment or this response is specific to apple juice.
It is unlikely that heat shock proteins are produced as a result of the presence of
chlorine denatured proteins in the cytoplasm for two reasons. Several studies have shown
that hypochlorous acid does not react with cytoplasmic components before cell
inactivation occurs. (1, 59). The second reason is related to the way the rpoH heat shock
regulon is activated. DnaK, DnaJ and GrpE bind to σ32, the protein product of rpoH, and
facilitate its destruction. As environmental temperature increases, denatured proteins
begin to compete for binding of these regulatory proteins whose job it is to fold proteins,
refold denatured proteins and facilitate the destruction of proteins too badly damaged to
be repaired. This decreased affinity for σ32 allows it to accumulate and transcribe heat
shock proteins including its regulatory proteins and a new equilibrium is achieved. Thus
rpoH is regulated by denatured cytoplasmic proteins by post-translationa1control and

27. 23
requires about five minutes to reach maximal levels of heat shock proteins when activated
in this manner (30, 49, 53, 105). However, heat shock proteins do not achieve maximal
levels until one hour after exposure to chlorine (25).
We propose that activation of rpoE regulon is the cause of increased production of
heat shock proteins in E. coli O157:H7. The gene rpoE produces the alternate sigma
factor σE(42) and is activated in an unclear manner by increasing levels of denatured
outer membrane proteins (OMPs) (61, 80). RpoH is one of only two genes known to be
activated by (105). Hypochlorous acid reacts with the outer membrane proteins
causing lost ability to transport carbohydrates, loss of ATPase activity and loss of
electron transport ability (1, 13, 34, 71). Thus chlorine exposure could activate
transcription of rpoE and this activation would increase transcription of rpoH. This type
of rpoH regulation takes place at the transcriptional level, so the time required for
maximal heat shock proteins would be more consistent with those reported by Dukan et
al. (25).
Chlorine treatment reduces the thermal resistance of E. coli O157:H7 heated in
buffer and apple juice. Increased rates ofE. coli inactivation are more evident in buffer
than apple juice where there is a possible thermotolerance response in a small portion of
the population. Untreated cells require twice as much heating time to achieve kill similar
to treated cells due to a hump in the inactivation curve. It is important to note though that
chlorine treated cells were exposed to apple juice for only twenty minutes prior to
heating, not the hour that is reported (25) to be required for maximum induction of heat
shock proteins. Thus given more adaptation time E. coli O157:H7 might become more
thermotolerant. However, Ingham and Uljas (44) have found that E. coli O157:H7
exposed to apple juice for two to six hours exhibit reduced thermotolerance, but the effect
of longer resident times in apple juice on the thermotolerance of chlorine treated E. coli
O157:H7 needs further investigation. Our study indicates that prior exposure to
hypochlorous acid need not be accounted for in developing heating regimens to kill E.
coli O157:H7.

28. Figures
Figure 1: Mean stationary phase Escherichia coli O157:H7 survivors after chlorine
treatment (O), and after both chlorine treatment and heating for 3.0 mins at 60°C (■).
Figure 2: Mean death curves of stationary phase Escherichia coli O157:H7 in phosphate
buffer heated at 58°C: Cells treated with 0.6 mg/L total available chlorine (O), untreated
cells (■). Solid lines: regression lines.
Figure 3: Mean death curves of stationary phase Escherichia coli O157:H7 in applejuice
heated at 58°C: Cells exposed to 0.6 mg/L total available chlorine (O), unexposed cells
(■). Solid lines: regression line of linear population.
24

29. 25

30. 26

31. 27

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