2. Characteristics of the
Enterobacteriaceae
• Gram-negative rods
• Glucose is fermented with strong acid
formation and often gas
• Cytochrome oxidase activity is
negative
• Nitrate is reduced to nitrite
3. Gram’s Stain for Bacterial
Morphology
• Crystal violet binds to cell wall
peptidoglycan with Gram’s iodine as a
mordant
• Safranin or basic fuchsin counterstains
bacterial cells decolorized by alcohol-
acetone
4. Gram’s Stain for Bacterial
Morphology
• Thick cell-wall peptidoglycan layer of gram-
positive bacteria strongly binds crystal violet
and resists decolorization by alcohol-acetone
• Thin cell-wall peptidoglycan layer of gram-
negative bacteria located beneath a thick
lipid-rich outer membrane weakly binds
crystal violet that is readily removed by
alcohol-acetone decolorization
5. Gram’s Stain Procedure
• Flood surface of smear with crystal violet solution
• After 1 min thoroughly rinse with cold tap water
• Flood smear with Gram’s iodine for 1 min
• Rinse smear with acetone-alcohol decolorizer until
no more crystal violet in rinse effluent
• Rinse with cold tap water
• Flood smear with safranin (regular Gram’s stain) or
basic fuchsin (enhanced Gram’s stain)
• Rinse with cold tap water
• Dry smear in slide rack
• Microscopically examine stained smear using oil-
immersion light microscopy
6. Glucose Fermentation
• Oxidation-reduction of glucose in the absence of molecular oxygen
(anaerobic glycolysis)
• Energy from hydrolysis of chemical bonds in anaerobic glycolysis
captured as high energy phosphate bonds of adenosine triphosphate
(ATP)
• NAD is reduced to NADH2 by accepting electrons during glycolytic
conversion of glucose to pyruvate
• NADH2 in turn reduces pyruvate with oxidation of NADH2 to NAD which
supports continued anaerobic glycolysis, and generation from
pyruvate of alcohols, carboxylic acids, and CO2 gas
• End products of glucose fermentation: organic acids and CO2 gas
• Fermentation detected by acidification of glucose-containing broth
(color change in broth or agar medium containing pH indicators), and
(for aerogenic species) production of gas (fractures in agar, gas
bubbles in inverted Durham tube)
• pH indicators: phenol red (yellow at acid pH), methyl red (red at acid
pH), neutral red (red at acid pH), bromcresol purple (yellow at acid pH)
7. Spot Cytochrome Oxidase Test
• The spot cytochrome oxidase test is
the first test performed with gram-
negative bacteria recovered in culture
• The optimal plate medium for a spot
cytochrome oxidase test is a trypticase
soy agar (TSA) containing 5% sheep
blood
• Bacterial colonies should be 18 to 24 hr
old
8. Spot Cytochrome Oxidase Test
• In a positive test, bacterial cytochrome
oxidase oxidizes the colorless reduced
substrate tetramethyl-p-phenylenediamine
dihydrochloride (TPDD) forming a dark
purple oxidized indophenol product
• Streak a small portion of bacterial colony to
filter paper soaked with a 1% solution of
TPDD
• If the streak mark turns purple in 10 sec or
less, the spot oxidase test is interpreted as
positive
9. Nitrate Reduction
• Enterobacteriaceae extract oxygen from
nitrate (NO3)producing nitrite (NO2)
• NO2 detected by reaction with α-
naphthylamine and sulfanilic acid producing
a red colored complex
• Absence of red color indicates either no
reduction of NO3 or reduction to products
other than NO2 (denitrification)
• Confirmation of true negative test: addition
of zinc ions which reduce NO3 to NO2
producing a red color in the presence of α-
naphthylamine and sulfanilic acid
10. Enterobacteriaceae: Genetic
Properties
• Chromosomal DNA has 39-59%
guanine-plus-cytosine (G+C) content
• Escherichia coli is the type genus and
species of the Enterobacteriaceae
• Species of Enterobacteriaceae more
closely related by evolutionary
distance to Escherichia coli than to
organisms of other families
(Pseudomonadaceae,
Aeromonadaceae)
12. Enterobacteriaceae:
Microbiological Properties
• Gram-negative and rod shaped (bacilli)
• Ferment rather than oxidize D-glucose
with acid and (often) gas production
• Reduce nitrate to nitrite
• Grow readily on 5% sheep blood or
chocolate agar in carbon dioxide or
ambient air
• Grow anaerobically (facultative
anaerobes)
13. Enterobacteriaceae:
Microbiological Properties
• Catalase positive and cytochrome oxidase
negative
• Grow readily on MacConkey (MAC) and eosin
methylene blue (EMB) agars
• Grow readily at 35o
C except Yersinia (25o
-
30o
C)
• Motile by peritrichous flagella except Shigella
and Klebsiella which are non-motile
• Do not form spores
21. Triple Sugar Iron (TSI) Agar
• Yeast extract 0.3% (% = grams/100 mL)
• Beef extract 0.3%
• Peptone 1.5%
• Proteose peptone 0.5%
Total Protein = 2.6%
• Lactose 1.0%
• Sucrose1
1.0%
• Glucose 0.1%
Carbohydrate = 2.1%
1
Absent in Kligler Iron Agar
22. Triple Sugar Iron (TSI) Agar
• Ferrous sulfate
• Sodium thiosulfate
• Sodium chloride
• Agar (1.2%)
• Phenol red
• pH = 7.4
23. TSI Reactions of the
Enterobacteriaceae
• Yellow deep, purple slant: acid deep due to glucose
fermentation , no lactose or sucrose fermentation with alkaline
slant due to production of amine’s from protein
• Black deep, purple slant: acid deep due to glucose
fermentation with H2S production, no lactose or sucrose
fermentation
• Yellow deep and slant: acid deep and slant due to glucose as
well as lactose and/or sucrose fermentation
• Black deep and yellow or black slant: acid deep and slant with
glucose and lactose and/or sucrose fermentation with H2S
production
• Fracturing or lifting of agar from base of culture tube: CO2
production
24.
25. TSI Reactions of the
Enterobacteriaceae
• A/A + g = acid/acid plus gas (CO2)
• A/A = acid/acid
• A/A + g, H2S = acid/acid plus gas, H2S
• Alk/A = alkaline/acid
• Alk/A + g = alkaline/acid plus gas
• Alk/A + g, H2S = alkaline/acid plus gas, H2S
• Alk/A + g, H2S (w) = alkaline/acid plus gas,
H2S (weak)
33. MacConkey (MAC) Agar
• Peptone 1.7%
• Polypeptone 0.3%
• Lactose1
1.0%
• Bile salts2
0.15%
• Crystal violet2
• Neutral red3
• Sodium chloride 0.5%
• Agar 1.35%
• pH=7.1
1
Differential medium for lactose fermentation
2
Inhibit gram positives and fastidious gram-negatives; MAC agar selective for
gram-negatives
3
Red color at pH < 6.8
34.
35.
36. Eosin Methylene Blue (EMB)
Agar (Levine)
• Peptone 1.0%
• Lactose1
0.5%
• Eosin y2
• Methylene blue2
• Agar
• pH = 7.2
1
Modified formula also contains sucrose (0.5%)
2
Inhibit gram-positives and fastidious gram-negatives; selective
for gram-negatives. Eosin y and methylene blue form a
precipitate at acid pH; differential for lactose fermentation
37.
38.
39. Bacterial Utilization of Lactose
• Presence of β-galactoside permease:
Transport of β-galactoside (lactose)
across the bacterial cell wall
• Presence of β-galactosidase:
Hydrolysis of β-galactoside bond
(lactose⇒glucose + galactose)
• ONPG: Orthonitrophenyl-β-D-galacto-
pyranoside
40. Differential Reactions of the
Enterobacteriaceae by TSI,
ONPG, and MAC
• Escherichia coli Red colonies,
(A/A, ONPG+) pitted
• Klebsiella1
Red colonies,
(A/A, ONPG+) mucoid
• Enterobacter Red colonies
(A/A, ONPG+)
• Citrobacter2
Red or colorless
(A/A or Alk/A, ONPG+) colonies
• Serratia Colorless colonies
(A/A, ONPG+)
1
K. pneumoniae, indole –, K. oxytoca, indole +
2
C. freundii, indole – and H2S +, C. koseri, indole + and H2S –
41. Differential Reactions of the
Enterobacteriaceae by TSI,
ONPG, and MAC
• Shigella Colorless Colonies
(Alk/A; ONPG – A, B, and C1
; ONPG + D1
)
• Salmonella Colorless Colonies
(Alk/A + H2S; ONPG –)
• Proteus Colorless Colonies
(Alk/A + H2S2
; ONPG –)
• Edwardsiella tarda Colorless Colonies
(Alk/A + H2S; ONPG–)
• Yersinia Colorless Colonies
(A/A, ONPG +)
1
Shigella A, B, and C, ornithine –; Shigella D, ornithine +
2
Proteus mirabilis. P. vulgaris sucrose + with A/A + H2S on
TSI
42. Differential Reactions of the
Enterobacteriaceae by EMB
• Escherichia coli Colonies with metallic green
sheen
• Klebsiella Colonies with precipitate (ppt)
and mucoid appearance
• Enterobacter Colonies with ppt
• Citrobacter Colonies with/without ppt
• Serratia Colonies without ppt
• Shigella Colonies without ppt
• Salmonella Colonies without ppt
• Proteus Colonies without ppt
• Yersinia Colonies without ppt
45. XLD Agar: Growth of
Salmonella
• Salmonella selective due to bile salt.
• Xylose fermentation (except Salmonella
serotype Paratyphi A) acidifies agar
activating lysine decarboxylase. With
xylose depletion fermentation ceases,
and colonies of Salmonella (except S.
Paratyphi A) alkalinize the agar due to
amines from lysine decarboxylation.
• Xylose fermentation provides H+
for
H2S production (except S. Paratyphi A).
46. XLD Agar: Appearance of
Salmonella
• Ferric ammonium citrate present in
XLD agar reacts with H2S gas and
forms black precipitates within
colonies of Salmonella.
• Agar becomes red-purple due to
alkaline pH produced by amines.
• Back colonies growing on red-purple
agar-presumptive for Salmonella.
47.
48.
49. XLD Agar: Growth of Escherichia
coli and Klebsiella pneumoniae
Escherichia coli and Klebsiella pneumoniae are
lysine-positive coliforms that are also lactose
and sucrose fermenters. The high lactose and
sucrose concentrations result in strong acid
production, which quenches amines produced
by lysine decarboxylation. Colonies and agar
appear bright yellow. Neither Escherichia coli
nor Klebsiella pneumoniae produce H2S.
50. XLD Agar: Growth of Shigella
and Proteus
• Shigella species do not ferment xylose,
lactose, and sucrose, do not decarboxylate
lysine, and do not produce H2S. Colonies
appear colorless.
• Proteus mirabilis ferments xylose, and
thereby provides H+
for H2S production.
Colonies appear black on an agar unchanged
in color (Proteus deaminates rather than
decarboxylates amino acids). Proteus
vulgaris ferments sucrose, and colonies
appear black on a yellow agar.
54. HE Agar: Growth of Enteric
Pathogens and Commensals
• High bile salt concentration inhibits growth of gram-
positive and gram-negative intestinal commensals,
and thereby selects for pathogenic Salmonella (bile-
resistant growth) present in fecal specimens.
• Salmonella species as non-lactose and non-sucrose
fermenters that produce H2S form colorless colonies
with black centers.
• Shigella species (non-lactose and non-sucrose
fermenters, no H2S production) form colorless
colonies.
• Lactose and sucrose fermenters (E. coli, K.
pneumoniae) form orange to yellow colonies due to
acid production.
55.
56. Salmonella-Shigella Agar
• Beef extract 0.5%
• Peptone 0.5%
• Bile salts 0.85%
• Sodium citrate 0.85%
• Brilliant green dye Trace
• Lactose 1.0%
• Sodium thiosulfate 0.85%
• Ferric citrate 0.1%
• Neutral red
• Agar 1.4%
• pH = 7.4
57. Salmonella-Shigella (SS) Agar
• Bile salts, citrates, and brilliant green dye inhibit
gram-positives and most gram-negative coliforms
• Lactose the sole carbohydrate
• Sodium thiosulfate a source of sulfur for H2S
production
• Salmonella forms transparent colonies with black
centers
• Shigella forms transparent colonies without
blackening
• Lactose fermentative Enterobacteriaceae produce
pink to red colonies with bile precipitate for strong
lactose fermenters
58. Use of Selective-Differential Agars for
Recovery of the Enterobacteriaceae from
Different Types of Specimens
• Feces1
: MAC or EMB + XLD &/or SS or HE2
• Sputum and Urine1
: MAC or EMB
• Wound3
:MAC or EMB
• Peritoneal and pleural fluid4
: MAC or EMB
• Subculture of blood positive for gram-negative’s in broth
culture4
: MAC or EMB
• CSF, pericardial fluid, synovial fluid, bone marrow5
: Not
required
1
Heavy population of commensal bacteria
2
Utilized with enrichment broth containing selenite or mannitol to
differentially inhibit enteric commensals
3
Commensal bacteria (skin) and frequent polymicrobial etiology
4
Possible polymicrobial etiology (normally sterile fluids)
5
Normally sterile, unimicrobial etiology predominant
59. Selectivity of Differential Agars
for Salmonella1
and Shigella2
• HE or SS agar (absence of lactose
fermentation1,2
, H2S production1
)
• XLD agar (absence of lactose fermentation1,2
,
H2S production1
, lysine decarboxylation1
)
• MAC or EMB agar (absence of lactose
fermentation1,2
)
• TSI agar (glucose fermentation1,2
, absence of
lactose fermentation1,2
, H2S production1
)
Descending Order of Selectivity for Salmonella
and Shigella
60. Recommended Reading
Winn, W., Jr., Allen, S., Janda, W., Koneman, E.,
Procop, G., Schrenckenberger, P., Woods, G.
Koneman’s Color Atlas and Textbook of
Diagnostic Microbiology, Sixth Edition,
Lippincott Williams & Wilkins, 2006:
• Chapter 5. Medical Bacteriology: Taxonomy,
Morphology, Physiology, and Virulence.
• Chapter 6. The Enterobacteriaceae.
61. Recommended Reading
Murray, P., Baron, E., Jorgensen, J.,
Landry,
M., Pfaller, M.
Manual of Clinical Microbiology, 9th
Edition, ASM Press, 2007:
• Farmer, J.J., III, Boatwright, K.D., and
Janda J.M. Chapter 42.
Enterobacteriaceae: Introduction and
Identification
