The document discusses microbial control through various physical and chemical methods. It begins with a brief history of bubonic plague in Europe during the Middle Ages caused by the Yersinia pestis bacterium. It then defines key terms like sterilization, disinfection, bactericidal and bacteriostatic. The document outlines several conditions that influence the effectiveness of antimicrobial agents and describes various physical methods of microbial control including heat, filtration and radiation. It provides details on how these methods kill microbes and examples of their applications.

1. Microbial Control

2. Outline
• History
• Definition of terms
• Pattern of microbial death
• Antimicrobial agents
– Physical
– Chemical

3. History
Ring- a - ring of rosies
Pocketful of posies
Atichoo ! Atichoo !
We all fall down

4. Bubonic Plague or the
Black Death
• Epidemic swept thru Europe in the Middle
Ages (13th and 14th centuries)
• 40 million people were killed
– About 1/3 of the population of the continent
• Etiological agent:
– Yersinia pestis Gram (-) rod
• 2 Vectors
– Rat
– Flea

5. Yersinia pestis - Gram (-) bacillus
Vectors - Rat and Flea

6. Bubonic Plague Infection
1. Flea bite with Yersinia pestis
2. Bacteria multiply in the bloodstream
Bacteremia
3. Bacteria localize in lymph nodes,
especially axillary and groin areas

7. 4. Hemorrhaging occurs in lymph nodes,
resulting in reddish rash that will eventually
become “black and blue” swellings or Buboes
(hence the name Bubonic Plague or Black
Death)

8. Bubonic Plague Infection
5. If untreated, about 50 % Mortality Rate
6. If bacteria spread to the lungs, it becomes
Pneumonic Plague and is now highly
contagious (Almost a 99 % Mortality Rate)

9. History
Ring- a - ring of rosies
Pocketful of posies
Atichoo ! Atichoo !
We all fall down
A pouch of sweet smelling herbs or posies were carried due to
the belief that the disease was transmitted by bad smells

10. Humans vs. Microbes
• Most of History, microbes have been
winning the battle
• In the last 100 yrs or so the battle has
swung in our favor
– Why?
• Because of our increasing knowledge of how to
Control Microbial Growth

11. Smallpox
Variola virus
Eradicated in 1977 (Somalia)

12. Terms :
Sterilization vs. Disinfection
• Sterilization
– destroying all forms of life
• Disinfection
– destroying pathogens or unwanted
organisms

13. Disinfectant vs. Antiseptic
• Disinfectant
– antimicrobial agent used on inanimate objects
• Antiseptic
– antimicrobial agent used on living tissue

14. cidal vs. static
• Bactericidal - kills bacteria
• Bacteristatic - inhibits bacterial growth
• Fungicidal
• Fungistatic
• Algacidal
• Algastatic

15. — Commercial Sterilization: Killing C. botulinum endospores

16. Microbial death
“a microbe is defined DEAD if it does not
grow when inoculated into culture medium
that would normally support its growth”
Death is defined as the inability of the
organisms to form a visible colony
–James Jay
(Modern Food Microbiology 6th ed)

17. “It takes more
time to kill a
large population
of bacteria than
it does to kill a
small
population,
because only a
fraction of
organisms die
during a given
time interval.
Bacterial populations die at a
constant logarithmic rate

18. Conditions Influencing Effectiveness
of Antimicrobial Agent Activity
1. Population size
– Larger population requires a longer time to die
2. Population composition
– Microorganisms vary markedly on susceptibility
– Vegetative versus Spores
– Young versus Mature cells
5. Concentration or Intensity of an Antimicrobial Agent
– The more concentrated an agent the more rapidly
microbes can be destroyed
– BUT sometimes an agent may be more effective at lower
concentrations (e.g. 70% alcohol)

20. Conditions Influencing Effectiveness
of Antimicrobial Agent Activity
3. Duration of Exposure
– The longer the exposure to an agent the more they will be
killed
6. Temperature
– An increase in temperature at which a chemical acts often
enhances it activity
– Example: acids used in high T = more effective
7. Local environment
– pH, organic matter, etc
– Controls or Protects the pathogen

22. Targets of Antimicrobial Agents
1. Cell membrane
2. Enzymes & Proteins
3. DNA & RNA

23. Methods of microbial control
• Physical
– Heat
– Filtration
– Radiation
• Chemical

24. HEAT
• Flaming
• Incineration
• Hot-air sterilization
• Moist-air sterilization
Hot-air Autoclave
Equivalent treatments 170˚C, 2 hr 121˚C, 15 min

25. HOW DOES HEAT KILL
MICROBES
• MOIST HEAT
– Kill effectively by degrading nucleic acids and by
denaturing enzymes and other essential proteins
– May also disrupt cell membranes
• DRY HEAT
– Microbial death results from the oxidation of cell
constituents and denaturation of proteins

26. MOIST HEAT
STERILIZATION

27. • Thermal Death Point (TDP)
– The lowest temperature at which a microbial
suspension in killed in 10 minutes
• Thermal Death Time (TDT)
– The shortest time needed to kill all organisms
in a microbial suspension at a specific
temperature and under defined conditions

28. • Decimal Reduction Time or D value
– Time required to kill 90% of the
microorganisms or spores in a sample at a
specified temperature
– Time required for the line to drop by one log
cycle or tenfold
– Used to estimate the relative resistance of a
microbe to different temperatures
However, such a destruction is logarithmic and it is theoretically
NOT POSSIBLE to “completely destroy” microbes in a sample

29. The z Value
and
The F Value
• z Value
– The increase in temperature required to reduce D
to 1/10 its value or to reduce it by one log cycle
• F value
– Time in minutes at a specific temperature needed
to kill a population of cells or spores
– Usually 121°C

31. APPLICATION:
After a food have been canned, it must be heated to
eliminate the risk of botulism arising from the presence of
Clostridium spores
Example:
IF the D value = 0.204 minutes
It would take 12D or 2.5 minutes to
reduce the spore number by heating at
the specified temperature

32. APPLICATION:
• If the z value for Clostridium spores is 10°C
• It takes a 10°C change in temperature to alter
the D value tenfold
• THUS: if the cans are to be processed at
111°C rather than 121°C, the D value would
increase by tenfold t 2.04 minutes
• The 12D value = 24.5 minutes

33. THE USE OF PHYSICAL
METHODS: FILTRATION
• Applicable for heat-sensitive materials that needs sterilization
• 2 types of filters
– Depth filters: consist of fibrous or granular materials that have been
bonded into a thick layer filled with twisting channels of small diameter
– Membrane filters: porous membranes; 0.2 µm pore sizes
• Laminar flow hood versus biological safety cabinets (HEPA filters)
– High Efficiency Particulate Air
– Remove 99.97% particles
– for sterilizing AIR

35. THE USE OF PHYSICAL
METHODS: RADIATION
• IONIZING RADIATION
• X rays, gamma rays, electron beams
• Excellent as a sterilizing agent and penetrates deep into objects
• NON-IONIZING RADIATION
– UV (about 260nm)
– Quite lethal but does not penetrate glass, dirt films, water and
other substances very effectively
• Microwaves: kill by heat not usually antimicrobial