This document provides an overview of physical and chemical methods for controlling microbes. It discusses various methods including heat, radiation, filtration, and chemicals. Heat methods include moist heat sterilization using autoclaves at 121°C for 10-40 minutes and dry heat using higher temperatures like 150-180°C in dry ovens. Radiation methods are ionizing radiation like gamma rays and X-rays, and nonionizing UV light. Chemical methods cover categories like halogens (chlorine, iodine), phenols, alcohols, hydrogen peroxide, and aldehydes. Each works by disrupting microbial cells through different mechanisms like damaging cell membranes, denaturing proteins, or interfering with

1. Chapter 11
Physical and
Chemical Agents
for Microbial
Control

2. Control of microbes
Physical and chemical
methods to destroy or reduce
microbes in a given area

3. OVERVIEW
3

4. Why do we want to control
microbes?
How do we do it?

5. Relative resistance of microbes
Least resistance
Highest resistance
– Bacterial endospores
– Prions
Moderate resistance
– Pseudomonas sp.
– Mycobacterium
tuberculosis
– Staphylococcus aureus
– Protozoan cysts
–
–
–
–
–
most vegetative cells
Fungal spores
enveloped viruses
Yeast
Protozoan trophozoites

6. Methods that kill microbes
_______________ – a process that destroys all
viable microbes, including viruses & endospores
– Heat, sterilants
_______________– a process to destroy
vegetative pathogens, not endospores
– disinfectants or germicides – chemical - kills
pathogenic microorganisms
– _____________________ – destroy/inhibit vegetative
pathogens on exposed body surfaces
– Sepsis – infection/growth in body
– Asepsis – preventing entry of pathogens and infection

7. Methods that reduce numbers
_______________– any cleansing technique that
mechanically removes microbes and reduces
numbers to safe levels
– _______________– reduces the number of microbes
on skin
___________________________ – controls
numbers by preventing growth (multipication)

8. 8

9. Microbial death
Permanent termination of an organism’s
vital processes
microbiological definition:
Involves permanent loss of reproductive
capability, even under optimum growth
conditions

10. Factors that influence microbial killing
10

11. How antimicrobial agents work:
Cellular targets of control
(mode of action)
1. Cell wall
2. Cell membrane
3. Cellular synthetic processes (DNA, RNA)
4. Proteins

12. mode of action
Cell wall
Action:
– Block synthesis
– Digestion
– Disrupt surface
w/o cell wall, bacteria will lyse (especially
gram positive… remember?)
Examples: penicillin, detergents, alcohol

13. Surfactants disrupt
mode of action

14. mode of action
Affect on synthesis (DNA, RNA)
Proteins – have many functions in the cell!
Antimicrobials can block: DNA synthesis
(master code), transcription, translation
 Mutagens (radiation = permanent
inactivation of DNA)
 Antimicrobial therapy (drugs)
 Chemicals – some destroy nucleic acids

15. mode of action
Heat, pH, heavy metals can alter proteins

16. Practical concerns
Does the application require sterilization?
Is the item to be reused? (time, $$)
Can the item withstand heat, pressure, radiation,
or chemicals?
Is the method suitable?
Will the agent penetrate to the necessary
extent?
Is the method cost- and labor-efficient & is it
safe?

17. Types of Control:
I. Methods of Physical Control
1. Heat
2. Cold temperatures
3. Desiccation
4. Radiation
5. Filtration

18. Physical Control
1. Heat
Moist vs. dry
Moist heat = lower temp and shorter time
Causes coagulation/denaturation of
protein
Dry heat = higher temp/longer time
Dehydrates cell, removes water,
denatures proteins, oxidation (burning)

19. Physical Control

20. Physical Control
1. Heat – moist heat
Moist heat uses hot water or steam
sterilization
 _______________ 15 psi/121oC/10-40min (steam
under pressure)
 intermittent sterilization – 100oC 30-60 min for 3
days (unpressurized steam)
disinfection
 Boiling at 100oC for 30 minutes to destroy nonspore-forming pathogens
 _______________ kills Salmonella, Listeria &
overall microbe count

21. Physical Control

22. Pasteurization
Pasteurization – heat  kill
potential agents of infection and
spoilage without destroying the
food flavor or value
63°C–66°C for 30 minutes (batch
method)
71.6°C for 15 seconds (flash
method)
Not sterilization – kills non-sporeforming pathogens and lowers
overall microbe count; does not kill
endospores or many
nonpathogenic microbes
22

23. Physical Control
1. Heat – dry heat
Dry heat uses higher
temperatures than moist
heat, can also sterilize
incineration – 600-1200oC
combusts & dehydrates
cells
dry ovens – 150-180oCcoagulate proteins

24. Thermal death
Physical Control
Thermal death time (TDT) – shortest
length of time required to kill all test
microbes at a specified temperature
Thermal death point (TDP) – lowest
temperature required to kill all microbes in
a sample in 10 minutes

25. Physical Control
2. Cold temperatures
_______________– slows the growth of
microbes
refrigeration 0-15oC & freezing <0oC
used to ____________food, media and
cultures

26. Physical Control
3. Desiccation
gradual removal of water from cells, leads
to metabolic inhibition
not effective microbial control – many cells
retain ability to grow when water is
reintroduced
_______________ = freeze-drying
Note: cold and dessication ARE NOT good methods of
disinfection or sterilization.

27. Physical Control
4. Radiation
1. _______________ radiation – deep
penetrating power, breaks DNA
– gamma rays, X-rays, cathode rays
– used to sterilize medical supplies & food
products
1. _______________ radiation – little
penetrating power to sterilize air, water &
solid surfaces
– uv light creates thymine dimers, which
interfere with replication

28. Ionizing radiation
Physical Control

29. Nonionizing radiation - UV
Physical Control

30. UV treatment of wastewater
Physical Control

31. Physical Control
Other “waves”
Sound (high frequency)
Can also be used to disrupt cells
(vibrations) or generate heat
Ultrasonic devices are used clean dental,
medical instruments before sterilization

32. 5. Filtration
physical removal of
microbes by passing a
gas or liquid through
filter
Pores of filter large
enough for liquid but too
small for microbe (<1
μm)
used to
Physical Control

33. OVERVIEW
33

34. Types of Control:
II. Methods of Chemical Control
Categories:
•
Halogens
•
Phenolics
•
Chlorhexidine
•
Alcohols
•
Hydrogen peroxide
•
Detergents & soaps
•
Heavy metals
•
Aldehydes
Uses:
•Disinfectants
•antiseptics
•sterilants
•degermers
•preservatives

35. Chemical antimicrobials
10,000 manufactured today
About 1,000 routinely used
Society is obsessed with “killing germs” –
to the point of being excessive
Result: widespread overuse  resistance
of pathogens, death of natural flora

36. Qualities of chemical antimicrobials
Rapid action, even in low concentration
Water/alcohol soluble, stabile
Broad spectrum w/o being toxic
Penetration, sustained action
Resitance to inactivation
Noncorrosive, nonstaining
Sanitizing and deodorizing
Inexpensive and available

37. Levels of activity

38. Factors that affect activity
Type of microorganism being treated
Material being treated
Amt of contamination
Exposure time
Strength/action of germicide
 Appendix C shows procedures for testing
effectiveness

39. Ways to express
strength/concentration
Dilution (1:200 is one part chemical in 200
parts dilutant such as water)
ppm – parts per million
Percent – 70% alcohol, or mg/ml
_______________ solutions – have water
as the solvent
_______________– are dissolved in
alcohol

40. Chemical control - categories
1.
2.
3.
4.
5.
6.
7.
8.
Halogens
Phenolics
Chlorhexidine
Alcohols
Hydrogen peroxide
Detergents & soaps
Heavy metals
Aldehydes

41. 1. Halogens
Ionic (halide) or nonionic
Mostly _______________
Germicidal and sporicidial with long
exposure
Affect protein structure (bonds)

42. 1. Halogens
Chlorine – Cl2, hypochlorites (chlorine
bleach - OCl), chloramines
– In water – release hypochlorous acid (HOCl)
– Denaturation of proteins by disrupting disulfide
bonds
– Can be sporicidal
– 0.6-1 ppm Cl2 to clean water
– Bleach – sanitization/disinfection

43. 1. Halogens
Iodine - I2, iodophors (betadine)
– Denature proteins – similar to Chlorine but not
affected by organic matter or pH
– Broad spectrum microbicide, can be sporicidal
– Milder medical & dental degerming agents,
disinfectants, ointments, topical antiseptic
– Betadine (iodophor) = iodine + neutral
polymer; allows for slow release and
increased penetration – used as antiseptic

44. 44

45. 2. Phenolics
Phenol ring (aromatic carbon ring) +
groups
Disrupt cell membranes & precipitate
(denature) proteins; bactericidal,
fungicidal, virucidal, not sporicidal
– _______________
– ____________antibacterial additive to soaps
– Mouthwash! (thymol)
– Can be VERY toxic! Not typically used as
antiseptics.

47. _______________

48. 3. _______________
Hibiclens, Hibitane
A surfactant & protein
denaturant with broad
microbicidal properties
Not sporicidal
Used as skin _________
agents for preoperative scrubs,
skin cleaning & burns

49. 4. Alcohols
Ethyl, isopropyl in solutions of 50-90%
(water needed for protein coagulation)
Act as surfactants dissolving membrane
lipids and coagulating proteins of
vegetative bacterial cells and fungi
_______________
Isopropanol = rubbing alcohol, but vapors
can be toxic

50. 5. Hydrogen peroxide
Weak (3%) to strong (25%)
_______________ agent (steals electrons)
Produce highly reactive hydroxyl free
radicals that damage protein & DNA while
also decomposing to O2 gas (bubbles)
toxic to _______________ , overwhelms
catalase in aerobes
Strong solutions are sporicidal

51. colonoscope

52. 52

53. 6. Detergents & soaps
________ compounds that work as
_______________
Quaternary ammonia cpds (__________)
act as surfactants that alter membrane
permeability of some bacteria & fungi
– Not sporicidal, ineffective against TB,
hepatitis, pseudomonas
Soaps- mechanically remove soil and
grease containing microbes

55. Figure 11.17
55

56. 56

58. 7. Heavy metals
Solutions of silver & mercury kill
vegetative cells in low concentrations by
inactivating proteins
Metallic salts
Oligodynamic action
Not sporicidal
_________ to humans
– Not used on broken skin

59. 59

61. 8. Aldehydes
Glutaraldehyde & formaldehyde kill by
alkylating protein & DNA (-CHO is reducing
group)
_______________ = H on AA is replaced by
the aldehyde (and crosslinked)
_______________ in 2% solution (Cidex)
used as sterilant for heat sensitive
instruments
_______________ - disinfectant,
preservative, toxicity limits use (formalin is
aqueous solution)

63. Gases & aerosols
_______________, propylene oxide,
betapropiolactone & chlorine dioxide
Strong _______________ agents,
sporicidal (sterilization)

64. ________________________ –
uses ethylene oxide

65. 65

66. Miscellaneous antimicrobials
Dyes – crystal violet, acriflavine
Acids, bases (alkalies) – destroy but some
are corrosive and hazardous
– Pickling – acetic acid
– Sauerkraut, olives – lactic acid
– Benzoic, sorbic acid – in lots of processed
foods (preservatives = _______________ )