Chapter 11
Physical and
Chemical Agents
for Microbial
Control
Control of microbes
Physical and chemical
methods to destroy or reduce
microbes in a given area
OVERVIEW
3
Why do we want to control
microbes?
How do we do it?
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
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
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
Microbial death
Permanent termination of an organism’s
vital processes
microbiological definition:
Involves permanent loss of reproductive
capability, even under optimum growth
conditions
Factors that influence microbial killing
10
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
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
Surfactants disrupt
mode of action
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
mode of action
Heat, pH, heavy metals can alter proteins
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?
Types of Control:
I. Methods of Physical Control
1. Heat
2. Cold temperatures
3. Desiccation
4. Radiation
5. Filtration
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)
Physical Control
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
Physical Control
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
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
Physical Control
Thermal death
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
Physical Control
2. Cold temperatures
_______________– slows the growth of
microbes
refrigeration 0-15oC & freezing <0oC
used to ____________food, media and
cultures
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.
Physical Control
4. Radiation
1. _______________ radiation – deep
penetrating power, breaks DNA
– gamma rays, X-rays, cathode rays
– used to sterilize medical supplies & food
products
2. _______________ radiation – little
penetrating power to sterilize air, water &
solid surfaces
– uv light creates thymine dimers, which
interfere with replication
Ionizing radiation
Physical Control
Nonionizing radiation - UV
Physical Control
UV treatment of wastewater
Physical Control
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
Physical Control
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
OVERVIEW
33
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
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
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
Levels of activity
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
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
Chemical control - categories
1.
2.
3.
4.
5.
6.
7.
8.
Halogens
Phenolics
Chlorhexidine
Alcohols
Hydrogen peroxide
Detergents & soaps
Heavy metals
Aldehydes
1. Halogens
Ionic (halide) or nonionic
Mostly _______________
Germicidal and sporicidial with long exposure
Affect protein structure (bonds)
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
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
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.
_______________
3. _______________
Hibiclens, Hibitane
A surfactant & protein
denaturant with broad
microbicidal properties
Not sporicidal
Used as skin _________
agents for preoperative scrubs,
skin cleaning & burns
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
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
colonoscope
52
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
Figure 11.17
55
56
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
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)
Gases & aerosols
_______________, propylene oxide,
betapropiolactone & chlorine dioxide
Strong _______________ agents,
sporicidal (sterilization)
________________________
– uses ethylene oxide
65
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 = _______________ )