Transcript Foundations in Microbiology

Chapter 11
Physical and Chemical Agents
for Microbial Control
Controlling Microorganisms
The
methods of microbial control belong to the general
category of decontamination procedures
Physical,
chemical, and mechanical methods are used to
destroy or reduce undesirable or unwanted microbes
(contaminants) in a given area and time
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Controlling Microorganisms
Primary
targets of microbial control are microorganisms
capable of causing infection or spoilage that are constantly
present in the external environment and on the human
body:

bacterial vegetative cells and endospores

fungal hyphae and spores, yeast

protozoan trophozoites and cysts

worms

viruses

prions
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Relative Resistance of Microbes
Highest

resistance
bacterial endospores, prions
Moderate




viruses - naked viruses: hepatitis B virus, poliovirus
Mycobacterium
tuberculosis,
Staphylococcus
Pseudomonas sp.
protozoan cysts
some fungal sexual spores (zygospores)
Least




resistance
aureus,
resistance
most bacterial vegetative cells
fungal spores and hyphae, yeasts
enveloped viruses
protozoan trophozoites
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Terminology and Methods of Control
Through
the years, a growing terminology has emerged for
describing and defining measures that control microbes
These
measures include:

sterilization

disinfection

asepsis and antisepsis

sanitization

degermation
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Sterilization
Sterilization
is a process that destroys or removes all viable
microorganisms, including viruses and endospores
Any
material that has been subjected to this process is said
to be sterile
An
object is either “sterile” or “not sterile”
Control
methods that sterilize are generally reserved for
inanimate objects
Sterilized
products are essential to human well-being
Chemicals
called sterilants are also used as sterilizing
agents
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Microbicidal Agents
-cide
means to kill
Bactericide
Fungicide
Virucide
- destroys bacteria (except endospores)
- kills fungal spores, hyphae, and yeasts
- inactivate viruses especially on living tissue
Sporicide
- destroys bacterial endospores
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Microbistasis
Microbistasis
is a condition in which microbes are
temporarily prevented from multiplying but are not killed
outright
Bacteriostatic
agents prevent bacterial growth on tissues or
on objects in the environment
Fungistatic
chemicals inhibit fungal growth
Antiseptics
and drugs used to control microorganisms in the
body have microbistatic effects
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Disinfection
Disinfection
is the use of a physical process or a chemical
agent (disinfectant) to destroy vegetative pathogens but not
bacterial endospores
Normally
used only on inanimate objects
The
concentrations required to be effective can be toxic to
human and other animal tissue
Disinfection
processes also remove the harmful products of
microorganisms (toxins) from materials
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Asepsis and Antisepsis
Asepsis
refers to any practice that prevents the entry of
infectious agents into sterile tissues and thus prevent
infection
Antisepsis
is the application of chemical agents
(antiseptics) directly to exposed body surfaces, wounds,
and surgical incisions to destroy or inhibit vegetative
pathogens
Commonly
practiced in health care
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Sanitization
Sanitization
is referred to as any cleansing technique that
mechanically removes debris, microorganisms, and toxins
Sanitization
reduces the potential for infection and spoilage
Soaps
and detergents are the most commonly employed
sanitizers
Sanitization
is
in
general
far
less
expensive
than
sterilization
Applies
to inanimate objects and surfaces
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Degermation
Degermation
refers to the reduction in microbial load on a
living tissue through mechanical means
Usually
involves scrubbing the skin or immersing it in
chemicals, or both
Microorganisms
are removed both mechanically and,
indirectly by emulsifying the oils that coat the skin and
provide a habitat for their growth
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Microbial death
Permanent
loss of reproductive capability, even under
optimum growth conditions
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Factors That Affect Death Rate
The
effectiveness of a particular agent is governed by
several factors:

number of microbes

nature of microbes in the population

temperature and pH of environment

concentration or dosage of agent

mode of action of the agent

presence of solvents, organic matter, or inhibitors
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Practical Concerns in Microbial Control
Selection
of method of control depends on circumstances:

does the application require sterilization or is disinfection
adequate?

is the item to be reused or permanently discarded?

can the item withstand heat, pressure, radiation, or chemicals?

is the control method suitable for a given application?

will the agent penetrate to the necessary extent?

is the method cost- and labor-efficient and is it safe?
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Antimicrobial Agents’ Modes of Action
Cellular
targets of physical and chemical agents:

the cell wall – cell wall becomes fragile and cell lyses; some
antimicrobial drugs, detergents, and alcohol

the cell membrane - loses integrity; detergent surfactants

cellular synthetic processes (DNA, RNA) – prevention of
replication, transcription; some antimicrobial drugs, radiation,
formaldehyde, ethylene oxide

proteins – interfere at ribosomes to prevent translation, disrupt or
denature proteins; alcohols, phenols, acids, heat
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Methods of Physical Control
Heat
– moist and dry
Cold
temperatures
Desiccation
Radiation
Filtration
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Mode of Action and Relative Effectiveness
of Heat
heat – lower temperatures and shorter exposure time;
coagulation and denaturation of proteins
Moist
heat – moderate to high temperatures; dehydration,
alters protein structure; incineration
Dry
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Heat Resistance and Thermal Death
endospores are the most resistant – usually
require temperatures above boiling
Bacterial
Vegetative
states of bacteria and fungi are the least
resistant
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Thermal Death Measurements
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
Thermal
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Common Methods of Moist Heat Control
The
four ways that moist heat is employed to control
microbes are:

steam under pressure

nonpressurized steam

boiling water

pasteurization
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Steam Under Pressure
Steam
under pressure – sterilization
Autoclave
Steam
Item
15 psi/121oC/10-40min
must reach surface of item being sterilized
must not be heat or moisture sensitive
of action – denaturation of proteins, destruction of
membranes and DNA
Mode
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Nonpressurized Steam
– intermittent sterilization for substances that
cannot withstand autoclaving
Tyndallization
exposed to free-flowing steam for 30 – 60 minutes,
incubated for 23-24 hours and then subjected to steam
again
Items
Repeat
Used
cycle for 3 days
for some canned foods and laboratory media
Disinfectant
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Boiling Water
at 100oC for 30 minutes to destroy non-sporeforming pathogens
Boiling
Disinfection
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Pasteurization
– heat is applied to kill potential agents of
infection and spoilage without destroying the food flavor or
value
Pasteurization
63oC
– 66oC for 30 minutes (batch method)
71.6oC
for 15 seconds (flash method)
Not
sterilization - kills non-spore-forming pathogens and
lowers overall microbe count; does not kill endospores or
many nonpathogenic microbes
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Dry Heat
Dry
heat using higher temperatures than moist heat
Incineration

Dry
– flame or electric heating coil
ignites and reduces microbes and other substances
ovens – 150-180oC- coagulate proteins
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Cold
Microbiostatic
Refrigeration
Used
– slows the growth of microbes
0-15oC and freezing <0oC
to preserve food, media and cultures
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Desiccation
Gradual
removal of water from cells, leads to metabolic
inhibition
effective microbial control – many cells retain ability to
grow when water is reintroduced
Not
Lyophilization
– freeze drying; preservation
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Radiation
radiation – deep penetrating power that has
sufficient energy to cause electrons to leave their orbit,
breaks DNA,
Ionizing

gamma rays, X-rays, cathode rays

used to sterilize medical supplies and food products
radiation – little penetrating power – must be
directly exposed
Nonionizing
UV
light creates thymine dimers, which interfere with
replication
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Sterilization by Filtration
Physical
removal of microbes by passing a gas or liquid
through filter
Used
to sterilize heat sensitive liquids and air in hospital
isolation units and industrial clean rooms
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Chemical Agents in Microbial Control
Antimicrobial
chemicals occur in the liquid, gaseous, or
even solid state
They
serve as disinfectants, antiseptics, sterilants,
degermers, and preservatives (chemicals that inhibit the
deterioration of substances)
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Desirable Qualities in a Germicide
rapid
action in low concentrations
solubility
broad
in water or alcohol and long-term stability
spectrum, non toxic to human and animal tissues
penetration
of inanimate surfaces to sustain a cumulative or
persistent action
resistant
to becoming inactivated by organic matter
noncorrosive
sanitizing
or nonstaining properties
and deodorizing properties
affordability
and ready availability
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Levels of Chemical Decontamination
High-level

germicides – kill endospores; may be sterilants
devices that are not heat sterilizable and intended to be used in
sterile environments (body tissue)
– kill fungal spores (not endospores),
tubercle bacillus, and viruses
Intermediate-level

used to disinfect devices that will come in contact with mucous
membranes but are not invasive
– eliminate only vegetative bacteria, vegetative
fungal cells, and some viruses
Low-level

clean surfaces that touch skin but not mucous membranes
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Factors that Affect Germicidal Activity of
Chemicals
Nature
of the microorganisms being treated
Nature
of the material being treated
Degree
Time
of contamination
of exposure
Strength
and chemical action of the germicide
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Germicidal Categories
Halogens
Phenolics
Chlorhexidine
Alcohols
Hydrogen
peroxide
Detergents and soaps
Heavy metals
Aldehydes
Gases
Dyes
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Halogens
Chlorine
– Cl2, hypochlorites (chlorine bleach), chloramines

denaturate proteins by disrupting disulfide bonds

intermediate level

unstable in sunlight, inactivated by organic matter

water, sewage, wastewater, inanimate objects
Iodine
- I2, iodophors (betadine)

denature proteins

intermediate level

milder medical & dental degerming agents, disinfectants, ointments
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Phenolics
Disrupt
cell walls and membranes and precipitate proteins
Low
to intermediate level - bactericidal, fungicidal, virucidal,
not sporicidal

lysol

triclosan- antibacterial additive to soaps
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Chlorhexidine
A
surfactant and protein denaturant with broad microbicidal
properties
Low
to intermediate level
Hibiclens,
Hibitane
Used
as skin degerming agents for preoperative scrubs,
skin cleaning and burns
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Alcohols
Ethyl,
isopropyl in solutions of 50-95%
Act
as surfactants dissolving membrane lipids and
coagulating proteins of vegetative bacterial cells and fungi
Intermediate
level
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Hydrogen Peroxide
Weak
(3%) to strong (25%)
Produce
highly reactive hydroxyl-free radicals that damage
protein and DNA while also decomposing to O2 gas – toxic
to anaerobes
Antiseptic
at low concentrations; strong solutions are
sporicidal
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Detergents and Soaps
Quaternary
ammonia compounds (quats) act as surfactants
that alter membrane permeability of some bacteria and fungi
Very
low level
Soaps
- mechanically remove soil and grease containing
microbes
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Heavy Metals
Solutions
of silver and mercury kill vegetative cells in low
concentrations by inactivating proteins
Oligodynamic
Low
action
level
Merthiolate,
silver nitrate, silver
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Aldehydes
Glutaraldehyde
and formaldehyde kill by alkylating protein
and DNA
Glutaraldehyde
in 2% solution (Cidex) used as sterilant for
heat sensitive instruments
High
level
Formaldehyde
- disinfectant, preservative, toxicity limits
use

formalin – 37% aqueous solution
Intermediate
to high level
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Gases and Aerosols
Ethylene
Strong
High
oxide, propylene oxide
alkylating agents
level
Sterilize
and disinfect plastics and prepackaged devices,
foods
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