The Terminology of Microbial Control
1 The control of microbial growth can prevent infections and
food spoilage.
2 Sterilization is the process of destroying all microbial
life on an object.
3 Commercial sterilization is heat treatment of canned foods
to destroy C. botulinum endospores.
a. Resistant thermophilic endospores don’t germinate at food
storage temperatures – the problem comes at temperatures above 45° C.
4 Disinfection is the process of reducing or inhibiting
microbial growth on a nonliving surface.
5 Antisepsis is the process of reducing or inhibiting
microorganisms on a living tissue.
6 The suffix -cide means to kill; the suffix -stat means to
inhibit
7 Sepsis is bacterial contamination.
The Rate of Microbial Death
1 Bacterial populations subjected to heat or antimicrobial
chemicals usually die at a constant rate.
2 Such a death curve, when plotted logarithmically, shows this
constant death rate as a straight line.
3 The time it takes to kill a microbial population is
proportional to the number of microbes.
4 Microbial species and life cycles phases (endospores) have
different susceptibilities to physical and chemical controls.
5 Organic matter may interfere with heat treatments and
chemical control agents.
6 Longer exposure to lower heat can produce the same effect as
shorter time at higher heat.
Actions of Microbial Control Agents
1 The susceptibility of the plasma membrane is due to its
lipid and protein components.
2 Certain chemical control agents damage the plasma membrane
by altering its permeability.
Damage to Proteins and Nucleic Acids
1 Some microbial control agents damage cellular proteins by
breaking hydrogen bonds and covalent bonds.
2 Other agents interfere with DNA and RNA replication and protein
synthesis.
Physical Methods of Microbial Control
Heat
1 Heat is frequently used to eliminate microorganisms.
2 Most heat kills microbes by denaturing enzymes.
a. Heat works better at low pH.
3 Thermal death point (TDP) is the lowest temperature at which
all the microbes in a liquid culture will be killed in 10 minutes.
4 Thermal death time (TDT) is the length of time required to
kill all bacteria in a liquid culture at a given temperature.
5 Decimal reduction time (DRT) is the length of time in which
90% of a bacterial population will be killed at a given temperature.
6 Boiling (100ş C) kills many vegetative cells and viruses
within 10 minutes.
7 Autoclaving (steam under pressure) is the most effective
method of moist heat sterilization. The steam must directly contact the
material to be sterilized.
a. Fifteen psi, 121 ° C for fifteen minutes kills all vegetative bacteria and
endospores.
8 In HTST pasteurization, a high temperature is used for a
short time (72ş C for 15 seconds) to destroy pathogens without altering the
flavor of the food. Ultra-high-temperature (UHT) treatment (140ş C for 3
seconds) is used for dairy products.
a. Classic pasteurization was done at 63° C for 30 minutes
9 Methods of dry heat sterilization include direct flaming,
incineration, and hot -air sterilization (170° C for two hours). Dry heat kills by oxidation.
10 Different methods that produce the same effect (reduction in
microbial growth) are called equivalent treatments.
Filtration
1 Filtration is the passage of a liquid or gas through a
filter with pores small enough to remain microbes.
2 Microbes can be removed from air by high -efficiency
particulate air filters.
a. HEPA – 0.3 um pore size
b. Decreases numbers
3 Membrane filters composed of nitrocellulose or cellulose
acetate are commonly used to filter out bacteria, viruses, and even large
proteins.
a. Range from 0.22 um – 0.45 um for bacteria but sometimes
mycoplasma or spirochetes can pass through.
b. Sizes can go as low as 0.01 um to filter viruses and
proteins.
Low Temperatures
1 The effectiveness of low temperatures depends on the
particular microorganism and the intensity of the application.
2 Most microorganisms do not reproduce at ordinary
refrigerator temperatures (0-7ş C).
3 Many microbes survive (but do not grow) at subzero
temperatures used to store foods.
High Pressure
1
High pressure denatures proteins in vegtetative cells.
2
Endospores aren’t usually harmed, but can be allowed to grow out and the
vegetative form killed with high pressure.
3
Used in Japan and the U.S. to preserve fruit juices – preserves the
flavor, color, and nutrient values.
Desiccation
1 In the absence of water, microorganisms can grow but cannot
remain viable.
2 Viruses and endospores can resist desiccation.
a. Mycobacterium tuberculosis is fairly resistant to
desiccation but the gonorrhea bacterium is very sensitive.
3 Environmental conditions effect efficiency of desiccation –
pus, feces, and mucus are protective
Osmotic Pressure
1 Microorganisms in high concentrations of salts and sugars
undergo plasmolysis.
2 Molds and yeasts are more capable than bacteria of growing
in materials with low moisture or high osmotic pressure.
Radiation
1 The effects of radiation depend on its wavelength, intensity
and duration.
2 Ionizing radiation (gamma rays, X-rays and high-energy
electron beams) has a high degree of penetration and exerts its effect
primarily by ionizing water and forming highly reactive hydroxyl radicals.
a. Destroys DNA.
3 Non-ionizing radiation causes damage to DNA.
a. Ultraviolet causes formation of thymine dimers.
b. Best wavelength is 260 nm.
4 Microwaves heat water but it is possible to isolate
vegetative forms from microwave ovens – probably protected by uneven heating in
solid foods
Chemical Methods of Microbial Control
1 Chemical agents are used on living tissue (as antiseptics)
and on inanimate objects (as disinfectants).
2 Few chemical agents achieve sterility.
Principles of Effective Disinfection
1 Properties of the disinfectant – how it works determines
what it will be effective against.
2 Concentration of the disinfectant - requires proper hydration.
3 The presence of organic matter, which can act as a buffer
4 The degree of contact with microorganisms – if the surface
needs cleaning or is porous microorganisms can escape contact with the
disinfectant.
5 Temperature should also be considered, since increased
temperatures usually enhance the efficacy of disinfectants.
a. Time left in contact is also an important consideration.
Evaluating Disinfectants
1 In the use-dilution test, bacterial (S. choleraesuis, S. aureus, and
P. aeruginosa) survival in the
manufacturer’s recommended dilution of a disinfectant is determined.
a. Viruses, endospore-forming bacteria, mycobacteria, and fungi
can also be used in the use-dilution test.
2 In the disk-diffusion method, a disk of filter paper is
soaked with a chemical and placed on an inoculated agar plate; a clear zone of
inhibition indicates effectiveness.
3 Phenol coefficient – compares activity of phenol.
Types of Disinfectants
Phenol and Phenolics
1 Phenolics are derivatives of phenol that have been altered
to reduce irritating qualities or increase antimicrobial activity when combined
with detergents.
2 Phenolics exert their action by injuring plasma membranes.
a. Can also denature proteins – enzyme inactivation
3 Qualities:
a. Not inactivated by organic compounds
b. Stable for long periods
c. Persist for long periods after application
d. Good for disinfecting things like pus, saliva, and feces
4 Cresol is a phenolic that is derived from coal tar.
5 O-phenylphenol is the main ingredient in Lysol.
Bisphenols
1 Bisphenols such as triclosan (over the counter) and
hexachlorophene (prescription) are widely used in household products.
2 Hexachlorphene is used in pHisoHex
a. Particularly good against gram-positive strep and staph
(cause skin infections in infants)
b. Can cause neurological damage if exposure is high (bathing
several times per day)
3 Triclosan is used in soaps, toothpaste, and incorporated
into plastic kitchenware.
a. Broad spectrum, especially against gram-positive bacteria
and fungi
b. Resistant strains of bacteria have arisen and triclosan may
contribute to antibiotic resistance as well
Biguanides
1 Chlorhexidine
a. Binds well to skin and mucus membranes
b. Low toxicity
c. Surgical hand scrubs and pre-op skin prep
d. Can damage eyes
e. Not sporocidal
Halogens
1 Some halogens (iodine and chlorine) are used alone or as
components of inorganic or organic solutions.
2 Iodine may combine with certain amino acids to inactivate
enzymes and other cellular proteins.
3 Iodine is available in a tincture (in solution with alcohol)
or an iodophor (combined with an organic molecule).
a. Betadine, Isodine (povidone-iodines)
b. Povidone is surface active – improves wetting actions.
4 The germicidal action of chlorine is based on the formation
of hypochlorous acid (HOCl) when chlorine is added to water.
a. Good oxidizing agent.
b. Chlorine is used as a disinfectant in gaseous form (Cl2
) or in the form of a compound, such as calcium hypochlorite, sodium
hypochlorite (NaOCl, Clorox), sodium dichloroisocyanurate, and chloramines.
c. Used to disinfect drinking water and swimming pools
d. 2 drops Clorox per liter (4 if cloudy) and let sit 30
minutes
Alcohols
1 Alcohols exert their action by denaturing proteins and
dissolving lipids.
a. Not good for wound disinfection because proteins coagulate
and form a protective coat around bacteria.
2 In tinctures, they enhance the effectiveness of other
antimicrobial chemicals.
3 Aqueous ethanol (60-95%) and isopropanol are used as
disinfectants.
a. Denaturation requires H2O, which is why aqueous
preparations are better than pure.
b. 70% is best ethanol concentration.
Heavy Metals and Their Compounds
1 Silver, mercury, copper, and zinc are used as germicidals.
2 They exert their antimicrobial action through oligodynamic
action. When heavy metal ions combine
with sulfhydryl (—SH) groups, proteins are denatured.
Surface -Active Agents
1 Surface-active agents decrease the surface tension among
molecules of a liquid; soaps and detergents are examples.
2 Soaps have limited germicidal action but assist in the
removal of microorganisms through scrubbing.
a. Emulsify fats
3 Acid-anionic detergents are used to clean dairy equipment.
a. Interact with plasma membrane and may cause damage to
enzymes.
Quanternary Ammonium Compounds (Quats)
1 Quats are cationic detergents attached to NH4+.
2 By disrupting plasma membranes, they allow cytoplasmic
constituents to leak out of the cell.
a. Also denature proteins (inhibit enzymes) and are surface
active.
3 Quats are most effective against gram-positive bacteria.
a. Also good against fungi, amoeba, and enveloped viruses but
pseudomonads can grow in them.
4 Inactivated by anions, soaps, detergents, and organic
material.
Chemical Food Preservatives
1 SO2, sorbic acid, benzoic acid, and propionic
acid inhibit fungal metabolism and are used as food preservatives.
2 Good to prevent mold growth in acidic foods (molds grow well
at low pH)
3 Nitrate and nitrite salts prevent germination of Clostridium botulism endospores in meats
and preserve red color.
a. Nitrites combine with amino acids to produce nitrosamines, which
may be carcinogenic.
b. Nitrosamines are produced from other things in the body and
may not be as big a deal as once thought.
Antibiotics
1 Nisin and natamycin are antibiotics used to preserve foods,
especially cheese.
2 Not used for treatment of disease.
Aldehydes
1 Aldehydes such as formaldehydes and glutaraldehyde exert
their antimicrobial effect by inactivating proteins.
2 They are among the most effective chemical disinfectants.
Gaseous Chemosterilizers
1 Ethylene oxide is the gas most frequently used for
sterilization.
2 It penetrates most materials and kills all microorganisms by
protein denaturation.
Peroxygens
(Oxidizing Agents)
1 Ozone peroxide, and peracetic acid are used as antimicrobial
agents.
2 They exert their effect by oxidizing molecules inside the
cell.
Microbial Characteristics And Microbial Control
1 Temperature
2 Type of Microorganism
a. Gram-negative bacteria are generally more resistant than
gram-positive bacteria to disinfectants and antiseptics.
i) Pseudomonas and Burkholderia
have porins in the cell wall with characteristics that make them resistant to a
lot of chemicals and antibiotics. They
can actually grow in some disinfectants and antiseptics due to this resistance
combined with the ability to metabolize some unusual molecules.
b. Mycobacteria, endospores, and protozoan cysts and oocysts
are very resistant to disinfectants and antiseptics.
c. Nonenveloped viruses are generally more resistant than
enveloped viruses to disinfectants and antiseptics.
i) Lipid envelope is a target for chemical injury
d. Prions resist disinfection and autoclaving - an hour soak in
sodium hydroxide followed by an hour of autoclaving at 136 degrees is
"fairly effective".
3 Physiological State
a. Actively growing more susceptible.
b. Endospores least susceptible.
4 Environment
a. Organic material inhibits chemicals by binding them and can
insulate microorganisms against heat.