Ch 7

The Terminology of Microbial Control
 Define the following key terms related to microbial
control: sterilization, disinfection, antisepsis, degerming, sanitization, biocide, germic
ide, bacteriostasis, and asepsis.
 The control of microbial growth can prevent infections and food spoilage.
 Sterilization is the process of removing or destroying all microbial life on an object.
 Commercial sterilization is heat treatment of canned foods to destroy C.
botulinum endospores.
 Disinfection is the process of reducing or inhibiting microbial growth on a nonliving
surface.
 Antisepsis is the process of reducing or inhibiting microorganisms on living tissue.
 The suffix -cide means to kill; the suffix -stat means to inhibit.
 Sepsis is bacterial contamination.
The Rate of Microbial Death
 Describe the patterns of microbial death caused by treatments with microbial
control agents.
 Bacterial populations subjected to heat or antimicrobial chemicals usually die at a
constant rate.
 Such a death curve, when plotted logarithmically, shows this constant death rate as a
straight line.
 The time it takes to kill a microbial population is proportional to the number of
microbes.
 Microbial species and life cycle phases (e.g., endospores) have different susceptibilities
to physical and chemical controls.
 Organic matter may interfere with heat treatments and chemical control agents.

 Longer exposure to lower heat can produce the same effect as shorter time at higher
heat.
Actions of Microbial Control Agents
 Describe the effects of microbial control agents on cellular structures.
 Alteration of Membrane Permeability
 The susceptibility of the plasma membrane is due to its lipid and protein components.
 Certain chemical control agents damage the plasma membrane by altering its
permeability.
 Damage to Proteins and Nucleic Acids
 Some microbial control agents damage cellular proteins by breaking hydrogen bonds
and covalent bonds.
 Other agents interfere with DNA and RNA and protein synthesis.
Physical Methods of Microbial Control
 Compare the effectiveness of moist heat (boiling, autoclaving, pasteurization) and
dry heat.
 Describe how filtration, low temperatures, high pressure, desiccation, and osmotic
pressure suppress microbial growth.
 Explain how radiation kills cells.
Heat
 Heat is frequently used to kill microorganisms.
 Moist heat kills microbes by denaturing enzymes.
 Boiling (100°C) kills many vegetative cells and viruses within 10 minutes.
 Autoclaving (steam under pressure) is the most effective method of moist heat
sterilization. The steam must directly contact the material to be sterilized.
 In HTST pasteurization, a high temperature is used for a short time (72°C for 15 seconds)
to destroy pathogens without altering the flavor of the food. Ultra-high-temperature
(UHT) treatment (140°C for 4 seconds) is used to sterilize dairy products.
 Methods of dry heat sterilization include direct flaming, incineration, and hot-air
sterilization. Dry heat kills by oxidation.
 Different methods that produce the same effect (reduction in microbial growth) are
called equivalent treatments.

Filtration
 Filtration is the passage of a liquid or gas through a filter with pores small enough to
retain microbes.
 Microbes can be removed from air by high-efficiency particulate air (HEPA) filters.
 Membrane filters composed of cellulose esters are commonly used to filter out bacteria,
viruses, and even large proteins.
Low Temperatures
 The effectiveness of low temperatures depends on the particular microorganism and the
intensity of the application.
 Most microorganisms do not reproduce at ordinary refrigerator temperatures (0–7°C).
 Many microbes survive (but do not grow) at the subzero temperatures used to store
foods.
High Pressure
 High pressure denatures proteins in vegetative cells.
Desiccation
 In the absence of water, microorganisms cannot grow but can remain viable.
 Viruses and endospores can resist desiccation.
Osmotic Pressure
 Microorganisms in high concentrations of salts and sugars undergo plasmolysis.
 Molds and yeasts are more capable than bacteria of growing in materials with low
moisture or high osmotic pressure.
Radiation
 The effects of radiation depend on its wavelength, intensity, and duration.
 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.
 Ultraviolet (UV) radiation, a form of nonionizing radiation, has a low degree of
penetration and causes cell damage by making thymine dimers in DNA that interfere
with DNA replication; the most effective germicidal wavelength is 260 nm.
 Microwaves can kill microbes indirectly as materials get hot.

Chemical Methods of Microbial Control
 List the factors related to effective disinfection.
 Interpret the results of use-dilution tests and the disk-diffusion method.
Identify the methods of action and preferred uses of chemical disinfectants.
 Differentiate halogens used as antiseptics from halogens used as disinfectants.
 Identify the appropriate uses for surface-active agents.
 List the advantages of glutaraldehyde over other chemical disinfectants.
 Identify chemical sterilizers.
 Chemical agents are used on living tissue (as antiseptics) and on inanimate objects (as
disinfectants).
 Few chemical agents achieve sterility.
Principles of Effective Disinfection
 Careful attention should be paid to the properties and concentration of the disinfectant
to be used.
 The presence of organic matter, degree of contact with microorganisms, and
temperature should also be considered.
Evaluating a Disinfectant
 The use-dilution test is used to determine bacterial survival in the manufacturer’s
recommended dilution of a disinfectant.
 The use-dilution test can also be used to evaluate the effectiveness of agents against
viruses, endospore-forming bacteria, mycobacteria, and fungi.
 In the disk-diffusion method, a disk of filter paper is soaked with a chemical and placed
on an inoculated agar plate; a zone of inhibition indicates effectiveness.
Types of Disinfectants
You will not be asking this laundry list.
 Phenolics exert their action by injuring plasma membranes.
 The bisphenol hexachlorophene is used as a skin disinfectant.
 Biguanides damage plasma membranes of vegetative cells.
 Terpenes and phenolics in essential oils of plants have antimicrobial activity.
 Iodine may combine with certain amino acids to inactivate enzymes and other cellular
proteins.

 The germicidal action of chlorine is based on the formation of hypochlorous acid when
chlorine is added to water.
 Alcohols exert their action by denaturing proteins and dissolving lipids.
 In tinctures, alcohols enhance the effectiveness of other antimicrobial chemicals.
 Silver, mercury, copper, and zinc exert their antimicrobial action through oligodynamic
action. When heavy metal ions combine with sulfhydryl (—SH) groups, proteins are
denatured.
 Soaps have limited germicidal action but assist in removing microorganisms.
 Acid-anionic detergents are used to clean dairy equipment.
 Quats are cationic detergents attached to NH4+ that disrupt plasma membranes.
 SO2, sorbic acid, benzoic acid, and propionic acid inhibit fungal metabolism and are used
as food preservatives.
 Nitrate and nitrite salts prevent germination of C. botulinum endospores in meats.
 Nisin and natamycin are antibiotics used to preserve foods, especially cheese.
 Aldehydes such as formaldehyde and glutaraldehyde are among the most effective
chemical disinfectants. They exert their antimicrobial effect by inactivating proteins.
 Ethylene oxide is the gas most frequently used for sterilization. It penetrates most
materials and kills all microorganisms by protein denaturation.
 Free radicals in plasma gases are used to sterilize plastic instruments.
 Supercritical fluids, which have properties of liquid and gas, can sterilize at low
temperatures.
 Hydrogen peroxide, peracetic acid, benzoyl peroxide, and ozone exert their
antimicrobial effect by oxidizing molecules inside cells.
Microbial Characteristics and Microbial Control
 Explain how the type of microbe affects the control of microbial growth.
 Gram-negative bacteria are generally more resistant than gram-positive bacteria to
disinfectants and antiseptics.
 Mycobacteria, endospores, and protozoan cysts and oocysts are very resistant to
disinfectants and antiseptics.

 Nonenveloped viruses are generally more resistant than enveloped viruses to
disinfectants and antiseptics.
 Prions are resistant to disinfection and autoclaving