CHAPTER
9
Controlling Microbial Growth
in the Environment
Chapter Outline
Basic Principles of Microbial Control (pp. 261–264)
Terminology of Microbial Control
Microbial Death Rates
Action of Antimicrobial Agents
The Selection of Microbial Control Methods (pp. 264–266)
Physical Methods of Microbial Control (pp. 266–273)
Heat-Related Methods
Refrigeration and Freezing
Desiccation and Lyophilization
Filtration
Osmotic Pressure
Radiation
Chemical Methods of Microbial Control (pp. 274–280)
Phenol and Phenolics
Alcohols
Halogens
Oxidizing Agents
Chapter 9 Controlling Microbial Growth in the Environment
Chapter Summary
Basic Principles of Microbial Control (pp. 261–264)
In discussions of microbial control in the environment, precise terminology is important, as is an
understanding of the concept of microbial death rate and the action of antimicrobial agents.
Terminology of Microbial Control
Many terms of microbial control are familiar to the general public but often misused. Precise
definitions are as follows:
In its strictest sense, sterilization refers to the removal or destruction of all microbes,
including viruses and bacterial endospores, in or on an object. (The term does not apply to
Disinfection refers to the use of physical or chemical agents known as disinfectants to
inhibit or destroy microorganisms, especially pathogens. It does not guarantee elimination of
all pathogens and applies only to treatment of inanimate objects.
When a chemical is used on skin or other tissue, the process is called antisepsis, and the
agent is an antiseptic.
Degerming is the removal of microbes from a surface by scrubbing, such as when you wash
your hands.
Microbial Death Rates
Scientists define microbial death as the permanent loss of reproductive ability under ideal
environmental conditions. One technique for evaluating the efficacy of an antimicrobial agent is
Action of Antimicrobial Agents
The modes of action of antimicrobial agents fall into two basic categories. Physical or chemical
agents may disrupt the integrity of cells by altering their cell walls or cytoplasmic membranes
Alteration of Cell Walls and Membranes
A cell wall maintains cellular integrity in a hypotonic solution. Disruption of the cell wall makes
Damage to Proteins and Nucleic Acids
Proteins are critical to cellular metabolism, and only function when they are in their correct
shape, which is maintained by hydrogen and sulfide bonds. When these bonds are broken, the
The Selection of Microbial Control Methods (pp. 264–266)
A perfect antimicrobial method or agent would be inexpensive, fast acting, stable during
storage, harmless to humans, and effective against all types of microbes. Because no single
method or agent meets all these criteria, scientists consider several factors when evaluating
methods and agents.
Factors Affecting the Efficacy of Antimicrobial Methods
The nature of the sites to be treated, the degree of susceptibility of microbes involved, and the
environmental conditions must all be considered when deciding which antimicrobial method to
use.
Site to Be Treated
The nature of the site to be treated influences the choice. For example, harsh chemicals or
Relative Susceptibility of Microorganisms
Generally, scientists and medical personnel select a method to kill the hardiest microorganisms
present, assuming that more fragile microbes will be killed as well. Bacterial endospores are the most
Chapter 9 Controlling Microbial Growth in the Environment
Environmental Conditions
The conditions under which a disinfectant is used, such as temperature and pH, are also a
Biosafety Levels
Containment of living microbes while they are studied is another aspect of microbial control.
The Center for Disease Control and Prevention has established guidelines for four levels of
Physical Methods of Microbial Control (pp. 266–274)
Physical methods of microbial control include exposing the microbes to extremes of heat and
cold, desiccation, filtration, osmotic pressure, and radiation.
Heat-Related Methods
Heat is one of the older and more common means of microbial control. High temperatures
denature proteins, interfere with the integrity of cytoplasmic membranes and cell walls, and
disrupt the function and structure of nucleic acids. Microorganisms vary in their susceptibility to
Moist Heat
Moist heat is more effective than dry heat because water is a better conductor of heat than air.
Boiling kills the vegetative cells of bacteria and fungi, the trophozoites of protozoa, and most
viruses within 10 minutes at sea level. It is not effective when true sterilization is required.
Instructor’s Manual for Microbiology with Diseases by Taxonomy, 5e
Dry Heat
Substances that cannot be sterilized by moist heat, such as powders and oils, can be sterilized by
Refrigeration and Freezing
Refrigeration between 0°C and 7°C halts the growth of most pathogens, which are
Desiccation and Lyophilization
Desiccation, or drying, has been used for thousands of years to preserve foods such as fruits,
peas, and yeast. It inhibits microbial growth because metabolism requires liquid water, although
Filtration
When used as a method of microbial control, filtration is the passage of air or a liquid through a
material that traps and removes microbes. Some membrane filters manufactured of
Osmotic Pressure
High concentrations of salt or sugar inhibit microbial growth by osmotic pressure, drawing the
water out of cells they need to carry out their metabolic functions. Honey, jams, salted fish, and
pickles are examples of foods preserved by osmotic pressure. Fungi have a greater tolerance for
hypertonic environments than bacteria, which explains why refrigerated jams may grow mold.
Radiation
There are two types of radiation: Particulate radiation consists of high-speed subatomic
particles freed from their atoms, whereas electromagnetic radiation is atomic energy without
mass traveling at the speed of light. Ionizing radiation is electromagnetic radiation with
Chapter 9 Controlling Microbial Growth in the Environment
Chemical Methods of Microbial Control (pp. 274–280)
Nine major categories of antimicrobial chemicals are used as antiseptics and disinfectants.
1. Phenol and Phenolics. Phenolics are compounds derived from phenol molecules that have
been chemically modified by the addition of halogens or organic functional groups such as
2. Alcohols. Alcohols such as isopropanol (rubbing alcohol) denature proteins and disrupt
cytoplasmic membranes; they are used either as 70–90% aqueous solutions or in a tincture,
3. Halogens. Halogens are the four very reactive, nonmetallic chemical elements: iodine,
chlorine, bromine, and fluorine. Halogens are considered intermediate-level antimicrobial
chemicals. Halogens are used both alone and combined with other elements in organic and
inorganic compounds. Halogens exert their antimicrobial effect by unfolding and thereby
4. Oxidizing Agents. Oxidizing agents such as hydrogen peroxide, ozone, and peracetic acid
are high-level disinfectants and antiseptics that release oxygen radicals, which are toxic to
5. Surfactants. Surfactants are “surface active” chemicals. They include soaps, whose
molecules have both hydrophobic ends, which act primarily to break up oils during
6. Heavy Metals. Heavy metal ions such as arsenic, silver, mercury, copper, and zinc are low-level
disinfectants that denature proteins. For most applications, they have been superseded by less
7. Aldehydes. Aldehydes are compounds containing terminal –CHO groups that cross-link
8. Gaseous Agents. Many items, such as plastic laboratory ware, artificial heart valves,
mattresses, and dried foods, cannot be sterilized easily with heat or water-soluble chemicals,
9. Enzymes. Antimicrobial enzymes are enzymes that act against microorganisms. Lysozyme
is an enzyme that degrades peptidoglycan cell walls. Scientists, food processors, and medical
Antimicrobial Drugs
Antimicrobial drugs include antibiotics. Antibiotics by definition are naturally produced by
microorganisms. Semisynthetic antibiotics are chemically modified antibiotics. In addition,
Methods for Evaluating Disinfectants and Antiseptics
Scientists have developed several methods to measure the efficacy of antimicrobial agents:
Phenol Coefficient
Phenol was an antiseptic used during surgery in the late 1800s. Since then, scientists have
evaluated the efficacy of various disinfectants and antiseptics by calculating a ratio that
Chapter 9 Controlling Microbial Growth in the Environment
Use-Dilution Test
In the use-dilution test, a researcher dips several metal cylinders into broth cultures of bacteria,
briefly dries them, and then immerses each into a different dilution of the disinfectants being
Kelsey-Sykes Capacity Test
An alternative to the use-dilution test is the Kelsey-Sykes capacity test, the standard in the
In-Use Test
In-Use Tests provide accurate determination of an agent’s efficacy under realistic conditions,
Development of Resistant Microbes
There is little evidence that the extensive use of antimicrobial chemicals in household cleansers
and personal care products enhances human health; however, it does promote the development