BSC 310 Lecture Notes - Lecture 18: Osmosis, Pasteurization, Rickettsia
Introduction to Controlling Microbial Growth
The control of microbial growth may involve sterilization, disinfection, antisepsis, sanitization, or
degerming. Sterilization is the destruction of all forms of microbial life, with particular attention to bacterial
spores. Disinfection and antisepsis both refer to destruction of microbial pathogens, although some
organisms, such as bacterial spores, may remain alive. Disinfection refers to the destruction of
pathogenic organisms on an inanimate (lifeless) object, such as a table-top, while antisepsis refers to that
destruction on a living object, such as the skin surface.
Sanitization refers to the reduction in the number of pathogens to a level deemed safe by
public health guidelines. Degerming is the physical removal of microorganisms by using such
things as soaps or detergents.
Any chemical agent that kills microorganisms is known as a germicide. An agent that
destroys bacteria is called a bactericide, one that kills fungi is a fungicide, and one that kills
viruses is a viricide. A bacteriostatic agent prevents the further multiplication of bacteria
without necessarily killing all that are present.
Among the conditions affecting the use of a germicide are temperature, the type of
microorganism, and the environment. Germicides are more effective at high temperatures
because the chemical breaks down at lower temperatures. Microorganisms vary in their
susceptibility depending on such things as the composition of their cell wall, the presence or
absence of a capsule, and the ability to form spores or cysts. The environment can affect
the activity of a germicide, as, for example, when organic matter is present. This material
shields microorganisms from germicides and often reacts with the germicide.
Physical Methods of Control
Physical methods for controlling the growth of microorganisms can be divided into heat
methods and nonheat methods. The lowest temperature at which all microorganisms
are killed in 10 minutes is the thermal death point, while the minimum amount of time
required to kill microorganisms at a given temperature is known as the thermal death
time. The time for destruction of 90 percent of the microbial population is the decimal
reduction time.
Dry heat. Dry heat kills microorganisms by reacting with and oxidizing their proteins. Dry
heat can be used in incineration devices, such as the Bunsen burner or the hot-air
oven. In the hot-air oven, a temperature of about 170°C for two hours will bring about
sterilization.
Moist heat. Moist heat is used to kill microorganisms in such things as boiling water.Most
vegetating microorganisms are killed within two or three minutes, but over two or three
hours may be required for destruction of bacterial spores. In moist heat, the microbial
proteins undergo denaturation, a process in which the three-dimensional form of the protein
reverts to a two-dimensional form, and the protein breaks down.
Moist heat is used in the autoclave, a high-pressure device in which steam is superheated
(Figure 1 ). Steam at 100°C is placed under a pressure of 15 pounds per square inch,
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Document Summary
The control of microbial growth may involve sterilization, disinfection, antisepsis, sanitization, or degerming. Sterilization is the destruction of all forms of microbial life, with particular attention to bacterial spores. Disinfection and antisepsis both refer to destruction of microbial pathogens, although some organisms, such as bacterial spores, may remain alive. Disinfection refers to the destruction of pathogenic organisms on an inanimate (lifeless) object, such as a table-top, while antisepsis refers to that destruction on a living object, such as the skin surface. Sanitization refers to the reduction in the number of pathogens to a level deemed safe by public health guidelines. Degerming is the physical removal of microorganisms by using such things as soaps or detergents. Any chemical agent that kills microorganisms is known as a germicide. An agent that destroys bacteria is called a bactericide, one that kills fungi is a fungicide, and one that kills viruses is a viricide.
