BROCK BIOLOGY OF MICROORGANISMS Chapter 11 -- Introduction

Chapter 11: Microbial Growth Control
Introduction

OVERVIEW

Chapter 11 (pages 399-431) concerns the control of microbial growth. In previous chapters, the factors that promote growth have been discussed; this chapter deals with inhibition and prevention of microbial growth, including disinfection, sterilization and chemotherapy.

CHAPTER NOTES

Control of microbial growth can be by inhibition of growth, killing the microorganisms or removing them from an environment. Antimicrobial agents can be divided into agents that kill microorganisms (bactericidal) and agents that inhibit growth (bacteriostatic) of the microbes. Sterilization is the process of killing or removing all living organisms and viruses an environment. Sterilization, may be accomplished by several physical methods, including heat, filtration, and radiation. Of these methods, heat is by far the most common. As the temperature increases beyond the maximum temperature for growth of a microorganism, lethal effects occur. The rate of death is a function of both the temperature and the time of exposure. Microbial death is an exponential function (death is linear when plotted on a log scale. The decimal reduction time is the time required to reduce the population by a factor of 10. In the figure, the upper line represents a heat resistant microorganism. (See first illustration.) (See second illustration.)

The autoclave typically uses an operating temperature of 121°C. At this temperature, bacterial endospores have a decimal reduction time of 4 - 5 min, whereas vegetative cells have decimal reduction times in the range of 0.1 to 0.5 min. Because endospores are not killed boiling temperatures (100°C), the autoclave uses steam under pressure, typically 15 lb/in. Generally, the time after the autoclave reaches pressure is 10-15 min, however, bulky objects or large volumes of liquid require more time.

Pasteurization is a process that reduces the microbial population of a liquid (e. g., milk, wine, and fruit juices). The process is named for Louis Pasteur, whom first used the methodology for making wine (see the box). During bulk pasteurization, the liquid is held in vats at 63 to 65°C for 30 min. In flash pasteurization, which gives more satisfactory results and can be adapted to continuous-flow, the liquid is heated to 71°C for 15 sec, then rapidly cooled. Pasteurization extends the shelf life of a product and reduces the level of pathogens in the product. (See illustration.)

Electromagnetic irradiation is another effective way to sterilize or reduce microbial burden of almost any substance. Microwaves, ultraviolet (UV) radiation, X-rays, gamma rays and electrons are used although each type of irradiation has a specific mechanism. UV irradiation, which does not penetrate solid, opaque or light absorbing materials, is useful for disinfecting surfaces, air and liquids that do not absorb the UV waves. Gamma and X-rays, which are more penetrating, are more difficult and expensive to use but are finding application in food preservation and other industrial processes. Irradiation has become a useful alternative to ethylene oxide in preparation of surgical supplies.

Microorganisms may be removed from liquid media by filter sterilization. Depth filters, which are made from paper, asbestos or glass fibers, consist of a random array of overlapping fibers. Depth filters trap particles in the torturous paths created throughout the depth of the structure. Membrane filters are composed of cellulose acetate or cellulose nitrate and are made in such a way that the filter contains a large number of tiny holes. Thus, the microorganisms are trapped on the surface of the filter. Finally nucleation track filters are created by treating very thin polycarbonate films with nuclear radiation then etching the film with a chemical. The sizes of the holes can be precisely controlled. Sterilization by filtration may preserve biologically important molecules that are inactivated by heat.

Antimicrobial agents are chemicals that kill or inhibit microorganisms. Cidal agents kill microbes, thus the terms bactericidal, fungicidal, and viricidal. Static agents (bacteriostatic, fungistatic and viristatic) inhibit growth. Antimicrobial agents act in a number of ways as shown in the figure to the left.

The minimum inhibitory concentration (MIC) is the amount of the agent required to inhibit growth of the test organism. MIC's are often determined by the tube dilution technique, which uses various concentrations of antimicrobial agents in medium containing a constant inoculum of microorganisms. The method varies with the culture medium, inoculum size, incubation time, and nature of the test organism. Antimicrobial action may also be studied by the agar diffusion method. A Petri plate containing an agar medium is evenly inoculated with the test organism. Known amounts of the antimicrobial agent are added to filter paper disks that are placed on the surface of the agar. During incubation, the agent diffuses from the disk and results in a zone of clearing around the disk that can be measured -- larger zones of clearing imply more effective agents. Disinfectants are chemical antimicrobial agents, frequently termed germicides, that are used on inanimate objects, whereas antiseptics are agents used on living tissue.

Control of microbes is important in the food industry because some microbes spoil food or their growth in food produces toxins. The susceptibility of a product to food spoilage is a consequence of its suitability as a growth medium. Therefore, foods with low water activity are less susceptible to microbial spoilage. Food can be preserved by (1) lowering the storage temperature, (2) lowering the pH, or (3) adding sugar or salt to decrease water activity. Canning is a process in which a food is sealed and heated so as to kill all living organisms or at least to ensure that there will be no growth of residual organisms in the can. Thus, Canning is a type of heat sterilization.

Chemotherapeutic agents are agents used for control of disease. Growth factor analogs, are substances that are related to growth factors and block utilization of the growth factor in the infectious agent. The first of these to be discovered was the sulfa drugs. One of these, sulfanilmide acts as an analog of p-aminobenzoic acid and blocks synthesis of folic acid. (See illustration.)

Antibiotics are microbial or synthetic substances that inhibit other microbial species. Only a few of these are practical in treating infectious diseases, because they may cause toxic effects upon animals or are ineffective in the body. The effectiveness of an antibiotic may be enhanced by chemical modification after its microbial production. Targets of action include the bacterial cell wall, membrane, and ribosomes. Bacteria may be innately resistant to an antibiotic, or may acquire resistance. Resistance can result from (1) the absence of the target of action, (2) drug impermeability, or (3) production of an inactivating enzyme. Antibiotics are a unique class of chemotherapeutic agents because they are natural products rather than synthetic chemicals, such as the sulfa drugs. (See illustration.)