15 Notes - Topic 15 Fundamentals of Microbiology...

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Unformatted text preview: Topic 15 Fundamentals of Microbiology (Biology 140) Course notes Dr. Josh D. Neufeld Learning Objectives: To consider methods for controlling microorganisms and appreciate the challenges associated with these methodologies. Until now, we have been considering growth characteristics of microorganisms. Now, we'll look at how to inhibit their growth or kill them. The methods that are used to inhibit growth or sterilize take advantage of cell sensitivity to heat, radiation or chemicals, or involve removal of the cells by filtration. The components of microbial cells are denatured if the temperature rises high enough, resulting in loss of cell viability (Figure 26.1). A given organism will have a characteristic sensitivity to temperature, with death occurring more rapidly at higher temperatures. There is an exponential relationship between time of heating and extent of killing. Decimal reduction time (D) is the time required for a ten ­fold reduction in the population density, and is not dependent on the initial cell concentration. Endospores are much more resistant to heat than are vegetative cells. For this reason, heat sterilization is aimed at ensuring that endospores are killed. At the autoclave temperature of 121°C, endospores have a decimal reduction time of 4 ­5 minutes. Moist heat is more effective than dry heat, since it can penetrate the object being sterilized more effectively. The high temperature within the autoclave, above the boiling point of water, is achieved under pressure (Figure 26.3). A very common method of reducing the numbers of microorganisms in foods is pasteurization. The goal of pasteurization is not to sterilize, but rather to reduce the cell numbers so that the incidence of pathogenic microorganisms and/or the likelihood of spoilage is reduced. Pasteurization involves raising the temperature for brief periods of time so that the microbial cell numbers are decreased while minimizing the adverse effects on the product. For example, milk can be pasteurized by treatment at 71°C for 15 seconds. Many different types of radiation can be used for sterilization. The mechanisms vary, depending on the type of radiation. Microorganisms also vary in their sensitivity to radiation (Table 26.1). There is an exponential relationship between radiation dose and survival (Figure 26.5). Sterilization by radiation has many applications, especially for objects that are heat sensitive or would be destroyed by heat sterilization. Some examples are spices, pharmaceuticals, and medical equipment. Fundamentals of Microbiology (Biology 140) Course notes Dr. Josh D. Neufeld Filtration is a very effective method for sterilizing liquids and gases. The cells are removed based on size, rather than being destroyed. The filters that are used for sterilization are designed to remove all cells, but do not remove virus ­sized particles. Membrane filters are commonly used in the microbiology laboratory to sterilize solutions or growth media (Figures 26.7, 26.8). Many chemical agents are available for killing or inhibiting the growth of microorganisms. These are called antimicrobial agents. A bacteriostatic agent would only inhibit the growth, while a bacteriocidal agent would kill (Figure 26.9). A bacteriolytic agent kills the cells by causing cell lysis. The sensitivity of a microorganism to a particular antimicrobial agent can be expressed as the minimal inhibitory concentration (MIC) (Figure 26.10). Sensitivity can also be determined by the agar diffusion method (Figure 26.11). Antiseptics: antimicrobial agents that can be applied to living tissue Disinfectant: antimicrobial agents that are used on non ­living objects Antimicrobics: antimicrobial agents produced either by microorganisms (antibiotics) or synthesized chemically. Please see example antiseptics and disinfectants in Tables 26.3 and 26.4 and example antibiotics in Figure 26.13. ...
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