Selection of an effective antimicrobial procedure is complicated by the fact that every procedure has parameters and drawbacks that limit its use. An ideal, multipurpose, non-toxic method simply does not exist. The ultimate choice depends on many factors including the type of microbe, extent of contamination, environmental conditions, and potential risk of infection associated with use of the item.
One of the most critical considerations in selecting a method of destroying microorganisms and viruses is the type of microbial population present on or in the product. Products contaminated with microorganisms more resistant to killing require a more rigorous heat or chemical treatment. Some of the more resistant microbes include:
■ Bacterial endospores. The endospores of Bacillus and Clostridium are by far the most resistant forms of life. Only extreme heat or chemical treatment ensures their complete destruction. Chemical treatments that kill vegetative bacteria in 30 minutes may require 10 hours to destroy their endospores. ■ endospores, p. 67
■ Mycobacterium species. The waxy cell walls of mycobacteria make them resistant to many chemical treatments. Thus, stronger, more toxic disinfectants must be used to disinfect environments that may contain Mycobacterium tuberculosis, the causative agent of tuberculosis. ■ tuberculosis, p. 580
■ Pseudomonas species. These common environmental organisms are not only resistant to some chemical disinfectants, but in some cases can actually grow in them. Pseudomonas species are of particular importance in a hospital setting, where they are a common cause of infection. ■ Pseudomonas infections, p. 697
■ Naked viruses. Viruses such as poliovirus that lack a lipid envelope are more resistant to disinfectants. Conversely, enveloped viruses, such as HIV, tend to be very sensitive to heat and chemical disinfectants. ■ naked viruses, p. 324
Numbers of Microorganisms Initially Present
The time it takes for heat or chemicals to kill a population of microorganisms is dictated in part by the number of organisms initially present. It takes more time to kill a large population of bacteria than it does to kill a small population, because only a fraction of organisms die during a given time interval. For example, if 90% of a bacterial population is killed during the first 3 minutes, then approximately 90% of those remaining will be killed during the next 3 minutes, and so on. Recall that during the death phase of a cell population, cells die at a constant rate. Exposure to heat or disinfectants increases that rate. ■ death phase, p. 103
In the commercial canning industry, the decimal reduction time, or D value, is the time required for killing 90% of a population of bacteria under specific conditions (figure 5.2). The temperature of the process may be indicated by a subscript, for example D121. A one D process reduces the number of organisms by one exponent. Thus, if the D value for an organism is 2 minutes, then it would take 4 minutes (2 D values) to reduce a population of 100 (102) organisms to only one (100)
Figure 5.2 The Relationship Between the Numbers of Initial Microorganisms and the Time It Takes to Kill Them The D value is the time it takes to reduce the population by 90%.
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