5.3.1: Antiseptics and Disinfectants

Controlling microbial growth in the environment around us is important in our homes, food industry, clinical and hospital setting among other areas. Antimicrobial chemicals are one tool that can be utilized to control microbial growth. Antimicrobial agents work to either kill or inhibit the growth of microorganisms. Agents that act to kill microbes are known as microbicidal (-cidal = kill) agents. Chemicals that work only to inhibit the growth of microorganisms are known as microbiostatic (-stasis = ). The suffixes -cidal and -stasis, can be combined with other terms to provide more detailed information about a specific type of microorganism being target. For example, chemicals that are referred to as bactericidal, specifically work to kill bacteria. Bacteriostatic chemicals would work to inhibit the growth of bacteria. Virucidal agents would specifically target and destroy viruses. Sporicidal chemicals, also known as sterilants would have the ability to destroy endospores.

Chemical agents do not work equally against all forms of microorganisms. This is because some organisms are more resistant due to their structural make up. For example, the two types of animal viruses, naked and enveloped viruses are not equally resistant to chemicals. Naked viruses are considered more resistant.

Enveloped viruses have a lipid membrane derived from their hosts that could be disrupted by any chemical that is damaging to lipids. Naked viruses, lacking the lipid envelope, would be unaffected by such agents. The most resistant form of microorganism is the endospore. Endospores, due to the protective layers around the genetic material, are resistant to heat, radiation, and many chemicals. In the microbiology lab, it is assumed that if the method is capable of killing/destroying endospores, then all other less resistant forms will have also been destroyed. Such methods are known as sterilizing methods because all forms of microorganisms have been killed.

Antiseptics are chemical agents that are used to reduce microbial numbers on living tissues. Antiseptics include substances such as isopropyl alcohol, iodine, and hydrogen peroxide. Disinfectant, on the other hand are reduce microbial numbers on inanimate objects, i.e. lab benches, kitchen counters, stethoscopes, etc. Bleach, Lysol, iodine, and alcohol are some examples of chemicals that can be utilized as disinfectants. Notice that some of the examples were listed as both an antiseptic and disinfectant. Not all chemical agents can classify as both, due to the fact that some of the agents used solely as disinfectants are too harsh to be applied to living tissues. Another group of chemical agents, known as sanitizers, are used primarily in the food industry, to reduce microbes to a safe level.

In this exercise, you test a variety of chemical agents that can be found in the household or in an introductory lab. You will soak filter disks in the chemical agents and then place the disks on agar plates that have been inoculated with a bacterium. After incubation, if the chemical agent has been successful in inhibiting or killing the microorganism, a clear zone will appear around the filter disk, known as a zone of inhibition. To assess the effectiveness of the chemical agents, you will measure the zones of inhibition.

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