|MadSci Network: Microbiology|
Dear Emma, Thanks for writing. Before I jump into the specific modes of action that different disinfectants have, I'll offer some background on bacterial structure to make sure we're thinking along the same track. Bacteria are single celled organisms. All the activities bacteria need to perform to live are contained within this single cell. The cell is delineated from the rest of the environment by a barrier known as the lipid membrane. The membrane is made up of lipid molecules that aggregate to exclude the fluid of the environment. To imagine this more clearly, think of the leftover fat in a frying pan (perhaps not a beautiful image, but an effective one). Fat is also made of lipids and is not soluble in water. Fat beads up and excludes the water, as you know if you've ever tried to clean a dirty frying pan with water alone. Within the lipid membrane are the life sustaining activities of the bacterial cell. The agents that carry out activities such as metabolism and reproduction are many different kinds of proteins that the cell must make from scratch. If a bacterium cannot maintain the lipid boundary or protein synthesis fails, the cell is doomed, and that's where disinfectants come in! Disinfectants are chemicals applied to non-living surfaces that rid these surfaces of bacterial contamination. All disinfectants are bactericidal; in other words, they kill the bacteria. However, only some disinfectants are bacteriostatic meaning that they prevent the growth of new bacteria. The amount of time the surface stays free of bacteria depends on the disinfectant. I'll give a sampling of common disinfectants and each mode of action in alphabetical order: Alcohol This disinfectant category contains specific kinds of pure alcohol (in other words not your common table wine). Ethanol (an ingredient in table wine) and Isopropanol (also known as rubbing alcohol and poisonous to humans) are very commonly used. These disinfectants disrupt the structure of proteins in the bacteria. If the proteins are disrupted, or denatured, they cannot perform their highly specific tasks, and the bacterium dies. Bleach Bleach is also known by its formal chemical name as sodium hypochlorite. Now we shall delve into some basic chemistry. In solution, the sodium hypochlorite dissociates, or dissolves, to produce atomic oxygen and chlorine. The chlorine and oxygen are very reactive, negatively-charged elements that oxidize components of the bacterial cell. This oxidation prevents effective operation of the cell's machinery, and the bacterium dies. Detergents I bet you've always wondered what the difference between a detergent and a soap is! Detergents act like soaps though detergents are not derived from fats like soaps are. Detergents are comprised of long molecules with very different ends. In some types of detergents, one end of the molecule is positively charged (insoluble in water), and the other is negatively charged (soluble in water). These molecules wiggle their way in between the lipids of the bacterium's membrane and cause the cell to spill its contents into the environment, killing the cell. Iodine Dilute solutions (as little as 1%) of iodine work in much the same way as the chlorine described above. Looking at a periodic chart, you'll even notice that chlorine and iodine are in the same halogen family, hence their common mode of action. Negatively charged iodine combines with proteins and disrupts their ability to function normally. Phenolics Phenolics is the generic name for a specific family of chemical structures. There are a variety of phenolics used as disinfectants. Lysol is perhaps the most famous. This class of disinfectants works by disrupting proteins, a common theme as we have seen, but also by disrupting the lipid membrane. A leaky or broken membrane in addition to faulty proteins are problems too severe for the bacterium to fix in time, and it dies. Phenolics are also bacteriostatic. All the disinfectants described above work in an indiscriminate manner. For example, the chlorine ions bind to many kinds of proteins, not just one. Phenolics cause many types of bacterial species to have leaky membranes. I refer to this as a "broad spectrum of action". As a consequence, it is highly unlikely that every protein in a cell will evolve an ability to resist the effects of chlorine. I'll call the following section "extra credit" because it moves beyond traditional disinfectants to a recent phenomenon on your grocery store shelves. Triclosan Here is a disinfectant that does discriminate, or is picky about its target in a bacterial cell. You have probably noticed quite a lot of "antibacterial" products on the store shelves in recent years. The active ingredient in these soaps and detergents is an antibiotic known as Triclosan. Triclosan, however, kills bacteria by acting on a single specific protein that is necessary for the bacterium to make an effective lipid membrane. If this one protein evolves to resist the action of Triclosan, Triclosan will no longer be able to kill those bacteria with the new version of the protein. Scientists are debating the effects of bacterial resistance to Triclosan, and the following article will give you more details on a hot topic in the scientific world. Heath, R.J., Y.T. Yu, M.A. Shapiro, E. Olson, C.O. Rock. 1998. "Broad spectrum antimicrobial biocides target the FabI component of fatty acid synthesis." Journal of Biological Chemistry. 273(46):30316-20. For some nitty gritty information on chemical structures of disinfectants, check out: Grayson, M., ed. 1982. “Antibiotics, Chemotherapeutics, and Antibacterial Agents for Disease Control.” New York: John Wiley & Sons, pp.435-473. http://www.cbc.umn.edu/~mwd/courses.html is a site that lists many kinds of biology courses available through the web, including microbiology. This would be a good starting point if you wanted to know more about bacteria or molecular biology.
Try the links in the MadSci Library for more information on Microbiology.