Inhibiting Microbial Growth

By Jennifer Stearns, Michael Surette

The terminology used to describe methods for reducing or removing microbes from a surface can sometimes be confusing. There are different reasons for wanting to get rid of microbes, but not all of them require sterilization (the complete eradication of all living things), which is needed for surgical equipment.

For the purposes of food preparation, for example, steps are used to ensure that foodborne pathogens that could make people sick have been removed. An example of this is commercial sterilization used on canned food, where the conditions are used to reduce the chances of including spores that cause botulism, but the food is not completely sterile.

When cleaning a place where food is served, like a restaurant, the process of sanitation gets rid of microbes that can be passed between people. Disinfection is the removal of growing microbial cells from a surface. When that surface is skin, the disinfecting agent is milder and is called an antiseptic.

Another distinction to make is whether a chemical is bacteriostatic (which stops the growth of bacteria but doesn’t kill it) or bacteriocidal (which does kill bacteria).

Physical methods

These physical methods disrupt proteins, killing microorganisms, and some have been used specifically to destroy bacterial endospores.

Heat, although useful for destroying microbes, isn’t enough to sterilize most things. It’s only in the combination of heat with pressure that resistant spores are killed. This is accomplished with special equipment like an autoclave or pressure cooker that uses steam under pressure.

In the absence of pressure, heat can kill most microbes, which is why boiling things in water for ten minutes is recommended to be fairly safe, although it won’t be free of the spores of some bacteria, if present.

If the object to be sterilized can withstand high heat, like metal can, then flaming can be used. With this method, the object is placed directly into a flame or is first dipped into alcohol that is then ignited. Needless to say, this is an effective way to sterilize an object.

Pasteurization is the heating of things like milk and beer at lower temperatures than what would be needed to sterilize them in order to kill pathogens and lower microbial numbers while still protecting the flavor of the product.

Two technological advances to the food industry include high-temperature short-time (HTST) pasteurization and ultra-high-temperature (UHT) treatment, the latter of which actually sterilizes the product without severely impacting its flavor.

Liquid solutions can be sterilized by filtration through a membrane with a pore size smaller than the microbes to be removed.

Low temperature serves to slow or halt the growth of most microbes. Some, however, can grow at low temperatures. If you want proof of this, just think of how leftovers can spoil in the refrigerator if left for too long.

There are several examples of radiation used to kill microorganisms. The first example is ionizing radiation such as X-rays and gamma rays that either directly damage cells or indirectly damage them via the production of free radicals. The second kind is nonionizing radiation that comes from ultraviolet (UV) light, which causes irreversible changes in a microbe’s DNA that make it useless.

Disinfectants

Examples of disinfectants are all around us. Some are easily identified and others are harder to spot. Here are some of them:

  • A type of phenol that comes from coal tar is called a cresol and is the main ingredient in Lysol. A bisphenol called triclosan has been used in the manufacture of kitchen utensils like cutting boards to deter microbial growth.

  • Halogens like iodine and chlorine have been used for centuries and are still used today to treat water (iodine tablets) and in the household disinfectant Clorox.

  • Alcohol is a convenient disinfectant that works for many microbes but isn’t able to get rid of endospores or unenveloped viruses.

  • Heavy metals like silver, copper, and mercury are very effective against microorganisms and along with their historical use in medicine and industry have recently been incorporated into sporting equipment and clothing to reduce the microbes that cause odor.