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Microbe Basics
Understanding the bugs that inhabit surgical facilities.
Dan Mayworm
Publish Date: October 10, 2007   |  Tags:   Infection Prevention

If it's useful to "know thine enemy," then it behooves us to know a little about bacteria and viruses, the two most common microorganisms found in healthcare facilities.

Bacteria usually occur as vegetative or growing cells; however, a few species also produce spores, complex entities which are highly resistant to biocides and most physical cleaning processes. At the center of the spore is the core, which contains RNA, DNA, minerals, and water. The more water it holds, the harder it is to inactivate the spore with moist heat.

Once it is formed within the bacterial cell, the spore remains dormant, often for many years, until the proper growth conditions cause it to germinate into a vegetative cell. Viable spores are so prevalent in nature that we must assume that they are also present in healthcare settings. Steam sterilization may be the only quick, effective, practical method for eliminating them.

Once bacteria are in a vegetative state, they are relatively easy to kill with disinfectants (note, however, that very few antiseptics boast the same effectiveness). This is partly because of their complexity and the many ways that they react with their environment. However, a few types of vegetative bacteria are particularly resistant. Tubercle bacillus has a waxy envelope that is comparatively resistant to aqueous germicides and almost completely resistant to quaternary ammonium compounds and hexachlorophene. The gram-positive staphylococci and enterococci and the gram-negative Salmonella and Pseudomos species are also somewhat more resistant than other varieties.

Viruses are simple acellular particles that consist of a protein coating surrounding either RNA or DNA. They show no metabolic processes outside a host cell. Furthermore, the complex interaction that occurs between one type of virus and a biocide does not necessarily occur with another type. These two factors make it much more difficult to predict viral kill. Some of the most resistant types are animal viruses called rotaviruses, which can withstand many commercially available disinfectants. Only a few agents are effective against human rotaviruses.

There are four classes of viruses:

  • Lipid-enveloped viruses have a high degree of sensitivity to all biocides. They include herpes simplex, influenza and mumps.
  • Non-lipid-enveloped small viruses are resistant to lipophilic germicides (isopropanol and QACs). They include parvoviruses and picornaviruses.
  • Larger non-lipid-enveloped viruses like adenoviruses and reoviruses are more resistant to biocides than lipid-enveloped viruses.
  • Unclassified viruses include hepatitis A and B. Retrovirusus like hepatitis C, hepatitis non-A and non-B, and HIV are readily destroyed by most disinfectants; hepatitis B is more resistant.

Steam kills most viruses. Most are susceptible to comparatively low moist heat temperatures (compared with the ones needed to kill bacterial spores).