Anaerobic bacteria and its Infection
An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It may react negatively or even die if free oxygen is present. In contrast, an aerobic organism (aerobe) is an organism that requires an oxygenated environment. Anaerobes may be unicellular (e.g. protozoans, bacteria) or multicellular. Most fungi are obligate aerobes, requiring oxygen to survive, however some species, such as the Chytridiomycota that reside in the rumen of cattle, are obligate anaerobes; for these species, anaerobic respiration is used because oxygen will disrupt their metabolism or kill them. Deep waters of the ocean are a common anoxic environment.
Anaerobic bacteria predominate on normal skin and among the bacterial flora of mucous membranes. Infections caused by anaerobic bacteria are common, arise from the sites where they are normal flora (endogenous), and can be serious or life-threatening. Anaerobic bacteria are fastidious, difficult to isolate, and often overlooked. Their recovery requires proper methods of collection, transportation, and cultivation. Their ubiquity on mucocutaneous surfaces often interferes with obtaining meaningful cultures. Although no universally accepted or simple and accurate way has been established to classify microorganisms according to conditions required for their replication, it presents a useful framework for the clinician. Nevertheless, differences among strains exist within the same species.
Anaerobic bacteria do not replicate in the presence of oxygen; however, they exhibit substantial differences in lethal effect of oxygen. In general, anaerobic organisms found exclusively as normal flora are strict anaerobes (i.e., die within minutes in <0.5% oxygen), whereas those of clinical significance are somewhat aerotolerant (i.e., tolerate 2% to 8% oxygen). Strict anaerobes do not grow in 10% carbon dioxide in air; microaerophilic bacteria can grow in 10% carbon dioxide in air or under aerobic or anaerobic conditions, and facultative organisms can grow in the presence or absence of air.
The physiologic basis for oxygen sensitivity is not well understood. Common teaching is that negative oxidation-reduction potential (Eh) of the environment is the critical factor. However, studies with Bacteroides fragilis reveal that oxygen has a direct toxic effect; chemical manipulation of oxidation-reduction potential has no effect if oxygen is not introduced. Furthermore, aerotolerance and possibly virulence of anaerobic bacteria correlate with the ability to induce the protective enzyme superoxide dismutase on exposure to oxygen.
Anaerobic bacteria range from those that die with very brief exposure to oxygen to those that can survive even in the presence of atmospheric oxygen (e.g., B. fragilis). Most anaerobes require an environment with a low oxidation-reduction potential (Eh gradient), which can be achieved in association with low pH, tissue destruction, byproducts from aerobic bacterial metabolism, or low oxygen content. Though not true anaerobes, some organisms such as microaerophilic streptococci and other capnophilic or hard-to-grow organisms are sometimes lumped together with anaerobes because of their fastidious nature. Some genera such as Lactobacillus and Actinomyces contain both aerobic and anaerobic species.
Anaerobic bacteria are found in the larynx, mouth, gastrointestinal tract, vagina, external genitalia, and skin. Anaerobic infections are either endogenous in origin or are from environmental organisms, e.g. Clostridium tetani.
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