Growth Requirement of Bacteria

The growth requirement of microorganism can be grouped on the basis of their need for oxygen to grow. Facultatively anaerobic bacteria can grow in high oxygen or low oxygen content and are among the more versatile bacteria, In contrast, structly anaerobic bacteria grow only in conditions where there is minimal or no oxygen present in the environment. Bacteria such as bacteroides found in the large bowel are examples of anaerobes. Strict aerobes only grow in the presence of significant quantitie of oxygen. Pseudomonas bacteria grwo under conditions of reduced oxygen and sometimes also require increased levels of carbon dioxide. Neisseria species (e.g. the cause of gonorrhea) are examples of micraerophilic bacteria.

Biochemical reactions: Clinical microbiology laboratories typically will identify a
pathogen in a clinical sample, purify the microorganism by plating a single colony of the
microorganism on a separate plate, and then perform a series of biochemical studies
that will identify the bacterial species.

Serologic systems: Selected antisera can be used to classify different bacterial
species. This may be based on either carbohydrate or protein antigens from the
bacterial cell wall or the capsular polysaccharide. (Group A streptococcal M proteins or
O and H polysaccharide antigens of salmonella).

Environmental Reservoirs: When considering likely pathogens it is also important to
know which of the different species are found in different locations. Environmental
reservoirs are generally divided into those that are endogenous (i.e., on or within the
human body) and exogenous (somewhere in the environment). When considering the
likely cause of an infection the likely source of the infection is important in your differential diagnosis. For example, an anaerobic organism resident in the large bowel is
the likely cause of an abdominal abscess that develops after large bowel surgery. A skin
rash developing in a hiker with a history of multiple tick bites is more likely to be borrelia, the agent of Lyme disease. An outbreak of food poisoning traced to imported
unpasteurized cheese might be due to listeria.

Endogenous reservoirs account for a large proportion of human infections. Many parts
of the body have their own normal flora. S. epidermidis is found on the skin. Viridans
streptococci are a part of the normal oropharyngeal flora and S. aureus is a commensal
of the anterior nares.


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