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Author Topic: Bacteria  (Read 72 times)



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« on: November 01, 2016, 09:33:08 AM »

A multiplicity of mycobacterial species, both saprophytes and potential pathogens, may be isolated from humans. The Level II or Level III diagnostic laboratory, through the use of a few reliable in vitro tests, should be able to provide a precise species identification of most acid-fast bacilli isolated from patients. Through the cooperative efforts of clinicians and bacteriologists over the past decade, it has been possible to determine statistically the potential clinical significance of most species of mycobacteria.

A clear-cut separation of pathogen from saprophyte is not always possible for the individual isolate except when M. tuberculosis is isolated. The isolation of a nontuberculous organism of potential clinical significance is not ipso facto evidence that the patient is diseased, nor, for that matter, should all isolates, which are usually clinically insignificant, be disregarded as saprophytes. Each mycobacterial isolate, as each patient, must be evaluated individually.

Mycobacterium tuberculosis, the causative agent of tuberculosis, usually is readily identified by its rough, nonpigmented, corded colonies on oleic-acid-albumin agars; a positive niacin test; generally weak catalase activity, which is lost completely by heating to 68 degrees C; and a positive nitrate reduction test. Drug-resistant (especially isoniazid-resistant) tubercle bacilli may grow poorly or not at all on laboratory media; they may lose or have diminished catalase activity; and they may fail to produce progressive disease in guinea pigs, although not necessarily in humans. Preliminary screening of all strains of M. tuberculosis for catalase activity may provide the clinician with valuable information relative to isoniazid resistance even before susceptibility tests are performed.

M. bovis is indistinguishable from M. tuberculosis except by culture followed by in vitro tests. If an isolate proves to be other than M. tuberculosis, its precise identification should be established. This may require that the culture be referred to a large reference laboratory such as a state health department laboratory, a Veterans Administration reference laboratory, or the Centers for Disease Control (CDC) in Atlanta for definitive identification.

Published with kind permission of CENTER for DISEASE CONTROL
« Last Edit: November 08, 2016, 16:08:13 PM by auntiebiotic »


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