Clinical Laboratory - Microbiology

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Microbiologic Testing

Usage and Limitation of Staining Techniques

Fluids with cells and tissue samples may be obtained that can be placed on a slide and viewed with a microscope. These cells and tissues can be stained to show micro-organisms.

Stains such as gram stains on fluids and tissues obtained are rapid and simple to perform, but may not yield highly specific information about a causative micro-organism. Sometimes, just knowing that some microbiologic agent is present can have significance, and treatment can begin empirically on that basis until more specific information becomes available.

Gram stains do not provide speciation. Reporting "gram-negative rods" means that the organism could be a member of the Enterobacteriaciae, such as E. coli, or be a Pseudomonas organism. The gram stain helps determine if organisms are present and in what general category they may be for instituting empiric antimicrobial therapy.

Normal flora may have staining characteristics similar to pathogens.

If samples obtained from body sites that are normally sterile, such as the subarachnoid space containing CSF, yield any organisms, then that finding by itself is significant.

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