01b_Septic_Enrich_PreClin - CLINICAL SPECIMEN UNKNOWN[Day 1...

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CLINICAL SPECIMEN - “UNKNOWN” [Day 1] LABORATORY EXERCISE #1 Rev: 3/10/10 Day 1 - Gram Stain and Dilution Streaking - One of the first steps in the identification of a bacterium is the “ GRAM STAIN ” of the specimen. This provides an initial “identification” of the nature of the pathogen - such a “presumptive diagnosis” is often useful to the physician in selecting the most appropriate antimicrobial drug for the immediate treatment of a patient. Usually, the actual identity of the pathogen (in terms of genus and species) and its antimicrobial “sensitivity” profile will not be available until the next day. The information from a Gram stain, together with the physician’s ability to “read” the patient’s symptoms and prescribe “empirical” chemotherapy, may be lifesaving! The other first step in identification is the “ DILUTION STREAK ” of the specimen on to various solid media. The purpose of this step is to produce isolated colonies - those pure cultures can next tested biochemically [to determine genus and species] and tested for antimicrobial sensitivity testing. The ISOLATION OF PURE CULTURES IS ESSENTIAL since clinical specimens may contain a mixture of bacterial species , including the pathogenic organism responsible for disease and normal microbiota [“normal flora ”] that naturally occur at some body sites. If biochemical and sensitivity testing are preformed on mixed cultures, they will yield useless data - pure cultures are essential! In a clinical lab, the various media that the technologist will use [for the each of various types of specimens] have already been determined. In each clinical lab, there is a “protocol” that specifies the types of growth media to be inoculated for each type of patient specimen. The choice of media is based upon the site of infection and hence the most likely organisms that would be causing an infection. The media are chosen to support the growth of even the most “fastidious” of microbes [i.e. the most demanding in terms of growth requirements] and the media are “differential ” [i.e. the patterns of growth on certain media facilitate the identification of the organism] The next day [Day 2], the patterns of growth on the various media, the information from the initial Gram stain, and a second Gram stain of the newly isolated colonies would be used to determine the most appropriate biochemical tests - optimally, these would allow a final identification of the pathogen. The technologist would set up those tests, read their results the following day [Day 3], and report the results back to the physician, two days after the initial clinical specimen was received.
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