Microbiologic Studies

Microbiologic studies that allow for direct examination of a specimen (e.g., sputum, blood, or urine) also may aid in a presumptive diagnosis and give an indication of the characteristics of the infecting organism. Generally, microbial cultures are obtained with a Gram stain of the cultured material.

A Gram stain of collected specimens can give rapid information that can be applied immediately to patient care. A Gram stain is performed to identify if bacteria are present and to determine morphologic characteristics of bacteria (such as gram-positive or gram-negative or shape—cocci, bacilli). Certain specimens do not stain well or at all and must be identified by alternative staining techniques (Mycoplasma spp., Legionella spp., Mycobacterium spp.). Figure 69-2 identifies bacterial pathogens as classified by Gram stain and morphologic characteristics. The presence of WBCs on a Gram stain indicates inflammation and suggests that the identified bacteria are pathogenic. The Gram stain may be useful in judging a sputum specimen's adequacy. For example, the presence of epithelial cells on sputum Gram stain suggests that the specimen is either poorly collected or contaminated. A poor specimen can give misleading information regarding the underlying pathogen and is a waste of laboratory personnel time and patient cost.

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