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Environmental Analysis

Gram stain & morphology [GS&M]

Published: July 8th, 2009

Revised: March 27th, 2019

This test provides a semiquantitative direct microscopic evaluation of a range of matrices, including swabs, MCEM air samples, bulk materials and fluids, to determine the presence and form of bacterial contaminants. Results are reported as 1+, 2+, 3+, etc. Because this test does not require a culture step, it can be completed on a rapid turn-around. This test cannot provide bacterial identifications to genus or species. However, it can be a useful tool to determine the presence of bacterial in a suspect material where a more detailed characterization is not necessary to guide appropriate actions. This test cannot determine whether the bacteria we observed were living or dead. If you require information about cell viability, the Gram Stain, Morphology & Viability test [D201] may be more appropriate.

This test provides a semiquantative result, which tells you if we have observed bacteria, their shapes, their response to the Gram Stain procedure, and the abundance of the different types according to a logarithmic scale, indicated as 1+ up to 4+. The number before the plus sign denotes the power of 10 that corresponds to the approximate number of each type of bacterium per microscopic field at 1000 x magnification. For example, 1+ would mean that 10 or fewer of a particular type of bacterium were observed per field over the number of fields we examined (50 or more). Similarly, 3+ would mean that we typically saw up to 1000 in the same field.

Gram staining is a routine procedure in microbiology that allows bacterial to be classified into two groups depending on their response to the test. This relatively simple test is extremely useful, since much can be determined or predicted based on the Gram staining reaction. For example, Gram-negative bacteria include a number of extremely common environmental general such as Pseudomonas and Legionella, as well as a large number of bacteria associated with the human gut (including E. coli, and many enteric pathogens like Salmonella, Shigella and Listeria). Gram-negative bacteria also characteristically produce endotoxin in their outer membrane. Endotoxin is a lipopolysaccharide that elicits a potent inflammatory response in humans upon inhalation exposure or blood contact. Gram-positive bacteria, by contrast, do not produce endotoxin in significant amounts. There are, however, many notable human pathogenic Gram-positive bacteria, including species in the familiar genera Staphylococcus and Streptococcus.

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