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Beyond bacterial shape, the staining characteristics of bacteria offer additional insight on which species are present in a sample. Using a technique originally described by the Danish bacteriologist Hans Christian Gram in the 1880s (which today bears his name) most bacteria can be divided into 2 broad categories. To perform this test specimens fixed to a glass microscope slide are sequentially exposed to 2 dyes; crystal violet followed by basic fuchsin. When the bacterial cell wall contains high peptidoglycan and low lipid levels, the crystal violet dye is avidly retained and the bacteria appear blue-purple when examined through a microscope. These are known as gram-positive bacteria. When bacteria do not retain crystal violet they appear red to pink due to basic fuchsin staining. These bacteria are known as gram-negative species.

A few strains can not be categorized as either gram-positive or gram-negative, examples include Mycobacterium tuberculosis. For these bacteria, alternate staining processes are helpful in pathogen identification.



Major strains in each category are listed below

Gram-positive species
Staphylococcus spp
Streptococcus spp
Clostridium spp
Listerial spp
Bacillus spp
Gram-negative species
Klebsiella spp
Acinetobacter spp
Enterobacteriaceae, such as E coli
Pseudomonas spp
Legionella spp
Neisseria spp
Salmonella spp
Haemophilus spp
Helicobacter pylori


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