Gram Negative Bacteria

    The gram negative cell wall is very thin, composed of about 10% peptidoglycan, lipoproteins, lipopolysaccharides, and proteins associated with the peptidoglycan layer.  This wall is too thin to retain most of the crystal violet-mordant complex, but does retain the safranin counterstain, so gram negative cells stain red.  There is an outer cell membrane attached to the wall by lipoproteins, and a periplasmic space beween the two.  The destain in the gram staining process is a mix of organic solvent compounds which remove this membrane, as well as any excess primary stain, accentuating the ability of the secondary stain to bind to the cell wall.

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Escherichia coli (1000X).  This organism is a coccobacillus commonly associated with the gastointestinal tract of mammals.  Some strains of this species are agents of gastroenteritis, urinary tract infections (UTIs), meningitis, and pneumonia.  One enterotoxigenic strain, E. coli O157:H7 (the alphanumeric code represents the O cell antigen serotype 157, and the H flagellar antigen serotype 7),  is associated with severe and potentially fatal forms of gastroenteritis and UTIs acquired through the consumption of undercooked meats and un- or underpasteurized apple juice.

Salmonella choleraesuis serovar typhimurium (1000X). This rod causes food infections in humans, and has been implicated in outbreaks of gastroenteritis associated with undercooked poultry and poultry products such as eggs. It can also be carried by reptiles such as turtles and iguanas which are kept as pets and spread to foods by improper sanitary practices.

Serratia marcescens (1000X).  This bacillus is an opportunistic hman pathogen associated with lower respiratory tract infections.  Its most characteristic feature is the phenotypic expression some strains have to produce the red pigment prodigiosin when grown at 25-30o C, but not at 37o C (human body temperature).

Proteus vulgaris (1000X).  This bacillus is an opportunistic pathogen associated with urinary tract infections.

Enterobacter aerogenes (1000X).  This rod is a common inhabitant of vegetable matter.  In the laboratory, it is often mistaken for E. coli owing to morphological similarity and its ability to ferment lactose.  However, this species can be separated from E. coli on the basis of the production of an alkaline intermediate by-product of glucose metabolism, acetyl methyl carbinol (acetoin) and the utilization of citrate as a carbon source through a group of tests called the IMViC (I ndole Methyl Red Voges-Proskauer Citrate) series.

Rhodospirillum rubrum (1000X).  This large (0.7-1.0 mm), nonpathogenic gram negative spirillum exhibits polar amphitrichous flagellation and a characteristic red pigment.

Neisseria sicca (1000X).  This diplococcus is an inhabitant of the mucus membranes of mammals.  It occurs most frequently in pairs or short chains, and has a distinct bean-shaped appearence.  It is related to N. gonorrheoea, the pathogen of the sexually transmitted venereal disease gonorrhoea, and N. meningitidis, the pathogen of bacterial meningitis.

Pseudomonas aeruginosa(1000X). This rod is an inhabitant of soil and plant matter, but is an opportunistic pathogen of wounds.