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 (Indole 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.