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The majority of bacterial pathogens can be divided into seven groups: the spirochetes, the gram-positive rods, the mycobacteria, the gram-positive cocci, the gram-negative rods, the gram-negative cocci, and the mycoplasmas. The main differentiating factor between pathogens, and a factor that is used in diagnostic microbiology, is whether the bacterium is gram positive or gram negative.

The gram-positive bacteria contain a thick murein (or peptidoglycan) layer on their surfaces that functions as a cell wall. The murein is composed of cross-linked sugar chains that are connected via peptides, and is the direct target of many antibiotics such as penicillin. The murein sac encloses the bacterium and allows it to survive in environments of lesser osmolarity. The gram-positive cocci morphologically resemble strings of pearls, and include the streptococci and staphylococci. The gram-positive rods are abundant in the environment and infrequently cause disease. They include the diptheroids, Bacillis anthracis, the Clostridia, and Listeria monocytogenes.

A special kind of modefication of gram-postive bacteria are mycobacteria. Their cell wall is the acid-fast wall, in which bacterial incorporate large amounts of waxes with murein, polysaccharides and lipids to form a highly protective cell wall. Mycobacterium employ this method of protection.

The gram-negative bacteria have an outer membrane that encloses a thin murein cell wall. The outer membrane is a lipid bilayer, with its outer structure being composes of lipopolysaccharide, or LPS. The LPS has three components: a lipid A, which anchors LPS to the membrane, the core polysaccharide, which is composed of two characteristic sugars, and the O-antigen, which is a long hydrophilic carbohydrate chain. The O-antigen gives the bacteria a unique serological specificity. The LPS, when releases into the body as a result of bacterial lysis, acts as an endotoxin inducing shock and also induces the alternative complement pathway. The gram-negative rods include the enteric bacteria such as Escherichia coli, Salmonella, Shigella, Pseudomonas, Vibrio cholerae, Helicobacter pylori, and Campylobacter jejuni. The gram-negative cocci include Neisseria spp which can cause meningitis and gonorrohea.

The spirochetes are gram-negative and cork-screw shaped with endoflagellum that are used in motility to send the bacterium ‘spiraling’ through medium. Spirochetes include Treponema pallidum, which causes syphilis, and Borrelia burgdorferi, the agent of Lyme disease.

Mycoplasmas form a distinct group of bacteria as they lack any rigid wall. They are evolutionary distinct from other bacteria, and have unique nutritional requirements. They include Mycoplasma pneumoniae.

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