infections and chancroid. Vibrio cholerae cause cholera. Legionella is a respiratory pathogen of this classification. Other species and genuses in the category of gamma-proteobacteria include Enterobacter, which thrives in the mammalian intestinal tract. There are two subcategories. One is Escherichia coli, which is a coliform bacterial organism, and noncoliforms, which include Shigella, Salmonella, and Yersinia pestis. E. coli is generally mutualistic but it can be pathogenic because of strains that produce certain dangerous toxins. One of these is the Shiga toxin, which leads to serious disease in humans. The delta-proteobacteria is a small classification of organisms that use sulfate as part of their electron transport chain. Very few of these are pathogenic to humans. The subclassification includes myxobacteria, which is a soil organism that scavenges on inorganic compounds. They have the capacity to create metabolically inactive spores called myxospores, which look like fruiting bodies and contain other organisms. The epsilon-proteobacteria are an extremely small group that are considered microaerophilic, meaning that they do not require much in the way of oxygen for their survival. The two bacteria most clinically important are Helicobacter and Campylobacter species. Campylobacter can cause food poisoning and Helicobacter causes chronic gastritis and stomach ulcers. Helicobacter pylori can survive in the acidic environment of the stomach and will make the stomach less acidic.
GRAM-NEGATIVE BACTERIA Most gram-negative bacteria are also proteobacteria. There are those, however, that do not belong to this category. One of these is the spirochetes, which are long, thin, and spiral. They are not easy to see, even after staining. For this reason, they are often observed through darkfield fluorescent technologies. They are extremely difficult to grow in cultures and move using an axial filament, which wraps around the cell itself, unlike a flagellum. Figure 20 shows what spirochetes look like under darkfield microscopy:
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