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E. coli has been perhaps the most studied bacterium since it was first described in 1886 by Theodor Escherich (1857–1911). Many strains of E. coli are in mutualistic relationships with humans. However, some strains produce a potentially deadly toxin called Shiga toxin , which perforates cellular membranes in the large intestine, causing bloody diarrhea and peritonitis (inflammation of the inner linings of the abdominal cavity). Other E. coli strains may cause traveler’s diarrhea , a less severe but very widespread disease.
The genus Salmonella , which belongs to the noncoliform group of Enterobacteriaceae , is interesting in that there is still no consensus about how many species it includes. Scientists have reclassified many of the groups they once thought to be species as serotypes (also called serovars ), which are strains or variations of the same species of bacteria. Their classification is based on patterns of reactivity by animal antisera against molecules on the surface of the bacterial cells. A number of serotypes of Salmonella can cause salmonellosis , characterized by inflammation of the small and the large intestine, accompanied by fever, vomiting, and diarrhea. The species S. enterobacterica (serovar typhi ) causes typhoid fever , with symptoms including fever, abdominal pain, and skin rashes ( [link] ).
[link] summarizes the characteristics of important genera of Gammaproteobacteria.
| Class Gammaproteobacteria | ||
|---|---|---|
| Example Genus | Microscopic Morphology | Unique Characteristics |
| Beggiatoa | Gram-negative bacteria; disc-shaped or cylindrical | Aquatic, live in water with high content of hydrogen disulfide; can cause problems for sewage treatment |
| Enterobacter | Gram-negative bacillus | Facultative anaerobe; cause urinary and respiratory tract infections in hospitalized patients; implicated in the pathogenesis of obesity |
| Erwinia | Gram-negative bacillus | Plant pathogen causing leaf spots and discoloration; may digest cellulose; prefer relatively low temperatures (25–30 °C) |
| Escherichia | Gram-negative bacillus | Facultative anaerobe; inhabit the gastrointestinal tract of warm-blooded animals; some strains are mutualists, producing vitamin K; others, like serotype E. coli O157:H7, are pathogens; E. coli has been a model organism for many studies in genetics and molecular biology |
| Hemophilus | Gram-negative bacillus | Pleomorphic, may appear as coccobacillus, aerobe, or facultative anaerobe; grow on blood agar; pathogenic species can cause respiratory infections, chancroid, and other diseases |
| Klebsiella | Gram-negative bacillus; appears rounder and thicker than other members of Enterobacteriaceae | Facultative anaerobe, encapsulated, nonmotile; pathogenic species may cause pneumonia, especially in people with alcoholism |
| Legionella | Gram-negative bacillus | Fastidious, grow on charcoal-buffered yeast extract; L. pneumophila causes Legionnaires disease |
| Methylomonas | Gram-negative bacillus | Use methane as source of carbon and energy |
| Proteus | Gram-negative bacillus (pleomorphic) | Common inhabitants of the human gastrointestinal tract; motile; produce urease; opportunistic pathogens; may cause urinary tract infections and sepsis |
| Pseudomonas | Gram-negative bacillus | Aerobic; versatile; produce yellow and blue pigments, making them appear green in culture; opportunistic, antibiotic-resistant pathogens may cause wound infections, hospital-acquired infections, and secondary infections in patients with cystic fibrosis |
| Serratia | Gram-negative bacillus | Motile; may produce red pigment; opportunistic pathogens responsible for a large number of hospital-acquired infections |
| Shigella | Gram-negative bacillus | Nonmotile; dangerously pathogenic; produce Shiga toxin, which can destroy cells of the gastrointestinal tract; can cause dysentery |
| Vibrio | Gram-negative, comma- or curved rod-shaped bacteria | Inhabit seawater; flagellated, motile; may produce toxin that causes hypersecretion of water and electrolytes in the gastrointestinal tract; some species may cause serious wound infections |
| Yersinia | Gram-negative bacillus | Carried by rodents; human pathogens; Y. pestis causes bubonic plague and pneumonic plague; Y. enterocolitica can be a pathogen causing diarrhea in humans |
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