MicroBiology-Draft/Sample

both). The noncoliforms include some notable human pathogens, such asSalmonellaspp., Shigellaspp.,andYersinia
pestis.

E. colihas been perhaps the most studied bacterium since it was first described in 1886 by Theodor Escherich
(1857–1911). Many strains ofE. coliare 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). OtherE. colistrains may cause
traveler’s diarrhea, a less severe but very widespread disease.

The genusSalmonella, which belongs to the noncoliform group ofEnterobacteriaceae, 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 asserotypes(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 ofSalmonellacan cause salmonellosis, characterized by inflammation
of the small and the large intestine, accompanied by fever, vomiting, and diarrhea. The speciesS. enterobacterica
(serovartyphi) causes typhoid fever, with symptoms including fever, abdominal pain, and skin rashes (Figure 4.10).

Figure 4.10 Salmonella typhiis the causative agent of typhoid fever.

Table 4.4summarizes 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 serotypeE. coliO157:H7, are pathogens;E. colihas

been a model organism for many studies in genetics and molecular

biology

Table 4.4

Chapter 4 | Prokaryotic Diversity 153