Microbiology, 2021

4.2 • Proteobacteria 145
Figure 4.8
(a) Legionella pneumophila, the causative agent of Legionnaires disease, thrives in warm water. (b) Outbreaks of Legionnaires
disease often originate in the air conditioning units of large buildings when water in or near the system becomes contaminated with L.
pneumophila. (credit a: modification of work by Centers for Disease Control and Prevention)
Enterobacteriaceae is a large family of enteric (intestinal) bacteria belonging to the Gammaproteobacteria.
They are facultative anaerobes and are able to ferment carbohydrates. Within this family, microbiologists
recognize two distinct categories. The first category is called the coliforms, after its prototypical bacterium
species, Escherichia coli. Coliforms are able to ferment lactose completely (i.e., with the production of acid and
gas). The second category, noncoliforms, either cannot ferment lactose or can only ferment it incompletely
(producing either acid or gas, but not both). The noncoliforms include some notable human pathogens, such as
Salmonella spp., Shigella spp., and Yersinia pestis.
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. Shiga toxin is one of the most potent bacterial toxins
identified. Upon entering target cells, Shiga toxin interacts with ribosomes, stopping protein synthesis. Lack of
protein synthesis leads to cellular death and hemorrhagic colitis, characterized by inflammation of intestinal
tract and bloody diarrhea. In the most severe cases, patients can develop a deadly hemolytic uremic syndrome.
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 (Figure 4.9).
Figure 4.9
Salmonella typh is the causative agent of typhoid fever. (credit: Centers for Disease Control and Prevention)