Microbiology, 2021

4.2 • Proteobacteria 147
Class Gammaproteobacteria
Example
Genus
Microscopic
Morphology
Unique Characteristics
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
Table 4.4
CHECK YOUR UNDERSTANDING
• List two families of Gammaproteobacteria.
Deltaproteobacteria
The Deltaproteobacteria is a small class of gram-negative Proteobacteria that includes sulfate-reducing
bacteria (SRBs), so named because they use sulfate as the final electron acceptor in the electron transport
chain. Few SRBs are pathogenic. However, the SRB Desulfovibrio orale is associated with periodontal disease
(disease of the gums).
Deltaproteobacteria also includes the genus Bdellovibrio, species of which are parasites of other gramnegative
bacteria. Bdellovibrio invades the cells of the host bacterium, positioning itself in the periplasm, the
space between the plasma membrane and the cell wall, feeding on the host’s proteins and polysaccharides.
The infection is lethal for the host cells.
Another type of Deltaproteobacteria, myxobacteria, lives in the soil, scavenging inorganic compounds. Motile
and highly social, they interact with other bacteria within and outside their own group. They can form
multicellular, macroscopic “fruiting bodies” (Figure 4.10), structures that are still being studied by biologists
and bacterial ecologists. 11 These bacteria can also form metabolically inactive myxospores.
11 H. Reichenbach. “Myxobacteria, Producers of Novel Bioactive Substances.” Journal of Industrial Microbiology & Biotechnology
27 no. 3 (2001):149–156.