4.2 Proteobacteria  (Page 6/11)

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 gram-negative 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 ” ( [link] ), structures that are still being studied by biologists and bacterial ecologists. H. Reichenbach. “Myxobacteria, Producers of Novel Bioactive Substances.” Journal of Industrial Microbiology&Biotechnology 27 no. 3 (2001):149–156. These bacteria can also form metabolically inactive myxospores .

[link] summarizes the characteristics of several important genera of Deltaproteobacteria.

Epsilonproteobacteria

The smallest class of Proteobacteria is Epsilonproteobacteria , which are gram-negative microaerophilic bacteria (meaning they only require small amounts of oxygen in their environment). Two clinically relevant genera of Epsilonproteobacteria are Campylobacter and Helicobacter , both of which include human pathogens. Campylobacter can cause food poisoning that manifests as severe enteritis (inflammation in the small intestine). This condition, caused by the species C. jejuni , is rather common in developed countries, usually because of eating contaminated poultry products. Chickens often harbor C. jejuni in their gastrointestinal tract and feces, and their meat can become contaminated during processing.

Within the genus Helicobacter, the helical, flagellated bacterium H. pylori has been identified as a beneficial member of the stomach microbiota, but it is also the most common cause of chronic gastritis and ulcers of the stomach and duodenum ( [link] ). Studies have also shown that H. pylori is linked to stomach cancer. S. Suerbaum, P. Michetti. “ Helicobacter pylori infection.” New England Journal of Medicine 347 no. 15 (2002):1175–1186. H. pylori is somewhat unusual in its ability to survive in the highly acidic environment of the stomach. It produces urease and other enzymes that modify its environment to make it less acidic.

[link] summarizes the characteristics of the most clinically relevant genera of Epsilonproteobacteria.

Key concepts and summary

• Proteobacteria is a phylum of gram-negative bacteria discovered by Carl Woese in the 1980s based on nucleotide sequence homology.
• Proteobacteria are further classified into the classes alpha-, beta-, gamma-, delta- and epsilonproteobacteria, each class having separate orders, families, genera, and species.
• Alphaproteobacteria are oligotrophs . The taxa chlamydias and rickettsias are obligate intracellular pathogens, feeding on cells of host organisms; they are metabolically inactive outside of the host cell. Some Alphaproteobacteria can convert atmospheric nitrogen to nitrites, making nitrogen usable by other forms of life.
• Betaproteobacteria are eutrophs . They include human pathogens of the genus Neisseria and the species Bordetella pertussis .
• Gammaproteobacteria are the largest and the most diverse group of Proteobacteria. Many are human pathogens that are aerobes or facultative anaerobes. Some Gammaproteobacteria are enteric bacteria that may be coliform or noncoliform. Escherichia coli , a member of Gammaproteobacteria, is perhaps the most studied bacterium.
• Deltaproteobacteria make up a small group able to reduce sulfate or elemental sulfur. Some are scavengers and form myxospores, with multicellular fruiting bodies.
• Epsilonproteobacteria make up the smallest group of Proteobacteria. The genera Campylobacter and Helicobacter are human pathogens.

Fill in the blank

Short answer