Cytophaga are motile aquatic bacteria that glide. Fusobacteria inhabit the human mouth and may cause severe infectious diseases. The largest genus of the CFB group is Bacteroides , which includes dozens of species that are prevalent inhabitants of the human large intestine, making up about 30% of the entire gut microbiome ( [link] ). One gram of human feces contains up to 100 billion Bacteroides cells. Most Bacteroides are mutualistic. They benefit from nutrients they find in the gut, and humans benefit from their ability to prevent pathogens from colonizing the large intestine. Indeed, when populations of Bacteroides are reduced in the gut—as often occurs when a patient takes antibiotics—the gut becomes a more favorable environment for pathogenic bacteria and fungi, which can cause secondary infections.
Only a few species of Bacteroides are pathogenic. B. melaninogenicus , for example, can cause wound infections in patients with weakened immune systems.
- Why are Cytophaga , Fusobacterium , and Bacteroides classified together as the CFB group?
Planctomycetes
The Planctomycetes are found in aquatic environments, inhabiting freshwater, saltwater, and brackish water. Planctomycetes are unusual in that they reproduce by budding, meaning that instead of one maternal cell splitting into two equal daughter cells in the process of binary fission, the mother cell forms a bud that detaches from the mother cell and lives as an independent cell. These so-called swarmer cells are motile and not attached to a surface. However, they will soon differentiate into sessile (immobile) cells with an appendage called a holdfast that allows them to attach to surfaces in the water ( [link] ). Only the sessile cells are able to reproduce.
[link] summarizes the characteristics of some of the most clinically relevant genera of nonproteobacteria.
| Nonproteobacteria | ||
|---|---|---|
| Example Genus | Microscopic Morphology | Unique Characteristics |
| Bacteroides | Gram-negative bacillus | Obligate anaerobic bacteria; abundant in the human gastrointestinal tract; usually mutualistic, although some species are opportunistic pathogens |
| Cytophaga | Gram-negative bacillus | Motile by gliding; live in soil or water; decompose cellulose; may cause disease in fish |
| Fusobacterium | Gram-negative bacillus with pointed ends | Anaerobic; form; biofilms; some species cause disease in humans (periodontitis, ulcers) |
| Leptospira | Spiral-shaped bacterium (spirochetes); gram negative-like (better viewed by darkfield microscopy); very thin | Aerobic, abundant in shallow water reservoirs; infect rodents and domestic animals; can be transmitted to humans by infected animals’ urine; may cause severe disease |
| Sphingobacterium | Gram-negative bacillus | Oxidase positive; nonmotile; contain high amounts of sphingophospholipids; rarely cause disease in humans |
| Treponema | Gram-negative-like spirochete; very thin; better viewed by darkfield microscopy | Motile; do not grow in culture; T. pallidum (subspecies T. pallidum pallidum ) causes syphilis |