"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. 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 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. Only the sessile cells are able to reproduce. Table 2.12 summarizes the characteristics of some of the most clinically relevant genera of nonproteobacteria: Chlamydia are gram-negative, coccoid or ovoid bacteria and are obligate intracellular bacteria; some cause chlamydia, trachoma, and pneumonia. Bacteroides are gram-negative bacilli that are obligate anaerobes, abundant in the human gastrointestinal tract, and usually mutualistic, although some species are opportunistic pathogens. Cytophaga are gram-negative bacilli that move by gliding, live in soil or water, decompose cellulose, and may cause disease in fish. Fusobacterium are gram-negative bacilli with pointed ends; they are anaerobic, form biofilms, and some species cause disease in humans, including periodontitis and ulcers. Leptospira are very thin, spiral-shaped, gram-negative-like bacteria that are better viewed by darkfield microscopy. They are aerobic, abundant in shallow-water reservoirs, infect rodents and domestic animals, can be transmitted to humans by infected animals’ urine, and may cause severe disease. Borrelia are very thin, gram-negative-like spirochetes that are better viewed by darkfield microscopy; B. burgdorferi causes Lyme disease and B. recurrens causes relapsing fever. Treponema are very thin, gram-negative-like spirochetes that are better viewed by darkfield microscopy. They are motile, do not grow in culture, and T. pallidum, subspecies T. pallidum pallidum, causes syphilis."