|Bacteroides spp. anaerobically cultured in blood agar medium|
Castellani & Chalmers 1919
Bacteroides is a genus of Gram-negative, obligate anaerobic bacteria. Bacteroides species are nonendospore-forming bacilli, and may be either motile or nonmotile, depending on the species. The DNA base composition is 40–48% GC. Unusual in bacterial organisms, Bacteroides membranes contain sphingolipids. They also contain meso-diaminopimelic acid in their peptidoglycan layer.
Bacteroides species are normally mutualistic, making up the most substantial portion of the mammalian gastrointestinal flora, where they play a fundamental role in processing of complex molecules to simpler ones in the host intestine. As many as 1010–1011 cells per gram of human feces have been reported. They can use simple sugars when available; however, the main sources of energy for Bacteroides species in the gut are complex host-derived and plant glycans. Studies indicate that long-term diet is strongly associated with the gut microbiome composition—those who eat plenty of protein and animal fats have predominantly Bacteroides bacteria, while for those who consume more carbohydrates the Prevotella species dominate.
One of the most important clinically is Bacteroides fragilis.
Bacteroides species also benefit their host by excluding potential pathogens from colonizing the gut. Some species (B. fragilis, for example) are opportunistic human pathogens, causing infections of the peritoneal cavity, gastrointestinal surgery, and appendicitis via abscess formation, inhibiting phagocytosis, and inactivating beta-lactam antibiotics. Although Bacteroides species are anaerobic, they are transiently aerotolerant and thus can survive in the abdominal cavity.
In general, Bacteroides are resistant to a wide variety of antibiotics—β-lactams, aminoglycosides, and recently many species have acquired resistance to erythromycin and tetracycline. This high level of antibiotic resistance has prompted concerns that Bacteroides species may become a reservoir for resistance in other, more highly pathogenic bacterial strains.
An alternative fecal indicator organism, Bacteroides, has been suggested because they make up a significant portion of the fecal bacterial population, have a high degree of host specificity that reflects differences in the digestive system of the host animal, and have a small potential to grow in the environment. Over the past decade, real-time polymerase chain reaction (PCR) methods have been used to detect the presence of various microbial pathogens through the amplification of specific DNA sequences without culturing bacteria. One study has measured the amount of Bacteroides by using qPCR to quantify the host-specific 16S rRNA genetic marker. This technique allows quantification of genetic markers that are specific to the host of the bacteria and allow detection of recent contamination. A recent report found temperature plays a major role in the amount of time the bacteria will persist in the environment, the life span increases with colder temperatures (0–4 °C).
There are data suggesting that members of Bacteroides affects the lean or obese phenotype in humans. In this article, one human twin is obese while the other is lean. Their fecal microbiota is transplanted into germ-free mouse and, interestingly, the phenotype in mouse-model corresponds to that in human.
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