Bacteroides species are normally
mutualistic, making up the most substantial portion of the mammalian
gastrointestinal microbiota,[4] where they play a fundamental role in processing of complex molecules to simpler ones in the host intestine.[5][6][7] As many as 1010–1011 cells per gram of human feces have been reported.[8] 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.[9] 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.[10]
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.[14] Although Bacteroides species are anaerobic, they are transiently
aerotolerant[15] 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.[16][17] It has been often considered susceptible to
clindamycin,[18] but recent evidence demonstrated an increasing trend in clindamycin resistance rates (up to 33%).[19]
In cases where Bacteroides can move outside the gut due to gastrointestinal tract rupture or intestinal surgery, Bacteroides can infect several parts of the human body. Bacteroides can enter the
central nervous system by penetrating the
blood brain barrier through the
olfactory and
trigeminal cranial nerves and can cause
meningitis and brain abscesses.[20]Bacteroides has also been isolated from abscesses in the neck and lungs. Some Bacteroides species are associated with
Crohn's disease,
appendicitis and
inflammatory bowel disease. Bacteroides species play multiple roles within the human gut microbiome.[5]
Microbiological applications
An alternative fecal indicator organism, Bacteroides, has been suggested because they make up a significant portion of the fecal bacterial population,[3] have a high degree of host specificity that reflects differences in the digestive system of the host animal[21] 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.[22] This technique allows quantification of genetic markers that are specific to the host of the bacteria Bacteroides 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).[23]
"A new study has found that there is a three-way relationship between a type of gut bacteria, cortisol, and brain metabolites. This relationship, the researchers hypothesize, may potentially lead to further insight into autism, but more in-depth studies are needed."[24]
Another study showed a 5.6-times higher risk of osteoporosis fractures in the low Bacteroides group of Japanese postmenopausal women.[25]
Human
Members of the
Bacillota and
Bacteroidota phyla make up a majority of the bacterial species in the human intestinal microbiota (the "gut microbiome"). The healthy human gut microbiome consists of 109 abundant species of which 31 (19.7%) are members of the Bacteroidetes while 63 (40%) and 32 (20%) belong to
Bacillota and
Actinomycetota.[26]
Bacteroides species' main source of energy is fermentation of a wide range of sugar derivatives from plant material. These compounds are common in the human colon and are potentially toxic. Bacteroides such as Bacteroides thetaiotaomicron[5] converts these sugars to fermentation products which are beneficial to humans. Bacteroides also have the ability to remove side chains from bile acids, thus returning bile acids to the hepatic circulation.[27]
There is data suggesting that members of Bacteroides affect the lean or obese phenotype in humans.[28] In this article, one human twin is obese while the other is lean. When their fecal microbiota is transplanted into germ-free mice, the phenotype in the mouse model corresponds to that in humans.[citation needed]
Bacteroides are symbiont colonizers of their host intestinal niche and serve several physiological functions, some of which can be beneficial while others are detrimental. Bacteroides participate in the regulation of the intestinal micro-environment and
carbohydrate metabolism with the capacity to adapt to the host environment by hydrolyzing
bile salts.[29] Some Bacteroides produce
acetate and
propionate during sugar fermentation. Acetate can prevent the transport of toxins from the gut to the blood while propionate can prevent the formation of tumors in the human colon.[30]
Bacteroides such as Bacteroides uniformis may play a role in alleviating
obesity. Low abundance of B. uniformis found in the intestine of formula-fed infants were associated with a high risk of obesity.[31] Administering B. uniformis orally may alleviate metabolic and immune dysfunction which may contribute to obesity in mice. Similarly, Bacteroides acidifaciens may assist the activating fat oxidation in
adipose tissue and thus could protect against obesity.[32][30]
^Finegold SM, Sutter VL, Mathisen GE (1983). Normal indigenous intestinal flora (pp. 3-31) in Human intestinal microflora in health and disease. Academic Press.
ISBN978-0-12-341280-5.
^Appleman MD, Heseltine PN, Cherubin CE (Jan 1990). "Epidemiology, antimicrobial susceptibility, pathogenicity, and significance of Bacteroides fragilis group organisms isolated at Los Angeles County-University of Southern California Medical Center". Reviews of Infectious Diseases. 13 (1): 12–18.
doi:
10.1093/clinids/13.1.12.
PMID2017610.
^Ryan KJ, Ray CG, eds. (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill.
ISBN978-0-8385-8529-0.
^Baughn A, Malamy M (2004). "Molecular Basis for Aerotolerance of the Obligately Anaerobic Bacteroides Spp.". In Nakano M, Zuber P (eds.). Strict and Facultative Anaerobes: Medical and Environmental Aspects. CRC Press. p. 161.
ISBN978-1-904933-03-8.
^Salyers AA, Gupta A, Wang Y (September 2004). "Human intestinal bacteria as reservoirs for antibiotic resistance genes". Trends in Microbiology. 12 (9): 412–416.
doi:
10.1016/j.tim.2004.07.004.
PMID15337162.
^Löfmark S, Jernberg C, Jansson JK, Edlund C (December 2006). "Clindamycin-induced enrichment and long-term persistence of resistant Bacteroides spp. and resistance genes". The Journal of Antimicrobial Chemotherapy. 58 (6): 1160–1167.
doi:
10.1093/jac/dkl420.
PMID17046967.
^
abWang C, Zhao J, Zhang H, Lee YK, Zhai Q, Chen W (2021-11-30). "Roles of intestinal bacteroides in human health and diseases". Critical Reviews in Food Science and Nutrition. 61 (21): 3518–3536.
doi:
10.1080/10408398.2020.1802695.
PMID32757948.
S2CID221036664.