Bacteroides form a genus of obligate anaerobic, unflagellated – and thus mostly immobile – bacteria that are part of the natural bacterial flora in the human digestive tract and have important functions in certain metabolic processes. The proportion of gram-negative bacteria in the large intestine is particularly high. They utilize complex carbohydrates in a fermentative metabolism in which, for example, salts and esters of acetic acid are formed as the end product.
What are Bacteroides?
Bacteroides is the name given to a genus of gram-negative, pleomorphic, unflagellated bacteria that make up a large part of the natural flora of the digestive tract. They make up a particularly large proportion of the intestinal flora in the mucosa of the colon, where they dominate in numbers. They are rod-shaped gram-negative, mostly immobile, bacteria that can adapt their shape to the habitat in which they are found. Bacteria, which live exclusively anaerobically, perform important functions and tasks that benefit humans. They obtain their energy through fermentation. They are able to synthesize a series of enzymes that control the relevant fermentation processes by catalytic means. In particular, they help in the absorption and hydrolysis of otherwise indigestible polysaccharides and proteins. They make some of their metabolic capabilities available to the body’s metabolism via the secretion of certain enzymes. Only a few species of Bacteroides also occur facultatively as pathogenic germs. The composition of the intestinal flora has a great influence on the utilization of the consumed food. For example, the proportion of Bacteroides in the intestinal flora of severely overweight individuals is significantly lower than in normal-weight individuals.
Occurrence, distribution, and characteristics
Obligate anaerobic bacteria of the genus Bacteroides are only slightly pathogenic, and infection by externally introduced bacteria of this genus is extremely rare. When the immune system is intact, Bacteroides live quasi-symbiotically as a dominant component of the intestinal flora, especially as part of the bacterial association in the colon. Strikingly, many species of Bacteroides have branched fatty acid chains incorporated into their lipid membranes. In addition, some species are capable of synthesizing sphingolipids. This is a group of substances of special lipids that play a role in signal transduction in nervous tissues. Sphingolipids also play important roles in intercellular and intracellular communication. In rare cases – especially in the presence of disease-related or artificially induced immunosuppression – endogenously caused infections may occur, i.e., by Bacteroides that have not previously shown any pathogenicity as colonizers of mucous membranes.
Significance and function
One of the most important characteristics and functions of Bacteroides is not in their pathogenicity, but in their digestive support of humans. Some of the very large protein molecules and polysaccharides that cannot be broken down and thus absorbed in the small intestine because of a lack of enzymes, pass through the “fermentation section” in the large intestine and can usually be broken down and subsequently absorbed by enzymes of the bacteria. Minerals and trace elements are also isolated from the rest of the food pulp with the help of Bacteroides and made available to the body’s metabolism via absorption in the intestinal villi. The bacteria thus take on an important extension of the body’s digestive capabilities. Without the activity of the Bacteroides or the entire bacterial flora, we would not be able to survive in the long term. Interestingly, the intestinal flora in newborns and infants consists mainly of bifidobacteria, which are present in breast milk and perform an important protective function. Because complex polysaccharides and proteins are not present in the sole food source of milk, Bacteroides are not needed until the transition to other food components. It is therefore important to make the dietary change gradually to avoid digestive problems. The intestinal flora then has sufficient time to adapt accordingly.
Diseases and ailments
Bacteroides, which live strictly anaerobically, do not form spores. They can therefore hardly survive outside their habitat because atmospheric oxygen has a toxic effect on them.Infections involving Bacteroides are therefore mostly endogenous mixed infections in which facultative aerobic bacteria are responsible for the consumption of oxygen. This type of endogenous infection can occur when, in addition to a weakened immune system, there is, for example, a lesion of the mucous membranes that the germs can use as a portal of entry. In the rare cases in which endogenous infection with (facultatively) pathogenic Bacteroides occurs, it usually involves inflammation of the peritoneum (peritonitis) and abscesses on the liver and in the upper abdomen. In principle, the inflammations can originate from those mucous membranes that are colonized with Bacteroides, i.e. from the oral cavity, the intestine or the urogenital tract. If the rod-shaped bacteria reach deeper tissues through corresponding lesions, they find ideal conditions for their survival. This can lead to suppurating abscesses and, in addition, to tissue necrosis. Since the infection develops in the absence of air, the dead tissue may develop a very unpleasant odor. In very rare cases, when the necrotic tissue degradation products enter the bloodstream and the immune system is overwhelmed with the punctate load, an immediately life-threatening sepsis can develop, which – similar to allergic reactions – corresponds to an exuberant immune reaction. A test for Bacteroides can be performed by detecting species-specific organic acids or enzymes using gas chromatography. Diagnosis and detection of the bacterium via the establishment of a culture is also safe, but it must be borne in mind that the material containing Bacteroides must be kept strictly under exclusion of air, otherwise the pathogens will die.
