Bacteroides

 Source - Sheila Patrick, in Molecular Medical Microbiology.

Bacteroides is the predominant genus within the lower human intestinal tract, as evidenced by its prevalence in the product of this open-ended culture system, faeces. Within the intestinal tract Bacteroides spp. Host molecular interaction can influence host function, for example in relation to immune system development. Given the opportunity to escape from the intestinal tract into the normally uncolonized peritoneal cavity as a result of, for example, either trauma or surgery, selected Bacteroides species, in particular B. fragilis, can cause life-threatening infection including bacteraemia.

Although aerotolerance is evident in some Bacteroides, in the medical diagnostic setting they are effectively obligate anaerobes. Bacteroides exhibit an unprecedented level of surface component diversity, both within and between strains, exemplified by B. fragilis surface polysaccharides. Extensive multiple DNA inversion events, conjugative and mobilizable transposons and multiple extracytoplasmic function (ECF) sigma factors contribute to this diversity and the adaptability of Bacteroides. Metronidazole has been an effective antibiotic for both treatment and prophylaxis, but the potential global spread of multidrug resistance is a major cause for concern.

The Definition of Bacteroides as a Genus and a Phylogenetic Group-

Bacteroides species were the first Gram-negative anaerobes to be viewed as potentially serious pathogens. Accori-dngly, more is known about their traits and ability to cause disease than about the other Gram-negative anaerobes. In the premolecular era, the definition of Bacteroides as a genus was problematic. Bacteroides were defined are Gram-negative obligate anaerobes that were nonmotile and did not produce spores. Bacteroides spp. Used carbohydrates as energy sources and produced acetate, propionate, and succinate. This was a rather vague definition.

Even the end products that defined Bacteroides spp. Were not very informative in the sense that they were produced by many anaerobic bacteria. Many of the oral anaerobes such as Porphyromonas spp. And Prevotella species were originally classified as Bacteroides spp. Differences between the oral species and the colonic Bacteroides spp. Had already been noted. Most evident was the black pigmentation of the oral strains. Also, they were much less aerotolerant. A careful phylogenetic analysis based on 16S rRNA genes has finally sorted out these different genera.

A result of the early phylogenetic analysis that was surprising at the time was the realization that Bacteroides and related genera were members of a completely separated phylogenetic group from the more familiar enterics and other proteobacteria. For a long time, the big phylogenetic divide was thought to be the divide between the Gram-negative and Gram-positive bacteria. After all, these two groups had very different cell wall structures. To find that within the Gram-negative bacteria, there were even bigger phylogenetic divides was unexpected. Now, of course, it is well known that there are numerous phyla of Gram-negative bacteria. The Gram-positive bacteria are unusual in that they seem to cluster in a single phylum.

A number of genome sequences of members of the Bacteroidetes are beginning to appear. These genome sequences confirm the original 16S rRNA conclusions about the relatedness of the various genera in this group. A notable feature of the genomes of such numerically major species as Bacteroides thetaiotaomicron is the presence of numerous glycosidases and polysaccharidases, a finding consistent with the well-known ability of many Bacteroides species to use oligo- and polysaccharides as sources of carbon and energy.