Bacteroides: the Good, the Bad, and the Nitty-Gritty – PDF

By Hannah M. Wexler

By a variety of measures, the species Homo sapiens is more microbial than human. Microorganisms comprise only a small, albeit significant, percentage of the body weight (between 2 and 5 pounds of live bacteria). However, in terms of cell numbers, we are about 10% human and 90% bacterial! Further, the number of genes in our microbiome may exceed the number of human genes by two orders of magnitude, making us genetically 1% human and 99% bacterial! Consequently, bacteria play a major role in bodily functions, including immunity, digestion, and protection against disease. Colonization of the human body by microorganisms occurs at the very beginning of human life, and many of these organisms become truly indigenous to the host.

The human colon has the largest population of bacteria in the body (in excess of 1011 organisms per gram of wet weight), and the majority of these organisms are anaerobes; of these, 25% are species of Bacteroides, the bacterial genus that is focus of this review. This review will summarize the current state of knowledge about Bacteroides species, the most predominant anaerobes in the gut. The aspects of these organisms that will be covered will include their role as commensal organisms (The Good); their involvement in human disease (The Bad); and information about their physiology, metabolism, and resistance mechanisms as well as a brief overview of clinical characteristics (The Nitty-Gritty).

Bacteroidetes is one of the major lineages of bacteria and arose early during the evolutionary process. Bacteroides species are anaerobic, bile-resistant, non-spore-forming, gram-negative rods. The taxonomy of Bacteroides has undergone major revisions in the last few decades (see “Taxonomy” below), but the genus is now limited to species within the Bacteroides fragilis group, which now number 20. Names of species within the Bacteroides or Parabacteroides group to date are listed in Table 1. Many of these species were isolated as single strains from human feces. The percentages of anaerobic infections that involve particular species of Bacteroides are indicated in Fig. 1 and were calculated from the Wadsworth Anaerobe Collection database, including more than 3,000 clinical specimens from which a Bacteroides species was isolated. The proportions of the most important species for the most common sites of isolation are indicated in Table 2. The numbers of B. fragilis isolates are 10- to 100-fold lower than those of other intestinal Bacteroides species, yet B. fragilis is the most frequent isolate from clinical specimens and is regarded as the most virulent Bacteroides species.

Bacteroides may be passed from mother to child during vaginal birth and thus become part of the human flora in the earliest stages of life. The bacteria maintain a complex and generally beneficial relationship with the host when retained in the gut, and their role as commensals has been extensively reviewed. A quote in a recent publication captured this attribute: “. . .with B. fragilis, as with real estate, it’s location, location, location”. When the Bacteroides organisms escape the gut, usually resulting from rupture of the genes. Third, both species exhibit multiple paralogous groups of genes, i.e., genes that seem to have derived from a common ancestral gene and have since diverged from the parent copy by mutation and selection or drift. The reasons for this seemingly inefficient use of genetic space are not completely clear, but it would seem that Bacteroides species are genetic “pack rats” that prefer to have all possibly needed versions of relevant proteins at hand and therefore will not need to rely on unpredictable mutations.

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Originally published in the American Society for Microbiology

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