Commentary: Remembrance of microbes past

Remarkably, NCS mice were tolerant to parenteral doses of endotoxin that were lethal within a few days of administration to SS mice. Exposure of NCS animals to SS mice during early life, or prior injections with small doses of heat-killed cells of gram-negative bacteria that thus exposed the animals to sublethal amounts of endotoxin, increased the susceptibility of NCS mice to match that of SS animals. Dubos and colleagues concluded that the enterobacteria, present in relatively high numbers in the gut of infant SS mice, sensitized the animals so that, as adults, they were highly susceptible to endotoxin. These observations led Dubos and colleagues to devote almost 20 years to the study of gut microbiota composition and gut microbial ecology. They established experimental methods and observations that, as Savage has appropriately stated, ‘should endure in history’. 2 Many clues to the influences of bacteria on the mammalian host have been obtained from comparisons of the biochemical and physiological characteristics of germ-free (raised in the absence of demonstrable microbes) and conventional (colonized by microbes) animals. 3 The impact of even a single bacterial species on the host animal can also be ascertained using this approach. Gnotobiotic (‘defined biota’) work like this can now be done at a sophisticated level because of the availability of advanced imaging technology, as well as genome sequences of experimental animal species and the consequent preparation of DNA microarrays that can be used to measure gene expression. The potential for obtaining exciting knowledge of the mechanistic influences of gut microbiota on a host using this approach has been demonstrated by the work of Gordon and colleagues, who have studied the impact of colonization of formerly germ-free mice by the bacterial species Bacteroides thetaiotaomicron. 4 Perhaps most striking have been their observations concerning angiogenesis in the murine gut. Quantitative three-dimensional imaging studies showed that a plexus of branched and interconnected blood vessels developed postnatally in small bowel villi of conventional mice. Angiogenesis coincided with the establishment of the gut microbiota. Vascular development was arrested in germ-free mice, but could be restarted by colonization of the gut by a conventional gut microbiota or by B. thetaiotaomicron, which had been shown in other experiments to up-regulate expression of the murine angiogenin-3 gene in the ileal mucosa. Other observations showed that interaction between the gut microbiota and Paneth cells was essential for the regulation of angiogenesis. 4,5