The gut microbiome: scourge, sentinel or spectator?

Abstract

The gut microbiota consists of trillions of prokaryotes that reside in the intestinal mucosa. This long-established commensalism indicates that these microbes are an integral part of the eukaryotic host. Recent research findings have implicated the dynamics of microbial function in setting thresholds for many physiological parameters. Conversely, it has been convincingly argued that dysbiosis, representing microbial imbalance, may be an important underlying factor that contributes to a variety of diseases, inside and outside the gut. This review discusses the latest findings, including enterotype classification, changes brought on by dysbiosis, gut inflammation, and metabolic mediators in an attempt to underscore the importance of the gut microbiota for human health. A cautiously optimistic idea is taking hold, invoking the gut microbiota as a medium to track, target and treat a plethora of diseases.

Keywords: diseases; gut enterotypes; inflammation; metabolism; microbiome.

Fig. 1

A schematic representation of the lower intestinal tract showing common bacteria found in various parts of the GI tract and bacterial abundance in cfu/ml. Main intestinal functions and pH values found along the GI tract are also shown.(cfu – colony forming unit).

Fig. 2

The human gut flora has recently been classified as belonging to one of three different clusters termed enterotypes. This diagram postulates that properties of the microbiome unique to each enterotype (hatched-hexagon, hatched-triangle and hatched-rhombus) give rise to enterotype-specific effects. Each enterotype likely also harbors common essential functions that may then facilitate bacterial survival in the gut. Both enterotype-specific and common functions can collectively impact on host response.

Fig. 3

The gut microbiota can affect a variety of host responses. In this schematic diagram, host genotype, the environment, and diet can all impact on the microbiota and the microbiome. The microbiota exerts its effects on the intestine through bacterial signaling molecules and metabolic products (microbiome). Components of the microbiome can also enter the circulation, be transported to various organs, and potentiate multiple effects in these organs (gray arrow). At the same time, the gut–brain axis can circumvent intestinal absorption and may allow the microbiome to directly affect the brain.