What is microbiome?
Microorganisms often live in the form of a community. Furthermore, they can live in close association with complex organisms, such as plants and humans, by establishing commensal, ammensal, mutualistic, parasitic and/or pathogenic relationships with their hosts. The collection of such microorganisms is called microbiome or microbiota. Microflora has also been used but flora represents the kingdom Plantae therefore it is a misnomer.
In the original version, microbiome referred to the collection of microbes and their genomic contents. Microbiota indicated the microbial community in their host. But “microbiome” has frequently been used interchangeably with microbiota [
1]. In this review, we focused mainly on bacterial microbiome with reference to either the collection of bacteria or their genomes, unless otherwise specified.
Microbiome can be found throughout the human body, ranging from the skin to the gut, and to previously considered as sterile environments such as the blood in circulation [
2]. Various reports indicated that over 10,000 microbial species have been shown to occupy various parts of the human body [
3,
4]. While diversity of microbes in the skin and vaginal sites are relatively low, great diversity can be found in other sites, e.g. the gut [
6]. Consequently, impact of microbiome in human diseases and vice versa can be extensive. For example, chronic lung diseases can alter composition of lung microbiome which can subsequently influence host defense and immunity, thus leading to further exacerbation of the diseases [
5]. Infection status has also been found to influence microbiome in the blood or the lung [
6‐
9].
What is the gut microbiome?
The gut microbiome is the genetic material of all the microbes, e.g. bacteria, fungi, protozoa and viruses which live on and inside the digestive tracts of humans and other animals, including insects. In this review, we focused on the human gut microbiome and on bacterial composition.
The human gut microbiome has co-evolved with its host for millennia and, therefore, has been extensively involved with a variety of essential activities in the host, e.g. digestion and nutrition [
10,
11], detoxification and body defense [
12], maturation of the host immune system [
11] and disease mediation [
13‐
17]. Consequently, a large number of microbes with high diversity can be found in the mammalian gut, with most of them being
Firmicutes and
Bacteroidetes [
18]. Such observation has been confirmed in different populations: Europeans and Americans [
19], Koreans [
20], Africans [
21,
22], Danish but not Chinese [
23]. The diversity can have specific implications for disease in different populations. For example, European and Chinese citizens with type 2 diabetes had different gut microbiome compositions [
24], with the Chinese having more diverse species [
24]. However, the reason of the major difference between the two populations, e.g. as related to age, environmental and genetic factors needs further investigation [
25].
With diverse microorganisms, the gut microbiome contains millions of different genes [
19]. Some of them may be acquired from environmental bacteria [
10], indicating their metabolic diversity and versatility. Accordingly, three major genera have been reported as enterotypes:
Bacteroides,
Prevotella and
Ruminococcus in the human gut as observed from 22 Europeans, 13 Japanese and 4 Americans [
26]. Interestingly, similar bacterial ecosystems were also identified in mice and chimpanzees [
27‐
29]. Their content in the human gut has been reported to be mainly influenced by their evolving change in the host and much less by age, gender, body weight, or race [
26,
30]. However, a recent study reported that diet had more influence on metabolome than microbiome. In another context, some studies reported that
Ruminococcus was a major ecotype [
30‐
32], including one which analyzed data from native populations from different countries [
33]. In particular,
Enterobacteriaceae belonged to the third major ecotype among Taiwanese [
34]. However, these discrepancies need to be clarified with more attention to sample size, and sampling methods and variations.
There are two major categories of microbes in the gut microbiota: (1) autochthonous microbes that seem to reside on the epithelium of colonic mucosa, and (2) allochthonous microbes that transiently pass the lumen as part of the digesta [
35]. The functional roles of these “residents” and “passengers” are believed to be very different. Indeed, the ratio of autochthonous to non-autochthonous microbes has been proven useful to assess cirrhosis progression [
36].
In general, host diet and phylogeny contribute to modifying the composition of gut microbial community in mammals and other species [
18,
37,
38]. Indeed, genome-scale metabolic modeling show that variations in the diet of the host significantly modified the composition of the three representative human gut bacteria (
B. thetaiotaomicron,
E. rectale and
M. smithii) [
39]. For example, alcohol is a common dietary modulator of intestinal microbiota, as shown in experimental animals and humans [
40‐
44]. In return, different composition of the three representative human gut bacteria influenced host metabolism and related diseases.
There are many reports which indicate that host genetics played an important role in determining the composition of microbiome [
15,
45‐
57]. For example, several susceptibility loci were shared by inflammatory bowel disease [
16,
17,
52], with infectious mycobacterial and staphylococcal organisms. These associations were validated from studies using the Gene Co-expression Network Analysis [
58]. Therefore, investigations on the relationships among susceptibility, microbiome composition and disease development can provide valuable evidence to develop disease prevention protocols.
In the gut, a typical microbial product is lipopolysaccharide (LPS) which are produced by Gram-negative bacteria [
6,
59‐
62] and are transported with chylomicrons [
63]. LPS has been shown to be strong stimulators of innate immunity in organisms from lower- to higher-order animals [
64]. For peritoneal dialysis patients, LPS level is used as an important indicator for survival. Indeed, a retrospective study of 300 patients show that plasma bacterial DNA levels were positively correlated with serum C-reactive proteins and LPS levels, and negatively correlated with survival rates [
7]. These results indicate that both plasma LPS and bacterial DNA levels can be used as indicators for systemic inflammation and for prognosis. Another important microbial product is Trimethylamine (TMA). The oxidation product of TMA by hepatic flavine monoxygenases, trimethylamine
N-oxide (TMAO), has influence on morbidity of patients [
65]. These observations indicate that localized microbiome can cause far-reaching consequences.