Proteobacteria: A Common Factor in Human Diseases

Received Jul 5, 2017; Accepted Oct 16, 2017

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1. Introduction

The gut is the most colonized human organ with up to 100 trillion microbes, about 10 times the number of the human cells [1]. At this level more than 50 phyla have been described with, however, the predominance of only 4 major phyla: Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria [2].

Notably, gastrointestinal tract (GIT) is also colonized by fungi and virus, which constitute, respectively, the gut mycobiome and the gut virome [3–5].

Metagenomics allowed estimating the number of genes of the microbiota, the so-called microbiome, with a number exceeding by more than 150 times the human genome (about 3.3 million in comparison to about 20.000 genes in humans) [6], thus representing a real second genome for the host.

The number of microbial cells displays a positive rostrocaudal gradient along all the GIT: from about 10–10³ microbes per gram in stomach and duodenum, 10⁴–10⁷ microbes per gram in jejunum and ileum, to 10¹¹-10¹² microbes per gram in the colon [7, 8].

Furthermore, microbiota composition also varies in the different GI tracts: anaerobes are predominant in the colon, in particular Bacteroidetes and Lachnospiraceae families which belong to the Firmicutes phylum [9]. On the other hand, facultative anaerobes are predominant in the small intestine [9].

Microorganisms colonization of the GIT begins at birth, with a dynamic microbiota that progressively stabilizes in the first years of life [10]. In adults, microbiota reaches higher complexity increasing in diversity [11]. Finally, in the elderly, microbiota composition displays reduced diversity with predominance of Proteobacteria and a decrease in Bifidobacterium [12].

Moreover, many factors influence the microbiota composition during lifetime, the most important being diet, delivery mode, feeding type, drugs use, especially antibiotics, and, as already mentioned above, age [13, 14].

Gut microbiota performs many important functions in the host,...