**1. Introduction**

*Escherichia coli* resides in the gastrointestinal (GI) tract of animals along with a few hundreds and thousands of different microbiota. In humans, *E. coli* is present at less than 1% of gut microbiota and it is not among the 25 most prevalent bacteria [1, 2]. But *E. coli* is the predominant *Enterobacteriaceae* species in humans [3]. Interestingly, *E. coli* is the first to colonize the intestines and persists all through life in humans [4]. Mucin layers do not allow any direct interaction of gut microbiota with the enterocytes. However, the diversity of gut microbiota is known to influence the intestinal permeability involving LPS, peptidoglycan, lipoproteins, deoxynucleic acid (DNA), and ribonucleic acid (RNA). Most *E. coli* strains are nonpathogenic and exist as commensals, but some pathogenic strains are associated with severe diseases [5]. Additionally, some *E. coli* strains known as pathobionts do not cause any disease in healthy individuals but exacerbate chronic inflammatory diseases [1]. The *E. coli* population in the GI tract is dynamic with a turnover in months to years [3, 6]. Humans contain five different strains of *E. coli* [7]. Oxygen diffusion from intestinal epithelium is favorable for *Enterobacteriaceae* members including *E. coli* to be present in close proximity to the mucus layer [8]. *E. coli* is known to play an important role in the maintenance by decreasing oxygen content,


#### **Table 1.**

*Characteristics of* E. coli *probiotic strains.*

by facilitating the colonization of anaerobes and vitamin K production, and protects against colonization of pathogens. *E. coli* produces microcins and colicins, which prevents the colonization of pathogens. In contrast to antibiotics, colicins act on bacteria related to the bacteria producing these antibacterial proteins. Many *E. coli* isolates from rat fecal matter were found to produce E1-, 1a/1b-, and B/D-type colicins with antimicrobial properties against enteropathogens [9].
