Escherichia coli (E. coli) is a Gram-negative, facultative anaerobic, rod-shaped bacterium of the genus Escherichia (Tenaillon et al. 2010).. It is commonly found in the lower intestine of warm-blooded organisms (Secher T 2016). E. coli is an extremely diverse bacterial species which forms part of the gut microbiome, a term which describes the ecological community of commensal, symbiotic and pathogenic microorganisms found in the intestine (Bull M J et al., 2014). Throughout life, the population of E. coli usually settles around 107108 colony-forming units per gram of faeces (Secher T 2016). The E. coli population of the intestines usually includes a set of durable core strains, as well as temporary transients that vary with health, nutrition, infection and antibiotic exposure (Blount 2015). E. coli grows in the thin mucous layer that lines the gut where it competes with many other microorganisms for nutrients. This results in it being a non-fastidious organism, with a wide-ranging diet (Blount 2015). E. coli is the primary aerobic microorganism in the gastrointestinal tract (Tenaillon et al. 2010). As E. coli is a facultative anaerobe it is able to optimise growth based on the availability of oxygen. It is able to utilise both anaerobic and microaerobic respiration in order to survive in the low oxygen environment in the intestine (Tenaillon O 2010). Although E. coli competes with other microorganisms for resources such as surface area an
d nutrition, it is suggested to also share a mutualistic relationship with some. It aids anaerobic commensals in the gut by consuming any oxygen that may enter the gut, helping the maintain an anaerobic environment. In exchange, it is believed that E. coli may benefit from the breakdown of mucosal polysaccharides and dietary fibres by obligate anaerobes in the intestines (Tenaillon O 2010). E. coli is one of the first commensal microorganisms that the human infant gains exposure to. This is thought to occur via exposure to maternal faecal microbiota during childbirth. Interestingly, this exposure appears to be reducing in industrialised countries due to more stringent hospital hygiene standards and an increase in the number of caesarean sections being performed (Nowrouzian et al. 2003). Although the relationship between E. coli and the host is often considered to be commensal in which one party notably benefits and the other is neither helped nor harmed, there is some evidence that E. coli provides some succour to the host in the form of the production of vitamins K and B12, as well as the competitive exclusion of pathogens from the gut (Katouli 2010) (Hudault, Guignot and Servin 2001). In return, the human intestinal tract provides E. coli with a steady supply of carbon and energy sources, a comfortable environment with a moderate pH and temperature as well as protection against certain stresses and transport and dissemination facilities.