Exploratory study into the culturability and viability of three strains of Escherichia coli after exposure to simulated gastric fluid of sub-lethal pHs

It has long been established that the acidic nature of gastric juice constitutes an important line of defence against invasion of the gut by microorganisms. Survival of the gastric barrier was traditionally tested using microbiological techniques (culturability), however, advances in viability studies have highlighted the viable but non-culturable (VBNC) state. The purpose of this study was to assess culturability and viability in the survival response of strains of Escherichia coli ( E. coli ) exposed to simulated gastric fluid (SGF) of varying pH. Bacteria were subjected to acid-stress conditions by incubation in SGF at various pH over 180 minutes. Survival was evaluated by assessing culturability, and viability via membrane integrity. The results showed that all strains of E. coli studied were able to remain viable in an injured state after exposure to various acidic SGF environments for the duration of incubation. These viable cells persisted in either a live or injured state. The Enterotoxigenic E. coli (ETEC) strain was both viable and culturable at all pHs, showing a high level of acid tolerance. The Enteropathogenic E. coli (EPEC) strain was not culturable at the lower pH of 1.5 and 2.5, however, showed a high viability percentage. This could show that the bacteria had entered a VBNC state at these low pHs. This exploratory study indicates that certain strains of pathogenic E. coli are able to endure the stomach pH and may enter into a VBNC state. This might prove to be a major public health concern due to the fact that these organisms can survive the harsh acidic environment of the stomach and may possibly return to the infectious state once in the favourable conditions of the intestines.