Investigating the development of acid tolerance in food-borne pathogens Escherichia coli, Salmonella spp., and Pseudomonas aeruginosa and the implication on the susceptibility to organic acids

Abstract

Various foodstuffs have a very low pH and bacteria have been reported to survive such products. Acid substances, such as organic acids are common food preservatives. These substances also lower the pH of processed foods. Decontamination with organic acids which could result in the emergence of acid tolerant food-borne pathogens is causing concern. The objectives of the study were to determine the development of acid tolerance in important food-borne pathogenic bacteria, to investigate evolving changes in the phenotypic characteristics as a result this acid tolerance, and to explore the possibility of repercussions in successful food preservation with organic acids. Bacterial strains were screened for acid-tolerance, by determining viable counts immediately before acid challenge and at various times after challenge. Strains were exposed to increasing concentrations of hydrochloric acid, acidic foodstuff and two organic acids (acetic and citric acid). Protein profiles were generated by SDS-PAGE examined for possible modification(s) as a result of acid tolerance development. Susceptibility to seven organic acids levels were scrutinised to evaluate the probability of a correlation between altered antimicrobial activity and acid tolerance. Salmonella enterica sv. Enteritidis ATCC 13076 and Escherichia coli ATCC 25922 indicated rapid development of acid tolerance, after 36h of acid exposure. In Salmonella enterica sv. Typhimurium ATCC 14028, E. coli 0111 and Pseudomonas aeruginosa ATCC 27853 intermediate intrinsic acid tolerance was obvious. On comparing susceptibility of these pathogens to the organic acids, it was demonstrated that pH played a significant role in inhibitory activity, as it is known that these compounds exhibit optimum antimicrobial activity at a lower pH (pH ≤ 5). Further investigations will be conducted to elucidate the two-way role of pH in foodstuff and the addition of an organic acid, in determining if microorganisms are losing their susceptibility for the preservative as a result of sub-optimal pH levels, or if they become acid-tolerant after surviving exposure to organic acids.