Increased acid tolerance of Escherichia coli O157:H7 as affected by acid adaptation time and conditions of acid challenge

Three strains of Escherichia coli O157:H7, ATCC 43889, 43895 and 933 were subjected to acid adaptation in Tryptic Soy Broth (pH 5.0) for 1, 2, 3, 4 and 6 h. Acid tolerance of the adapted cells was determined in subsequent acid challenge at pH 3.0, 4.0 and 5.0 (acidified with HCl) and in the presence of lactic, acetic or propionic acid. It was found that acid adaptation increased acid tolerance of the E. coli O157:H7 strains tested and was dependent on strain, acid adaptation time and pH of the challenge. Among the acid adaptation times tested, 4 h of adaptation enabled the test organism, regardless of strains, to exhibit the most pronounced acid adaptation response which was most marked at pH 3.0, followed by pH 4.0 and 5.0. The extent of increased acid tolerance varied with the strains of E. coli O157:H7 and challenge of organic acid. The 4-h acid-adapted cells of ATCC 43889 and 933 showed an increase in acid tolerance in the presence of lactic, acetic and propionic acids. An increase in tolerance was also noted with ATCC 43895 in the presence of acetic and lactic acid, but not in the presence of propionic acid.