Decreased resistance of sublethally injured Escherichia coil O157:H7 to salt, mild heat, nisin and acids induced by high pressure carbon dioxide

Resistance of sublethally injured cells (SICs) of Escherichia coli O157:H7 induced by high pressure carbon dioxide (HPCD) to salt, low temperature, mild heat, nisin, acids and low pHs was investigated in this study. The SICs of E. coli were obtained following HPCD at 5 MPa and 25 degrees C for 40-60 min or 5 MPa and 45 degrees C for 20 min. The untreated cells could survive normally while the HPCD-treated cells showed 2.87 log(10) cycles' reduction on tryptic soy agar (TSA) with 3% NaCl. The counts of the untreated cells were not significantly changed during 5 h incubation at 4 degrees C or 40 min incubation at 45 degrees C, and the HPCD-treated cells were also not affected by 5 h incubation at 4 degrees C but showed 1.75 log(10) cycles' reduction at 45 degrees C for 40 min. The antimicrobial nisin caused an extra 0.25-1.0 log(10) cycles' reduction of the HPCD-treated cells while the untreated cells was not inactivated by nisin. Except for oxalic acid (OA), citric acid (CA), malic acid (MA), tartaric acid (TA), lactic acid (LA), acetic acid (AA) and hydrochloric acid (HCl) did not inactivate the untreated cells, but all these seven acids caused a 0.74-1.53 log(10) cycles' reduction of HPCD-treated cells. These results indicated that HPCD-induced SICs had a decreased resistance to salt, mild heat, nisin and acids. Moreover, the recovery test was used to investigate the sensitivity of the SICs to different pHs. Results showed that the SICs could not recover below or equal to pH 4.0. These promising results would open up the possibility of exploring the combination of other technologies (eg. mild heat, nisin and acids) with HPCD as hurdle approaches to inactivate target pathogens in foods.