Acid adaptation increased the resistance of Escherichia coli O157:H7 in bok choy ( Brassica rapa subsp. chinensis ) juice to high-pressure processing

Abstract

ABSTRACT The adaptive response of Escherichia coli O157:H7 to acid stress, a major stressor in juice processing environments, has been shown to confer cross-resistance against pasteurization methods such as heat and X-ray. However, whether acid adaptation confers resistance against high-pressure processing (HPP) has not been explored. This study aimed to investigate the effect of acid adaptation on the pressure resistance of E. coli O157:H7 in a bok choy vegetable juice matrix. Acid adaptation was performed in tryptone soya broth without dextrose adjusted to pH 5.0 at 37°C for 24 h, while non-adapted cells were prepared in similar conditions, without the pH adjustment (pH 7.3). The C7927 strain was used as it exhibited the greatest increase in pressure resistance after acid adaptation among the three strains tested. Acid adaptation increased the D -value at 400 MPa from 0.8 ± 0.1 min in non-adapted cells to 1.2 ± 0.1 min in acid-adapted cells ( P < 0.05). Furthermore, significantly lower reductions in total viable counts were observed after 72 h of post-HPP refrigerated storage for acid-adapted cells (3.4 ± 0.2 log CFU/mL) as compared to non-adapted cells (5.0 ± 0.1 log CFU/mL). Sublethally injured populations were measured via plating on selective media, as well as differential fluorescent staining and flow cytometry. The levels of damages to membrane, DNA, proteins, and peptidoglycan were significantly lower in acid-adapted cells, suggesting that the acid adaptive response of E. coli O157:H7 may protect it against HPP-induced inactivation and injury by modulating the pressure resistance of these cellular targets. IMPORTANCE Based on the U.S. Food and Drug Administration regulations, E. coli O157:H7 is a pertinent pathogen in high acid juices that needs to be inactivated during the pasteurization process. The results of this study suggest that the effect of acid adaptation should be considered in the selection of HPP parameters for E. coli O157:H7 inactivation to ensure that pasteurization objectives are achieved.