Abstract
The effect of acids on the growth, survival and detection of Salmonella enteritidis PT4 is particularly important in view of the number of outbreaks in which mayonnaise has been implicated as a vehicle. Capacitance measurement was compared with colony counting procedures for the enumeration and determination of sub-lethal injury of Salmonella enteritidis during storage under varied conditions of pH, acidulant and temperature. Capacitance monitoring was shown to offer an improved technique for the measurement of sub-lethal injury in cell populations. Higher levels of sub-lethal injury were detected by the extension of capacitance detection time than were indicated by differential colony counts on selective and non-selective media. The extension of detection time noted with sub-lethally injured cell populations was shown to be due to an extended lag phase when cells were placed in the capacitance growth medium and not the result of delayed detection of the growth of a small, uninjured sub-population. Plots of percentage survival and extension of detection time in survivors gave similar curves for acetic and lactic acid. These acids showed both greater lethality and greater ability to inflict sub-lethal injury than the stronger citric or hydrochloric acid. Sub-lethal injury and lethality were not simply related, as little sub-lethal injury was observed with the stronger acids even under conditions that were ultimately highly lethal. The results indicate that weak organic acids cause more reversible damage to cellular sites prior to death: an observation that has implications for choice of resuscitation procedures when examining acidified foods. Injured cells were found to contain lower levels of ATP than healthy unstressed cells. Inhibition with chloramphenicol did not appear to increase injury and total protein patterns for injured and uninjured cells were similar, suggesting that protection afforded by the synthesis of stress proteins is not a significant factor in this case. The recovery of sub-lethally injured cells in various pre-enrichment and selective enrichment media using capacitance detection times and colony counts on selective and non selective media was determined. Buffered peptone water appeared more effective in recovering injured Salmonella compared to Lactose broth. Additionally, selenite cystine was shown to recover cells faster than the other two selective broths tested; the Muller-Kauffmann Tetrathionate broth and the Rappaport Vassiliadis enrichment broth. In this study, different injury conditions were applied; these included acid stress, heating and freezing. According to the lag phases of injured cell populations, short pre-enrichment is not recommended in the present study.