Exposure of Salmonella Enteritidis to ovotransferrin under egg-white conditions: cytoplasmic-membrane depolarization and increased pspG promoter activity

Salmonella enterica serovar Enteritidis is a major food-borne pathogen associated with the consumption of eggs and egg products. This association reflects the ability of S . Enteritidis to survive in egg white despite the presence of a large number of antimicrobial factors. One such factor is ovotransferrin, which plays a key role through its iron-chelating activity, although membrane-perturbating activity has also been observed. This study aimed to assess the membrane-damaging activity of ovotransferrin against S . Enteritidis and its contribution to egg-white antimicrobial activity. We investigated ovotransferrin’s ability to depolarize the cytoplasmic membrane of S . Enteritidis and its effect on pspG promoter activity. The pspG gene belongs to the phage shock protein (Psp) system, a membrane stress response involved in maintaining membrane integrity and proton motive force. Experiments were conducted under egg-white-like conditions using a synthetic medium mimicking the ionic composition at pH 7.9 and 9.3 (as encountered in egg white just after laying and a few days after, respectively). The effect of temperature was also investigated to consider conditions supporting bacterial growth (30 °C) or rapid bacterial killing (45 °C) in egg white. Membrane depolarization was monitored using the fluorescent probe DiSC 3 (5) and pspG promoter activity was assessed using a transcriptional fusion in the presence of 10% egg white. Ovotransferrin induced membrane depolarization in S . Enteritidis and increased pspG promoter activity, as did 10% egg white. While membrane depolarization was similar at both temperatures, it was enhanced at alkaline pH, whereas pspG promoter activity was reduced under combined heat and alkaline stress. Overall, this study shows that ovotransferrin contributes to egg-white-induced membrane perturbation in the S . Enteritidis strain tested and reveals that two key environmental factors (temperature and pH) modulate the bacterial stress response to egg white. These results provide new insights into the mechanisms involved in the response of S . Enteritidis to egg white conditions, including how environmental factors influence membrane perturbation and pspG promoter activity. They thereby improve our understanding of the physiology of S . Enteritidis in egg white, a major food-safety concern.