Acidification and extended storage at room temperature of mayonnaise reduce Salmonella Typhimurium virulence and viability

Abstract

Despite food safety recommendations, raw egg-based foods, such as mayonnaise, are frequently identified as the source of Salmonella during outbreaks. Acidification and storage temperature have been linked with reduced bacterial culturability. Raw egg-based sauces stored at 25 °C have historically been linked with faster decline of Salmonella culturability than preparations stored at 5 °C. This study aimed to determine whether reduced culturability in acidified mayonnaise correlated with reduced in vitro bacterial motility, invasiveness and viability as well as disease-causing capacity in BALB/c mice. Acidification of mayonnaise and incubation at 25 °C for 4 h significantly reduced culturability of Salmonella Typhimurium DT9 but was dependent on initial bacterial load. Bacteria recovered from acidified mayonnaise exhibited reduced invasiveness into polarized cultured intestinal epithelial cells and 12 h post inoculation were no longer invasive suggesting a reduced capacity to cause disease. To confirm this, BALB/c mice were inoculated with Salmonella Typhimurium contaminated mayonnaise stored at 5 °C or 25 °C for 12, 24, 48, 72, and 96 h. Mice inoculated with mayonnaise incubated at 5 °C for 12 and 24 h exhibited mild to moderate disease symptoms; all other mayonnaise treatment groups did not exhibit disease symptoms. In acidified mayonnaise, Salmonella Typhimurium DT9 exhibited a global downregulation of metabolism, stress response, and virulence genes upon addition to mayonnaise. After 4 h of incubation at both 5 °C and 25 °C, however, the vast majority of genes were upregulated which was maintained over the 96-hour experiment suggesting that bacteria were severely stressed. Salmonella Typhimurium DT9 cells were isolated from mayonnaise samples and ATP production was quantified. At both 5 °C and 25 °C, ATP production decreased in acidified mayonnaise preparations. At 25 °C, ATP production decreased more rapidly than at 5 °C. After 24 h, ATP production of bacteria in mayonnaise stored at 25 °C was not significantly different from the dead control group. Thus, the current recommendation of only serving freshly prepared raw egg-sauces or refrigerating immediately after preparation, could be placing consumers at higher risk for contracting salmonellosis.

Keywords: Acidification; BALB/c mice; Invasion; Raw egg sauces; Salmonella Typhimurium; Virulence.