The effect of growth atmosphere on the ability of Listeria monocytogenes to survive exposure to acid, proteolytic enzymes and bile salts

Four isolates of Listeria monocytogenes from food, human and environmental sources were grown separately in broth (pH 6.0 at 8 °C) under atmospheres of air, 100% N2, 40% CO2:60% N2 or 100% CO2. Exponential and stationary phase cells were harvested to determine if growth atmosphere and growth phase influenced this pathogenʹs ability to survive exposure to an acid environment coupled with proteolytic enzymes, and the activity of bile salts. In general, isolates were more resistant to the acid environment than the bile salts environment and stationary phase cells were significantly more resistant to both environments than exponential phase cells. Irrespective of prior growth atmosphere, none of the isolates when in exponential phase remained detectable following full exposure to the acid environment (110 min at 37 °C) or the bile environment (3 h at 37 °C). With the exception of one isolate grown under the atmosphere of 40% CO2:60% N2, all isolates when in stationary phase were detectable following full exposure to the acid environment but death rates varied significantly. Stationary phase cells of all isolates grown under 40% CO2:60% N2 and 100% CO2 were highly susceptible to the bile salts environment: cells were not detectable after a 2-min exposure whereas stationary phase cells grown under air or 100% N2 were recovered following full exposure to the bile environment. Survival curves were characterised by a population decline of at least 3 log10/ml (from an initial level of 7 log10 CFU/ml) in the first 15 min; thereafter a constant population number of approximately 4 log10/ml was maintained over the remaining exposure period. No survival was observed when stationary phase cells of L. monocytogenes FRRB 2538 grown in air and 100% N2 were subjected to the acid environment followed by immediate exposure to the bile salts environment. The results showed that growth atmosphere and growth phase could influence survival of this pathogen against conditions that imitate the extremes of the most important nonspecific defence mechanisms against microbial infection: the acid environment of the stomach coupled with the activity of proteolytic enzymes, and the activity of bile salts in the small intestine.