The effect of inoculum size and sublethal injury on the ability of Listeria monocytogenes to initiate growth under suboptimal conditions

Aims: To investigate the effect of inoculum size and physiological state on the ability of. Listeria monocytogenes cells to initiate growth under subo ptimal conditions of salt concentration and pH. Methods and Results: Cell suspensions were serially diluted in media of dif ferent salt concentration or pH and replicate inocula distributed into 96-w ell microplates. The proportion of wells showing growth at each dilution le vel was determined after incubation for 6 weeks for each set of conditions. Growth occurred from single cells up to a concentration of 1.2 mol l(-1) N aCl; above this threshold, the inoculum size needed to initiate growth beca me progressively larger. A similar effect was seen with decreasing pH but o nly very close to the growth/no growth boundary. The threshold for inoculum -dependent growth was lower in exponential phase cells than in stationary p hase ones and sublethal injury greatly decreased the probability of growth from small inocula. Conclusions: The growth/no growth boundary for L. monocytogenes is not an a bsolute cut-off point but represents a region where the probability of grow th rapidly decreases as conditions become more extreme. We interpret the re quirement for a critical inoculum size for growth as being due to death of a proportion of cells in the inoculum rather than to co-operative populatio n effects. Significance and Impact of the Study: Physiological heterogeneity within th e cell population and inoculum size will affect the risk of L. monocytogene s growing in food.