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Lett Appl Microbiol. 2001 Nov;33(5):357-61.

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

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School of Food Biosciences, University of Reading, PO Box 226, Whiteknights, Reading, RG6 6AP UK.



To investigate the effect of inoculum size and physiological state on the ability of Listeria monocytogenes cells to initiate growth under suboptimal conditions of salt concentration and pH.


Cell suspensions were serially diluted in media of different salt concentration or pH and replicate inocula distributed into 96-well microplates. The proportion of wells showing growth at each dilution level 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 NaCl; above this threshold, the inoculum size needed to initiate growth became progressively larger. A similar effect was seen with decreasing pH but only very close to the growth/no growth boundary. The threshold for inoculum-dependent growth was lower in exponential phase cells than in stationary phase ones and sublethal injury greatly decreased the probability of growth from small inocula.


The growth/no growth boundary for L. monocytogenes is not an absolute cut-off point but represents a region where the probability of growth rapidly decreases as conditions become more extreme. We interpret the requirement 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 population effects.


Physiological heterogeneity within the cell population and inoculum size will affect the risk of L. monocytogenes growing in food.

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