The effect of inoculum size on the lag phase of Listeria monocytogenes

The effect of inoculum size on population lag times of Listeria monocytogen es was investigated using the Bioscreen automated microtitre plate incubato r and reader. Under optimum conditions, lag times were little affected by i noculum size and there was little variation between replicate inocula even at very low cell numbers. However, in media containing inhibitory concentra tions of NaCl, both the mean lag time and variation between replicate inocu la increased as the inoculum size became smaller. The variation in lag time of cells within a population was investigated in more detail by measuring the distribution of detection times from 64 replicate inocula containing on ly one or two cells capable of initiating growth. The variance of the lag t ime distribution increased with increasing salt concentration and was great er in exponential than in stationary phase inocula. The number of cells req uired to initiate growth increased from one cell under optimum conditions t o 10(5) cells in medium with 1.8 M NaCl. The addition of spent medium from a stationary phase culture reduced the variance and decreased lag times. Th e ability to initiate growth under severe salt stress appears to depend on the presence of a resistant sub-fraction of the population, although high c ell densities assist adaptation of those resistant cells to the unfavourabl e growth conditions by some unspecified medium conditioning effect. These r esults are relevant to the prediction of lag times and probability of growt h from low numbers of stressed cells in food. (C) 2001 Elsevier Science B.V . All rights reserved.