Viable count estimates of lag time responses for Salmonella typhimurium M48 subjected to abrupt osmotic shifts.

Generally, relative lag times (RLT; lag time divided by generation time) become extended as conditions become less favourable for growth. Mellefont et al. (2003, 2004) [Mellefont, L.A., McMeekin, T.A., Ross, T., 2003. The effect of abrupt osmotic shifts on the lag phase duration of foodborne bacteria. Int. J. Food Microbiol. 83(3), 281-293; Mellefont, L.A., McMeekin, T.A., Ross, T., 2004. The effect of abrupt osmotic shifts on the lag phase duration of physiologically distinct populations of Salmonella typhimurium. Int. J. Food Microbiol. 92, 111-120] reported that abrupt osmotic shifts of Salmonella typhimurium M48 from optimal to low aw led to unexpectedly small RLTs at low aw. In this study, RLTs resulting from similar osmotic shifts were estimated by viable count (VC) and compared to turbidimetric estimates to test the hypothesis that the 'downturn' in RLT is an artefact of the turbidimetric technique used. No 'downturn' in RLT was observed with VC data and RLTs increased with increasing magnitude of osmotic shift. Anomalous turbidimetric estimates of lag time at low aw were confirmed as the likely source of the 'downturn' in RLT. The abrupt osmotic shifts resulted in a complex pattern of microbial population behaviour. Immediately after transfer from optimal aw to low aw, inactivation of a portion of the population occurred for all the conditions tested. The degree of inactivation became progressively larger with larger shifts in aw. The initial decline in population was followed by a period during which no change in numbers occurred, followed by growth that appeared, in most cases, to be exponential. At the lowest aws tested (< or =0.954), the growth response after the initial decline was at a rate slower than that of exponential phase growth. Due to the use of non-selective media containing pyruvate (to eliminate oxygen radicals), the observed patterns of inactivation, lag and regrowth at most aw conditions are unlikely to result from a temporary loss of culturability, but may represent inactivation of a portion of the population.