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The effects of nine common food industry stresses on the times to the turbidity (T d ) distribution of Listeria monocytogenes were determined. It was established that the main source of the variability of T d for stressed cells was the variability of individual lag times. The distributions of T d revealed that there was a noticeable difference in response to the stresses encountered by the L. monocytogenes cells. The applied stresses led to significant changes of the shape, the mean, and the variance of the distributions. The variance of T d of wells inoculated with single cells issued from a culture in the exponential growth phase was multiplied by at least 6 and up to 355 for wells inoculated with stressed cells. These results suggest stress-induced variability may be important in determining the reliability of predictive microbiological models.

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The Listeria monocytogenes genome encodes putative multidrug efflux transporters but only the MdrL transporter has been partially characterized in the wild-type LO28 strain. Here, we show in the LO28 strain, that the expression of MdrL is repressed at the transcriptional level, under standard growth conditions, by the product of a new gene ladR (lmo1408), and the expression of MdrL is induced in the presence of rhodamine. Phylogenetic analysis in related firmicutes shows that LadR, conserved in all sequenced Listeria genomes, forms an independent group from the large and diverse PadR transcriptional regulator family (PF03551). This is the first report of a bacterial multidrug transporter controlled by a member of the PadR family.

Salmonella serotype typhimurium transpositional mutants altered in resistance to biliary salts and detergents were isolated previously. We have characterized further the LX1054 mutant strain, the most sensitive of them. The chromosomal DNA segment flanking transposon insertion was cloned and sequenced. The highest level of identity was found for the acrB (formerly acrE) gene of Escherichia coli, a gene encoding a drug efflux pump of the Acr family. LX1054 exhibited a reduced capacity to colonize the intestinal tract. After passages in mice, the mutant strain lost the sensitive phenotype. In vitro, a resumption of growth appeared after 17 h of culture in medium with cholate or other tested biological or chemical detergents. Then, the acquired resistant phenotype seemed stable. The data suggested a role of S. typhimurium acrB-like gene in resistance to biliary salts and detergents and in mice intestinal colonization. However, the local and transient sensitivity observed in vivo, and the in vitro adaptations suggest that several detergent-resistance mechanisms operate in S. typhimurium.