Gene profiling-based phenotyping for identification of cellular parameters that contribute to fitness, stress-tolerance and virulence of Listeria monocytogenes variants

Microbial population heterogeneity allows for a differential microbial response to environmental stresses and can lead to the selection of stress resistant variants. In this study, we have used two different stress resistant variants of Listeria monocytogenes LO28 with mutations in the rpsU gene encoding ribosomal protein S21, to elucidate features that can contribute to fitness, stress-tolerance and host interaction using a comparative gene profiling and phenotyping approach. Transcriptome analysis showed differential expression of 319 genes of which 116 genes were upregulated and 114 genes were downregulated in both rpsU variants. Upregulated genes included a major contribution of SigB-controlled genes such as intracellular acid resistance-associated glutamate decarboxylase (GAD) (gad3), genes involved in compatible solute uptake (opuC), glycerol metabolism (glpF, glpK, glpD), and virulence (inlA, inlB). Downregulated genes in the two variants involved mainly genes involved in flagella synthesis and motility. Phenotyping results of rpsU variants matched the gene profiling data including enhanced freezing resistance conceivably linked to compatible solute accumulation, higher glycerol utilisation rates, and better adhesion to Caco 2 cells presumably linked to higher expression of internalins. Also, bright field and electron microscopy analysis confirmed reduced flagellation of the variants. The activation of SigB-mediated stress defence offers an explanation for the multiple-stress resistant phenotype in rpsU variants