Exploiting genomic differences between Listeria monocytogenes EGDe isolates reveals crucial roles for SigB and wall rhamnosylation in biofilm formation.

Listeria monocytogenes is a Gram-positive Firmicute that causes food-borne infections, in part due to its ability to use multiple strategies, including biofilm formation, to survive adverse growth conditions. To screen for genes required for biofilm formation we harnessed the ability of bacteria to accumulate mutations in the genome over time, diverging the properties of seemingly identical strains. By sequencing the genomes of four laboratory reference strains of the commonly used L. monocytogenes EGDe, we showed that each isolate contains SNPs compared with the reference genome. We discovered that two SNPs, contained in two independent genes within one of the isolates, impacted biofilm formation. Using bacterial genetics and phenotypic assays, we confirmed that rsbU and rmlA influence biofilm formation. RsbU is the upstream regulator of the alternative sigma factor, SigB and mutation of either rsbU or sigB increased biofilm formation. In contrast, deletion of rmlA, which encodes the first enzyme for TDP-L-rhamnose biosynthesis, resulted in a reduction in the amount of biofilm formed. Further analysis of biofilm formation in a strain that still produces TDP-L-rhamnose, but which cannot decorate the wall teichoic acid with rhamnose (rmlT mutant), showed that it is the decorated wall teichoic acid that is required for adhesion of the cells to surfaces. Together these data uncover novel routes by which biofilm formation by L. monocytogenes can be impacted, and reinforce the necessity to obtain whole genome sequencing data of isolates used in the laboratory.