Cold-shock proteins affect desiccation tolerance, biofilm formation and motility in Listeria monocytogenes

Listeria monocytogenes is a foodborne pathogen whose biofilm formation and desiccation tolerance may contribute to its survival in the food industry. L. monocytogenes possesses three cold-shock domain family proteins (CspA, CspB and CspD) known to be essential for adaptation against various food-relevant stress conditions including cold growth.

The role of Csps in desiccation tolerance and biofilm formation was investigated in csp mutants as well as twenty-one other wild-type (WT) strains. Mutants with a single (ΔcspA) or multiple (ΔcspAB, ΔcspAD and ΔcspABD) deletions of csp genes, in a desiccation sensitive WT background (L. monocytogenes EGD-e) were immotile and exhibited an elevated desiccation tolerance compared to the parent strain.

However, deletion of cspA in the more desiccation resistant food and outbreak related L. monocytogenes strains 568 and 08-5578 had no impact on desiccation tolerance although compared to the parent strains the mutants were also immotile. A correlation between lower motility and higher desiccation tolerance was observed among the 20 WT strains (Spearman rank correlation, rs = −0.56, p = 0.01), although exceptions occurred indicating that multiple factors influence the diverse desiccation tolerance among L. monocytogenes strains.

Expression of cspA was upregulated in WT EGD-e, 568 and 08-5578 strains after desiccation for seven days, while the 568 and 08-5578 ΔcspA mutants expressed elevated levels of cspD and cspB (>30 fold higher) compared to their WTs. This indicates that upregulation of the other csps compensates for the deleted cspA gene.

Although biofilm formation was improved in all EGDe csp mutants relative to the WT strain, the opposite was observed for 568 and 08-5578 WT strains and their cspA deletion mutants. Only motile strains formed biofilm in the peg lid assay but a significant negative correlation (rs = −0.60, p = 0.01) was seen between higher motility and higher biofilm formation of WT strains.

In conclusion, the survival of L. monocytogenes strains in the food processing environment may depend on the control of motility, which is a necessity for biofilm formation but disadvantageous for desiccation survival.