Transposon mutagenesis reveals genes involved in osmotic stress and drying in Cronobacter sakazakii

This study characterizes the growth in hyperosmotic media and the resistance to desiccation of a collection of fifteen Cronobacter sakazakii strains. C. sakazakii strains showed similar abilities to grow/persist under osmotic stress conditions to strains from other related Enterobacteriaceae, i.e. Cronobacter muytjensii, Cronobacter malonaticus, Enterobacter gergoviae, Enterobacter cloacae, Enterobacter aerogenes, and S. Typhimurium. Nevertheless, some degree of heterogeneity among C. sakazakii strains could be observed, and in general strains isolated from clinical sources showed the greatest robustness. A transposon mutagenesis approach was used to identify genetic systems involved in the response of C. sakazakii DPC 6529 to hyperosmotic conditions. We obtained evidence that de novo protein synthesis, repair of damage in macromolecules and maintenance of the structure and integrity of the cellular envelope are essential processes for the cell under osmotic stress. Moreover, some metabolic activities are also important, including the synthesis of glutamine as a compatible solute and the regulation of nucleotide and nucleoside pools. The Cpx system, known as an envelope stress response regulator, and the sigma factors RpoN and RpoS seem to be the main signals regulating the bacterial response to hyperosmotic conditions. Among the identified salt-sensitive mutants, only those disrupted in dnaK and dnaJ, encoding two molecular chaperones, were important for C. sakazakii survival under desiccation. This suggests that the systems and proteins involved in the desiccation response differ from those responsible for growth under hyperosmotic conditions, at least under the conditions tested in the current study.