Tanscriptional profiling of staphylococcus aureus during growth in 2 M NaCl leads to clarification of physiological roles for Kdp and Ktr K⁺ uptake systems

Staphylococcus aureus exhibits an unusually high level of osmotolerance and Na⁺ tolerance, properties that support survival in various host niches and in preserved foods. The genetic basis of these traits is not well understood. We compared the transcriptional profiles of S. aureus grown in complex medium with and without 2 M NaCl. The stimulon for growth in high-osmolality media and Na+ included genes involved in uptake of K⁺, other compatible solutes, sialic acid, and sugars; capsule biosynthesis; and amino acid and central metabolism. Quantitative PCR analysis revealed that the loci responded differently from each other to high osmolality imposed by elevated NaCl versus sucrose. High-affinity K⁺ uptake (kdp) genes and capsule biosynthesis (cap5) genes required the two-component system KdpDE for full induction by osmotic stress, with kdpA induced more by NaCl and cap5B induced more by sucrose. Focusing on K⁺ importers, we identified three S. aureus genes belonging to the lower-affinity Tr/Ktr family that encode two membrane proteins (KtrB and KtrD) and one accessory protein (KtrC). In the absence of osmotic stress, the ktr gene transcripts were much more abundant than the kdpA transcript. Disruption of S. aureus kdpA caused a growth defect under low-K⁺ conditions, disruption of ktrC resulted in a significant defect in 2 M NaCl, and a ΔktrC ΔkdpA double mutant exhibited both phenotypes. Protective effects of S. aureus Ktr transporters at elevated NaCl are consistent with previous indications that both Na⁺ and osmolality challenges are mitigated by the maintenance of a high cytoplasmic K⁺ concentration.