General stress sigma factor RpoS influences time required to enter the viable but non-culturable state in Salmonella enterica

In stressful conditions, bacteria enter into the viable but non-culturable (VBNC) state; in this state, they are alive but fail to grow on conventional media on which they normally grow and develop into colonies. The molecular basis underlying this state is unknown. We investigated the role of the alternative sigma factor RpoS (σ(38)) in the VBNC induction using Salmonella Dublin, Salmonella Oranienburg and Salmonella Typhimurium LT2. VBNC was induced by osmotic stress in LT2 and Oranienburg. Dublin also entered the VBNC state, but more slowly than LT2 and Oranienburg did. The LT2 rpoS gene was initiated from an alternative initiation codon, TTG; therefore, LT2 had smaller amounts of RpoS than Dublin and Oranienburg. Oranienburg had a single amino acid substitution (D118N) in RpoS (RpoS(SO)). Disruption of rpoS caused rapid VBNC induction. VBNC induction was significantly delayed by Dublin-type RpoS (RpoS(SD)), but only slightly by RpoS(SO). These results indicate that RpoS delays VBNC induction and that the rapid induction of VBNC in LT2 and Oranienburg may be due to lower levels of RpoS and to the D118N amino acid substitution, respectively. Reduced RpoS intracellular level was observed during VBNC induction. During the VBNC induction, Salmonella might regulate RpoS which is important for maintenance of culturablity under stresses.