INFLUENCE OF THE RPOS (KATF) SIGMA-FACTOR ON MAINTENANCE OF VIABILITYAND CULTURABILITY OF ESCHERICHIA-COLI AND SALMONELLA-TYPHIMURIUM IN SEAWATER

The sigma factor RpoS is essential for stationary-phase-specific, mult iple-stress resistance. We compared the viabilities (direct viable cou nts) and culturabilities (colony counts) in seawater of Escherichia co li and Salmonella typhimurium strains and those in which rpoS was dele ted or which were deficient in guanosine 3',5'-bispyrophosphate (ppGpp ) synthesis (relA spoT). RpoS, possibly via ppGpp regulation, positive ly influenced the culturability of these bacteria in oligotrophic seaw ater. This influence closely depended, however, upon the growth state of the cells and the conditions under which they were grown prior to t heir transfer to seawater. The protective effect of RpoS was observed only in stationary-phase cells grown at low osmolarity. A previous exp osure of cells to high osmolarity (0.5 M NaCl) also had a strong influ ence on the effect of RpoS on cell culturability in seawater. Both E. coli and S. typhimurium RpoS mutants lost the ability to acquire a hig h resistance to seawater, as observed in both logarithmic-phase and st ationary-phase RpoS(+) cells grown at high osmolarity. A previous grow th of S. typhimurium cells under anoxic conditions also modulated the incidence of RpoS on their culturability. When grown anaerobically at high osmolarity, logarithmic-phase S. typhimurium RpoS(+) cells partly lost their resistance to seawater through preadaptation to high osmol arity. When grown anaerobically at high osmolarity until stationary ph ase, both RpoS(+) and RpoS(-) cells retained very high levels of both viability and culturability and then did not enter the viable but nonc ulturable state for over 8 days in seawater because of an RpoS-indepen dent, unknown mechanism.