Differential expression of virulence genes and motility in Ralstonia (Pseudomonas) solanacearum during exponential growth

A complex network regulates virulence in Ralstonia solanacearum (formerly Pseudomonas solanacearum); central to this system is PhcA, a LysR- type transcriptional regulator. We report here that two PhcA-regulated virulence factors, endogluconase (Egl) and acidic exopolysaccharide I (EPS I), and motility are expressed differentially during exponential growth in batch cultures. Tests with strains carrying lacZ fusions in a wild-type genetic background revealed that expression (on a per-cell basis) of phcA was constant but expression of egl and epsB increased 20- to 50-fold during multiplication from 1 x 10⁷ to 5 x 10⁸ CFU/ml. Expression of xpsR, an intermediate regulator downstream of PhcA in the regulatory cascade for eps expression, was similar to that of epsB and egl. Motility track photography revealed that all strains were essentially nonmotile at 10⁶ CFU/ml. As cell density increased, 30 to 50% of wild-type cells were motile between 10⁷ and 10⁸ CFU/ml, but this population was again nonmotile at 10⁹ CFU/ml. In contrast, about 60% of the cells of phcB and phcA mutants remained motile at 10⁹ CFU/ml. Expression of phcB, which is not positively regulated by PhcA, was the inverse of epsB, egl, and xpsR (i.e., it decreased 20-fold at high cell density). PhcB is essential for production of on extracellular factor, tentatively identified as 3-hydroxypalmitic acid methyl ester (3-OH PAME), that might act as an exponential-phase signal to activate motility or expression of virulence genes. However, growth of the lacZ fusion strains in medium containing excess 3-OH PAME did not result in motility or expression of virulence genes at dramatically lower cell densities, suggesting that 3- OH PAME is not the only factor controlling these traits.