Influence of Na+, dicarboxylic amino acids, and pH in modulating the low-calcium response of Yersinia pestis

The virulence of yersiniae is promoted in part by shared approximate to 70-kb plasmids (pCD in Yersinia pestis and pYV in enteropathogenic Yersinia pseudotuberculosis and Yersinia enterocolitica) that mediate a low-calcium response. This phenotype is characterized at 37 degrees C by either bacteriostasis in Ca2+-deficient medium with expression of pCD/pYV-encoded virulence effectors (Yops and LcrV) or vegetative growth and repression of Yops and LcrV with >= 2.5 mM Ca2+ (Lcr(+)). Regulation of Yops and LcrV is well defined but little is known about bacteriostasis other than that Na+ Plus L-glutamate promotes prompt restriction of Y. pestis. As shown here, L-aspartate substituted for L-glutamate in this context but only Na+ exacerbated the nutritional requirement for Ca2+. Bacteriostasis of Y. pestis (but not enteropathogenic yersiniae) was abrupt in Ca2+-deficient medium at neutral to slightly alkaline pH (7.0 to 8.0), although increasing the pH to 8.5 or 9.0, especially with added Na+ (but not L-glutamate), facilitated full-scale growth. Added L-glutamate (but not Na+) favored Ca2+- independent growth at acidic pH (5.0 to 6.5). Yops and LcrV were produced in Ca2+-deficient media at pH 6.5 to 9.0 regardless of the presence of added Na+ or L-glutamate, although their expression at alkaline pH was minimal. Resting Ca2+-starved Lcr(+) cells of Y. pestis supplied with L-glutamate first excreted and then destroyed L-aspartate. These findings indicate that expression of Yops and LcrV is necessary but not sufficient for bacteriostasis of Ca2+-starved yersiniae and suggest that abrupt restriction of Y. pestis requires Na+ and the known absence of aspartate ammonia-lyase in this species.