ABSTRACT
The virulence of yersiniae is promoted in part by shared ≈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°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.
mxiA of Shigella flexneri 2a, which facilitates export of invasion plasmid antigens, encodes a homolog of the low-calcium-response protein, LcrD, of Yersinia pestis.