Effect of deletion of the lpxM gene on virulence and vaccine potential of Yersinia pestis in mice

Yersinia pestisundergoes an obligate flea–rodent–flea enzootic life cycle. The rapidly fatal properties ofY. pestisare responsible for the organism's sustained survival in natural plague foci. Lipopolysaccharide (LPS) plays several roles inY. pestispathogenesis, prominent among them being resistance to host immune effectors and induction of a septic-shock state during the terminal phases of infection. LPS is acylated with 4–6 fatty acids, the number varying with growth temperature and affecting the molecule's toxic properties.Y. pestismutants were constructed with a deletion insertion in thelpxMgene in both virulent and attenuated strains, preventing the organisms from synthesizing the most toxic hexa-acylated lipid A molecule when grown at 25 °C. The virulence and/or protective potency of pathogenic and attenuatedY. pestisΔlpxMmutants were then examined in a mouse model. The ΔlpxMmutation in a virulent strain led to no change in the LD50value compared to that of the parental strain, while the ΔlpxMmutation in attenuated strains led to a modest 2.5–16-fold reduction in virulence. LPS preparations containing fully hexa-acylated lipid A were ten times more toxic in actinomycin D-treated mice then preparations lacking this lipid A isoform, although this was not significant (P>0.05). The ΔlpxMmutation in vaccine strain EV caused a significant increase in its protective potency. These studies suggest there is little impact from lipid A modifications on the virulence ofY. pestisstrains but there are potential improvements in the protective properties in attenuated vaccine strains.