Mutation in the pla gene of Yersinia pestis alters the course of the plague bacillus-flea (Siphonaptera: Ceratophyllidae) interaction.

Yersinia pestis possesses a unique gene (pla) encoding coagulase and fibrinolysin which is implicated in the transmission of plague by fleas. This gene is encoded on the highly conserved but poorly characterized 'pesticin' plasmid pKYP1. The role of the pKYP1-encoded gene, pla, in plague transmission was addressed by feeding fleas on blood containing avirulent Y. pestis strain EV76-6 and three derivatives of this strain (K10-2, K10-3, and K10-5) carrying Tn801 insertions in pKYP1. One of these mutant strains, K10-5, contains an insertion within the pla gene that eliminates both coagulase and fibrinolysin activities, whereas strains K10-3 and K10-2 retain both pla-associated phenotypes. After feeding, it was found that flea mortality at 4 d after infection associated with strain K10-5 (26%) was significantly lower than the mortality observed with other strains (53-64%). These results suggest that expression of the pla gene product may contribute to the deleterious effects of plague bacilli on fleas that have been associated with flea blockage and plague transmission. This increased mortality is not caused simply by an increased bacterial load in fleas containing pla+ bacteria because fleas ingesting pla+ strains contained no more bacteria by flea blot hybridization analysis than did those that ingested the pla- strain K10-5. It is anticipated that further work in this area will clarify the mechanism by which pla acts and will reveal additional genetic loci in the plague bacillus which are required for transmission by fleas.