AN IRON-DEPENDENT MUTANT OF LISTERIA-MONOCYTOGENES OF ATTENUATED VIRULENCE

A bank of Tn 917-insertional mutants from the facultative intracellula r pathogen Listeria monocytogenes was screened by an original method b ased on bacterial growth on synthetic medium under iron-limiting condi tions. One mutant, whose in vitro growth in synthetic medium was speci fically dependent upon the availability of iron in its environment, wa s isolated and characterized. The insertional event occurred in a non- coding region, upstream of a rrn operon and located within a 1100-kb N otI fragment of the physical map, where the virulence genes already id entified in L. monocytogenes were also present. Protein analysis by SD S-PAGE revealed a pleiotropic effect of the insertional event on cell- associated proteins, suggesting a polar effect of the transposon on ad jacent unknown gene(s). The virulence in the mouse of this mutant was strongly impaired, although it was capable in vitro of growing intrace llularly and of spreading from cell to cell, as shown by the productio n of lytic plaques on cell culture.