Conjugative mobilization of the rolling-circle plasmid pIP823 from Listeria monocytogenes BM4293 among gram-positive and gram-negative bacteria

We determined the sequence and genetic organization of plasmid pIP823, whic h contains the dfrD gene; dfrD confers high-level trimethoprim resistance t o Listeria monocytogenes BM4293 by synthesis of dihydrofolate reductase typ e S2. pIP823 possessed all the features of the pUB110/pC194 plasmid family, whose members replicate by the rolling-circle mechanism. The rep gene enco ded a protein identical to RepU, the protein required for initiation of the replication of plasmids pTB913 from a thermophilic Bacillus sp. and pUB110 from Staphylococcus aureus. The mob gene encoded a protein with a high deg ree of amino acid identity with the Mob proteins involved in conjugative mo bilization and interplasmidic recombination of pTB913 and pUB110. The host range of pIP823 was broad and included L. monocytogenes, Enterococcus faeca lis, S. aureus, Bacillus subtilis, and Escherichia coli. In all these speci es, pIP823 replicated by generating single-stranded DNA and was stable. Con jugative mobilization of pIP823 was obtained by self-transferable plasmids between L. monocytogenes and E.faecalis, between L. monocytogenes and E. co li, and between strains off. coli, and by the streptococcal conjugative tra nsposon Tn1545 from L. monocytogenes to E. faecalis, and from L. monocytoge nes and E. faecalis to E. coli. These data indicate that the gene flux obse rved in nature from gram-positive to gram-negative bacteria can occur by co njugative mobilization. Our results suggest that dissemination of trimethop rim resistance in Listeria spp. and acquisition of other antibiotic resista nce determinants in this species can be anticipated.