Elimination of a virulence plasmid from Shigella sonnei and Escherichia coli by antibiotics.

Plasmid pWR105 is a non-self-conjugative plasmid conferring enteroinvasive properties on Shigella sonnei. Loss of this plasmid is accompanied by loss of the invasive phenotype as well as of the form I antigen expression. Thirteen antibiotics belonging to different chemical families were used at subinhibitory concentrations to eliminate pWR105 from S. sonnei and Escherichia coli K12. Rifampicin, novobiocin, chloramphenicol, cotrimoxazole and erythromycin eliminated the plasmid from both strains. Clindamycin eliminated pWR105 from E. coli only. Several other antibiotics gave a low rate of cure (ciprofloxacin, nalidixic acid, oxolinic acid, nifurzide, tilbroquinol, minocycline). We may expect that these antibiotics would destabilize plasmids from other Shigella species and enteroinvasive E. coli, as these extrachromosomal DNA molecules share a high degree of homology.