[Elimination of various virulence plasmids by antibiotics].

The pathogenicity of some enterobacteria is due to a plasmid encoding for outer membrane proteins or for toxins. The elimination of the plasmid gives a non-virulent strain. We have tried to eliminate plasmids encoding for the thermostable toxin of Escherichia coli (plasmid pCSltl) or for the enteroinvasive property of Shigella (plasmids pWR24, pHW401, pWR110), of Salmonella (plasmid pSD6) and of Yersinia (plasmids pYL4 and P4). The loss of plasmid was confirmed by agarose gel electrophoresis of a crude lysate. Fourteen antibiotics belonging to different chemical families were used at subinhibitory concentration. A control experiment without antibiotic was carried out to detect spontaneous loss of plasmids. No antibiotic was able to eliminate plasmids pCSltl, pHW401, pSD6, pYL4. Novobiocin eliminated pWR24 and pWR110, rifampicin eliminated pWR110 and P4. Three other antibiotics gave a cure of bacteria harbouring plasmids pWR110 and P4 but the percentage of cure was too low for a therapeutical interest.