Plasmid-mediated rifampicin resistance in Pseudomonas fluorescens

Rifampicin is an antibiotic mostly used to treat tuberculosis and leprosy, and, occasionally, other diseases. Resistance is due to alterations in membrane permeability or to mutation in the rpoB gene coding for mRNA polymerase. Both these mechanisms originate via chromosomal mutation. However, a rifampicin-resistant Pseudomonas fluorescens strain harboured a multiresistance plasmid which transferred rifampicin resistance when transformed into P. putida or Escherichia coli. Rifampicin readily diffused into the sensitive cells of the E. coli and P. putida recipients, but the transformants with the plasmid, pSCL were resistant to the drug and did not accumulate it. Potassium cyanide restored the diffusion of rifampicin into the resistant cells, indicating that an efflux pump was involved in the resistance mechanism. The resistance of the transformants and the wild strain was also abolished in sphaeroplasts generated by EDTA lysozyme treatment. Analysis of membrane proteins by SDS-PAGE revealed the presence of two new proteins in the plasmid-containing cells of E. coli, P. putida and P. fluorescens and not in the plasmid-free cells. These may be involved in the efflux of rifampicin.