Evidence of an efflux pump in Serratia marcescens

Spontaneous mutants resistant to fluoroquinolones were obtained by exposing Serratia marcescens NIMA (wild-type strain) to increasing concentrations o f ciprofloxacin both in liquid and on solid media. Frequencies of mutation ranged from 10(-7) to 10(-9). Active expulsion of antibiotic was explored a s a possible mechanism of resistance in mutants as well as changes in topoi somerase target genes. The role of extrusion mechanisms in determining the emergence of multidrug-resistant bacteria was also examined. Mutants resist ant to high concentrations of fluoroquinolones had a single mutation in the ir gyrA QRDR sequences, whereas the moderate resistance in the rest of muta nts was due to extrusion of the drug.