Background: Active efflux is a common mechanism of resistance to fluoroquinolones in Streptococcus pneumoniae. Two efflux systems have been described so far in this species: PmrA, a member of the major facilitator superfamily; and the two ABC transporters PatA and PatB. We studied the inducibility of expression of pmrA, patA and patB by using subinhibitory concentrations of fluoroquinolones.
Methods: A wild-type susceptible strain, two clinical isolates resistant to fluoroquinolones and two efflux mutants selected in vitro after exposure to ciprofloxacin were studied. MICs were determined for these strains and their mutants in which pmrA, patA or patB had been disrupted. Gene expression was determined after exposure to half the MIC of norfloxacin, ciprofloxacin, levofloxacin, moxifloxacin or gemifloxacin and quantified by real-time PCR.
Results: Increased MICs of norfloxacin, ciprofloxacin and levofloxacin (to a lesser extent) and increased expression of patA and patB were seen for all resistant strains; these were reduced in patA or patB disruptants or in the presence of reserpine. Exposure to any of the five fluoroquinolones caused a reversible increase in expression of patA and patB, but not of pmrA. Mitomycin C, an inducer of the competence system in S. pneumoniae, also induced patA and patB expression in the two strains tested.
Conclusion: The ABC efflux system PatA/PatB is induced upon exposure to subinhibitory concentrations of fluoroquinolones, whether substrates of the transporter or not. This effect, possibly resulting from the activation of the competence pathway, may contribute to resistance.