The multidrug-resistant mutant Streptococcus pneumoniae M22 constitutively overexpresses two genes (patA and patB) that encode proteins homologous to known efflux proteins belonging to the ABC transporter family. It is shown here that PatA and PatB were strongly induced by quinolone antibiotics and distamycin in fluoroquinolone-sensitive strains. PatA was very important for growth of S. pneumoniae, and it could not be disrupted in strain M22. PatB appeared to control metabolic activity, particularly in amino acid biosynthesis, and it may have a pivotal role in coordination of the response to quinolone antibiotics. The induction of PatA and PatB by antibiotics showed a pattern similar to that exhibited by SP1861, a homologue of ABC-type transporters of choline and other osmoprotectants. A second group of quinolone-induced transporter genes comprising SP1587 and SP0287, which are homologues of, respectively, oxalate/formate antiporters and xanthine or uracil permeases belonging to the major facilitator family, showed a different pattern of induction by other antibiotics. There was no evidence for the involvement of PmrA, the putative proton-dependent multidrug transporter that has been implicated in norfloxacin resistance, in the response to quinolone antibiotics in either the resistant mutant or the fluoroquinolone-sensitive strains.Large epidemiological studies of Streptococcus pneumoniae in clinical infections have associated mutations in the genes encoding gyrase and topoisomerase IV with fluoroquinolone resistance (13, 40). However, resistance to fluoroquinolones can also be mediated by active efflux (5, 9-11, 17, 18, 22, 34, 44, 53). Until recently, the only efflux pump implicated in pneumococcal fluoroquinolone resistance was PmrA (22), but it now appears that there must be other efflux pumps involved in this resistance phenotype (11,45). Multidrug-resistant strain M22, a mutant selected after exposure of S. pneumoniae NCTC 7465 (strain M4) to ciprofloxacin, appeared to have such an efflux-mediated resistance mechanism (46). The mutation frequency of 6.9 ϫ 10 Ϫ8 and stable resistance without antibiotic pressure suggested a mutation in a single gene (46). However, no mutations in the fluoroquinolone resistance-determining regions of the A subunits of DNA gyrase or topoisomerase IV have been detected (36). Strain M22 was at least fourfold more resistant than strain M4 to ciprofloxacin, norfloxacin, acriflavine, ethidium bromide, doxorubicin, tetracycline, erythromycin, and cetrimide. The MICs of clinafloxacin, gatifloxacin, grepafloxacin, levofloxacin, and sitafloxacin were reproducibly twofold higher for strain M22 than for strain M4, but those of moxifloxacin, ofloxacin, sparfloxacin, and chloramphenicol were identical for the two strains. The accumulation of ciprofloxacin, gatifloxacin, and ofloxacin by strain M22 was significantly less than that observed in strain M4, whereas the accumulation of norfloxacin and ethidium was consistently higher than in strain M4. Addition of reserpine increased the uptake...

Streptococcus pneumoniae M22 is a multidrug-resistant mutant selected after exposure of capsulated wildtype S. pneumoniae NCTC 7465 (strain M4) to ciprofloxacin. DNA microarray analysis comparing the gene expression profiles of strain M22 with those of strain M4 showed that strain M22 constitutively expressed 22 genes at levels higher than those observed in strain M4 under all conditions studied. These included the genes encoding the enzymes involved in branched-chain amino acid biosynthesis and two genes (patA and patB) with sequences suggestive of ABC transporter proteins. Expression of the patA and patB genes was induced by ciprofloxacin in both strains, but in strain M4 it only reached the levels observed in strain M22 after long incubation with high concentrations of ciprofloxacin. The altered expression profile observed with strain M22 suggested that the mutation or mutations acquired during resistance selection bring the cell into a state in which the expression of critical genes is preemptively altered to correct for the potential effects of ciprofloxacin on gene expression in the parent strain.