The MICs of trovafloxacin, ciprofloxacin, ofloxacin, and sparfloxacin at which 90% of isolates are inhibited for 55 isolates of pneumococci were 0.125, 1, 4, and 0.5 microgram/ml, respectively. Resistant mutants of two susceptible isolates were selected in a stepwise fashion on agar containing ciprofloxacin at 2 to 10 times the MIC. While no mutants were obtained at the highest concentration tested, mutants were obtained at four times the MIC of ciprofloxacin (4 micrograms/ml) at a frequency of 1.0 x 10(-9). Ciprofloxacin MICs for these first-step mutants ranged from 4 to 8 micrograms/ml, whereas trovafloxacin MICs were 0.25 to 0.5 microgram/ml. Amplification of the quinolone resistance-determining region of the grlA (parC; topoisomerase IV) and gyrA (DNA gyrase) genes of the parents and mutants revealed that changes of the serine at position 80 (Ser80) to Phe or Tyr (Staphylococcus aureus coordinates) in GrlA were associated with resistance to ciprofloxacin. Second-step mutants of these isolates were selected by plating the isolates on medium containing ciprofloxacin at 32 micrograms/ml. Mutants for which ciprofloxacin MICs were 32 to 256 micrograms/ml and trovafloxacin MICs were 4 to 16 micrograms/ml were obtained at a frequency of 1.0 x 10(-9). Second-step mutants also had a change in GyrA corresponding to a substitution in Ser84 to Tyr or Phe or in Glu88 to Lys. Trovafloxacin protected from infection mice whose lungs were inoculated with lethal doses of either the parent strain or the first-step mutant. These results indicate that resistance to fluoroquinolones in S. pneumoniae occurs in vitro at a low frequency, involving sequential mutations in topoisomerase IV and DNA gyrase. Trovafloxacin MICs for wild-type and first-step mutants are within clinically achievable levels in the blood and lungs of humans.

High throughput chemical file screening with an enzymatic assay to detect inhibitors of the ErmC methyltransferase enzyme from macrolide-lincosamide-streptogramin B (MLSB) resistant pathogenic bacteria identified low molecular weight compounds that had IC5Os (50% inhibitory concentration) in the nMolar to^Molar range. These sameinhibitors were assessed in vitro for their capacity to inhibit the liver enzyme,cathechol-0-methyltransferase and the prokaryotic enzyme, EcoBl methylase. Selective inhibitors of the ErmC methyltransferase were tested in tertiary assays to determine their minimal inhibitory concentrations (MICs), as single agents and in combination with the macrolide, azithromycin, against strains of pathogenic bacteria expressing MLSB-resistance. Compoundsthat were active in vitro, alone or in combination with azithromycin, against strains of macrolide-resistant pathogens were tested in a mouse model of infection using an MLSB-resistant strain of Staphylococcusaureus or a macrolide-susceptible strain of Streptococcus pyogenes.

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