Erythromycin-susceptible S. pneumoniae isolates were categorized as Group IV. Minimum inhibitory concentration (MIC) was determined by the Clinical and Laboratory Standards Institute (CLSI) (2008) broth microdilution method. In vitro susceptibility was tested for 19 antimicrobial agents including erythromycin, penicillin, amoxicillin–clavulanate, ceftriaxone, cefuroxime, cefixime, cefprozil, cefdinir, imipenem, ertapenem, ciprofloxacin, levofloxacin, moxifloxacin, gatifloxacin, clindamycin, tetracycline, trimethoprim–sulfamethoxazole, OSI-744 order rifampin,

and vancomycin. Three to five colonies from an overnight culture on 5% sheep blood agar plates (Becton-Dickinson, Sparks, MD) were resuspended in 10 mL of brain–heart infusion (BHI) broth (Difco Laboratories, Detroit, MI) and incubated for 6–8 h at 35 °C without shaking. For total viable count determination, a 100-μL aliquot selleck kinase inhibitor was diluted in 1 × phosphate-buffered saline (PBS) and plated onto 5% sheep

blood agar plates. The remaining culture was centrifuged for 5 min at 2500 g. The pellet was resuspended in 200 μL of BHI broth and plated onto 5% sheep blood agar plates containing 2 μg mL−1 rifampin (Sigma-Aldrich, St. Louis, MO). Mutation frequency values are reported as the proportion of rifampin-resistant colonies (detected after 48–72 h of incubation in a 5% CO2 atmosphere) vs. total viable cell counts (O’Neill & Chopra, 2002). Results correspond to the mean value obtained in triplicate experiments. An isolate was considered a mutator strain when its frequency was ≥7.5 × 10−8 (Morosini et al., 2003). Allelic replacement mutagenesis for determination of the recombination rate of S. pneumoniae isolates was performed and competent

cells were prepared as described previously (Song et al., 2005). A spr0476 gene that has already been reported as a nonessential gene was used as a target gene for Cediranib (AZD2171) homologous recombination (Thanassi et al., 2002). Amplification of the left and right flanking regions of spr0476 was performed using two pairs of primers, 0476L-F/L-R (5′-CAT CAG TGG AAG GAA TGG TTG ACC-3′/5′-GAC GAA CTC CAA TTC ACT GTT ATC TAC CCA CAA GAG CTT GA-3′) and 0476R-F/R-R (5′-AGA TTT AGA TGT CTA AAA AGC CAT GAA AAG CGT CGT TTG AC-3′/5′-GTT GCG ATT GCG TCC ACC TCC TCA-3′), generating PCR products of 500 and 430 bp, respectively. Primers 0476L-R and 0476R-F contained 21 nucleotides that are identical to the 5′- and 3′-ends of the kanamycin resistance gene cassette, followed by 23 bp of spr0476 gene-specific sequence. The resulting fused PCR product of 1.8 kb was directly transformed into each S. pneumoniae isolate, and homologous recombination between the construct and spr0476 in the chromosome was forced. Pneumococcal transformation was executed under the conditions described previously (Gutiérrez et al., 2004; Song et al., 2005). To estimate the rate at which the fused PCR products recombine with the chromosomal spr0476 in S.