, 1976; Mani et al., 1993). Briefly, 100 mL cultures of S. aureus growing exponentially (OD620 nm≈0.6) in TSB medium at 37 °C with aeration were pelleted, washed twice in cold 0.05 M Tris-HCl (pH 7.2) and then resuspended in 50 mL of 0.05 M Tris-HCl (pH 7.2) containing 0.05% (v/v) Triton X-100 (Sigma Chemical Co., St. Louis, MO). The cells were incubated at 37 °C with shaking and the OD620 nm was measured at 30-min intervals for 5 h. Values reported are averages of at least three
independent experiments. The statistical significance of the data was evaluated using a Student’s t-test. To proactively examine resistance to ramoplanin, we generated a resistant strain by serial passage of S. aureus NCTC 8325-4 in the presence of sub-MICs of ramoplanin. APO866 cost The results from each passage of NCTC 8325-4 are shown in Table 1. In general, multiple passages were required for S. aureus to be able grow in the next higher concentration of ramoplanin. During the 16th see more passage, growth was observed in a culture containing 5 μg mL−1 ramoplanin. A sample from this culture was plated on TSA. An isolated colony was selected and passed twice on TSA, and then a colony
was selected and named RRSA16 for ‘ramoplanin-resistant S. aureus 16th series.’ The nucleotide sequence of the 16s rRNA genes of RRSA16 were identical to those of its S. aureus NCTC 8325-4 progenitor. The susceptibility of RRSA16 to a panel of antimicrobials focused on cell wall active compounds was determined by broth microdilution (Table 2). The ramoplanin MIC increased from 0.75 μg mL−1 for NCTC 8325-4 to 8 μg mL−1 for RRSA16. Interestingly, RRSA16 had reduced susceptibility to two other antimicrobials 4-Aminobutyrate aminotransferase that act by binding peptidoglycan lipid intermediate II, vancomycin and nisin. The vancomycin MIC increased from 1.25 μg mL−1 for NCTC 8325-4 to 9 μg mL−1 for RRSA16, a level classified as VISA. The nisin MIC increased from 10 μg mL−1 for NCTC 8325-4 to >32 μg mL−1 for RRSA16. The MIC for oxacillin, which inhibits peptidoglycan at the transpeptidation step, increased slightly from 0.25 μg mL−1 for NCTC 8325-4 to 0.5 μg mL−1 for RRSA16. No changes in the
susceptibility were observed for bacitracin, phosphomycin, d-cycloserine, ciprofloxacin, erythromycin or rifampcin. The resistant RRSA16 was passed in an antibiotic-free medium for 18 days, generating R16-18d, a strain that was more sensitive to ramoplanin and vancomycin than RRSA16 (Table 2). These values are still higher than the MICs observed for NCTC 8325-4. The nisin MIC of R16-18d remained higher than 32 μg mL−1, the highest concentration tested. We next wished to examine RRSA16 for altered growth characteristics when grown in rich media. The doubling time of RRSA16 was 41 min, almost twice as long as the 23-min doubling time observed for NCTC 8325-4. The R16-18d doubling time of 26 min was similar to the doubling time of NCTC 8325-4.