Although the role of rifampicin as adjunctive MDV3100 concentration therapy is controversial [31], the combined therapy seems beneficial as long as the bacteria exhibit susceptibility to the antibiotics combined [30]. The distinctive phenotypic feature in the particular clone of ST-228 described here was the borderline resistance

to rifampicin that could be missed by some methods of antimicrobial susceptibility testing (i.e. disk diffusion or E-test). Hence our interest in studying whether this low-level RIF-R was an adaptive phenomenon or to the GSK1120212 research buy contrary, known rpoB mutations underlay such phenotype. Almost all isolates belonging to this multi-resistant MRSA clone (104/108) showed a low-level rifampicin

resistance (MICs, 1 to 4 mg/L) and carried the amino acid substitution 481His/Asn in the RNA polymerase. Only 4 isolates showed additional substitutions known to be involved in a high-level rifampicin resistance: two isolates (MICs, 128 mg/L) carried mutational change 477Ala/Thr, and one isolate (MIC, ≥ 256 mg/L) 468Gln/Lys [13, 17, 27, 32]. The fourth isolate (MIC, ≥ 256 mg/L) showed substitution 527Ile/Leu, the Capmatinib chemical structure only one which mutation was found in the rifampicin resistance-determining cluster II, described recently among Japanese MRSA isolates [32]. It is noteworthy that 20 isolates (19%) of the RIF-R isolates, carrying rpoB mutation resulting in amino acid substitution in position 481, were detected as rifampicin susceptible by the disk-diffusion test. However, the inhibition zones of these strains were between Edoxaban 20 and 23 mm, closer to the susceptibility breakpoint established by CLSI (susceptibility ≥ 20 mm) than inhibition zones among RIF-S MRSA isolates that were usually ≥ 30 mm. Therefore, if screening for rifampicin resistance is made only by disk diffusion,

special attention needs to be paid to strains borderline to the CLSI susceptibility breakpoint to avoid reporting false susceptibility results. MICs by E-test failed to detect rifampicin resistance following CLSI guidelines [11] in a group of 12 strains (MICs, 0.75-1 mg/L). These isolates showed MICs by microdilution of 2 mg/L and carried the rpoB mutation responsible for amino acid substitution in position 481. Thus, and according to other authors, it would be advisable to apply ≤ 0.5 and ≥ 8 mg/L as new breakpoints to classify rifampicin susceptibility or resistance in S. aureus clinical isolates [13, 17]. High-level rifampicin resistance could be attributable to double mutations within rpoB, as previously described [27]. We did not find in this particular clone that the presence of a prior mutational change (481His/Asn) increased the frequency of acquisition of additional mutations responsible for a higher level of rifampicin resistance, when compared to a reference strain.