As shown in Table 2, mutations in mefE-mel of the serotype 6B strains S15 and S125 resulted in a significant decrease in TEL-MIC to the level of ATCC 49619 (<0.015 μg mL−1), which is used as a standard drug-susceptible strain. EM-MICs were also reduced to the level of ATCC 49619 (<0.5 μg mL−1). It is therefore concluded that mefE-mel is the determinant solely responsible for reduced TEL susceptibility and EM resistance in these clinical isolates. The mefE-mel mutation in strain S88 (TEL-MIC 1 μg mL−1), harboring both mefE-mel and ermB, resulted in a moderate reduction in TEL-MIC to 0.12 μg mL−1. Independent disruption of
S88 ermB resulted in a similar effect on TEL susceptibility (MIC 0.12 μg mL−1). In Obeticholic Acid manufacturer contrast, disruption of both the mefE-mel and the ermB determinants further reduced TEL-MIC to the level of ATCC 49619 (MIC<0.015 μg mL−1). Similar results were obtained when the mutants were constructed independently from strains S120 and
S43, which carry both mefE-mel and ermB elements. Taken together, the results suggest that reduced TEL susceptibility (TEL-MIC 1 μg mL−1) in S. INCB024360 supplier pneumoniae may be caused by the acquisition of the mefE-mel element only and conferred additionally by the ermB element. The disruption of ermB resulted in drastic decreases in resistance to EM; MIC declined from >512 to 4 μg mL−1. However, the mefE-mel mutations did not significantly affect resistance. Additional mefE-mel mutations
in the ermB mutants reduced EM-MICs to the level of ATCC (MIC 0.5 μg mL−1). These results suggest that ermB is a predominant mechanism for high resistance to EM in the pneumococcal isolates harboring both ermB and mefE-mel determinants, although the efflux assembly confers low-level resistance. Sequence analyses of the five isolates revealed no mutations in 23S rRNA gene domains II or V. There were no mutations in the L4 ribosomal protein from any isolate, except that from strain S43, in which the S20N mutation was found (data not shown). No mutations were found in the L22 ribosomal protein from any isolate. It has been demonstrated that the mefE and mel carried by mega may be a part of Tn2009, a composite element in which mega is integrated into a Tn916-like transposon carrying tetM (Franke & Clewell, 1981; Del Grosso et al., 2004). The presence of tetM has been examined PKC inhibitor in isolates S15, S36, S89, S105 and S125, which express tetracycline resistance (MICs 16 μg mL−1), using PCR with the primers TETM1 and TETM2 (Del Grosso et al., 2004). This primer set produced an amplicon of approximately 2.0 kb, indicating the presence of tetM. The linkage between mefE-mel and tetM in these strains was investigated by Southern hybridization based on the restriction cleavage map constructed from the sequence (accession number AF376746). In these five isolates, mefE-mel and tetM were in close proximity, as shown in Tn2009 (data not shown).