In vitro activity of retapamulin against Staphylococcus aureus isolates resistant to fusidic acid and mupirocin.

OBJECTIVES: We determined the in vitro activity of retapamulin, a novel pleuromutilin antibiotic, against 664 Staphylococcus aureus isolates from the UK, including many resistant to fusidic acid and/or highly resistant to mupirocin. METHODS: MICs were determined on Mueller-Hinton agar in accordance with the CLSI guidelines. Susceptibility was categorized using CLSI criteria, where available; otherwise the European Committee for Antimicrobial Susceptibility Testing (EUCAST)/BSAC criteria were used (for mupirocin and fusidic acid). Mutations in the rplC gene, which encodes ribosomal protein L3, were sought by PCR and DNA sequencing. RESULTS: The S. aureus included 488 (73%) methicillin-resistant isolates (oxacillin MICs >2 mg/L), 336 isolates (51%) resistant to fusidic acid (MICs >1 mg/L) and 254 (38%) with high-level mupirocin resistance (MICs >256 mg/L); 103 (16%) isolates were resistant both to fusidic acid and to high levels of mupirocin. Retapamulin inhibited 663 (99.9%) isolates at < or =0.25 mg/L. A single methicillin-resistant S. aureus isolate, also with high-level mupirocin resistance, required a retapamulin MIC of 2 mg/L, but its reduced susceptibility to retapamulin was not associated with any mutation in ribosomal protein L3. CONCLUSIONS: Retapamulin demonstrated excellent activity in vitro against S. aureus isolates, irrespective of their level of resistance to other antibacterials. These results support the EUCAST epidemiological cut-off value for retapamulin of < or =0.5 mg/L against S. aureus.

IMP-4, a novel metallo-beta-lactamase from nosocomial Acinetobacter spp. collected in Hong Kong between 1994 and 1998.

Between 1994 and 1998, 97 imipenem-resistant Acinetobacter isolates were identified at the Prince of Wales Hospital, Hong Kong, China. A bla(IMP) PCR product was obtained from 23 of 35 viable cultures; 12 isolates belonged to genomic DNA group 3, 8 belonged to group 2 (Acinetobacter baumannii), 2 belonged to group 13TU, and 1 belonged to group 1. The bla(IMP) homologues were sequenced from two isolates from genomic DNA group 2 and one isolate each from groups 3 and 13TU. The four sequences included an identical 738-bp open reading frame, predicted to encode a polypeptide of 246 amino acids, with 95.6% homology to IMP-1 and 89.3% homology to IMP-2. The new enzyme, designated IMP-4, was partially purified. It had a pI of 8.0 and was strongly active against imipenem and meropenem, with V(max) values 53 and 8% of that for penicillin G, respectively. Strong activity was also seen against oxyimino-aminothiazolyl cephalosporins but not against aztreonam. Hydrolytic activity was inhibited by EDTA but not by clavulanate or tazobactam. Carbapenem MICs for most bla(IMP)-positive isolates were 4 to 32 microg/ml, but one isolate with the intact gene was susceptible, with imipenem and meropenem MICs of 0.25 and 0.5 microg/ml, respectively. The latter isolate did not produce the band with a pI of 8.0, and gene expression was inferred to have been lost. None of the isolates studied in detail contained extrachromosomal DNA, and carbapenem resistance was not transmissible to Escherichia coli. Nevertheless, the presence of bla(IMP-4) in different genomic DNA groups implies horizontal transfer, and sequences resembling a GTTRRRY integrase-dependent recombination motif were identified in the flanking regions of bla(IMP-4).

Characterization of OXA-25, OXA-26, and OXA-27, molecular class D beta-lactamases associated with carbapenem resistance in clinical isolates of Acinetobacter baumannii.

Carbapenem resistance in Acinetobacter spp. is increasingly being associated with OXA-type beta-lactamases with weak hydrolytic activity against imipenem and meropenem. Such enzymes were characterized from Acinetobacter isolates collected in Belgium, Kuwait, Singapore, and Spain. The isolates from Spain and Belgium had novel class D beta-lactamases that were active against carbapenems. These were designated OXA-25 and OXA-26, respectively, and had >98% amino acid homology with each other and with the OXA-24 enzyme recently described by others from an Acinetobacter isolate collected elsewhere in Spain. The isolate from Singapore had OXA-27 beta-lactamase, another novel class D type with only 60% homology to OXA-24, -25, and -26, but with 99% homology to OXA-23 (ARI-1), described previously from an Acinetobacter baumannii isolate collected in Scotland. Sequence data were not obtained for the carbapenem-hydrolyzing OXA enzyme from the isolate from Kuwait; nevertheless, the enzyme was phenotypically similar to OXA-25 and -26. The enzymes OXA-23, -24, -25, -26, and -27 retained the STFK and SXV motifs typical of class D beta-lactamases, but the YGN motif was altered to FGN. The KTG motif was retained by OXA-27 and -23 but was replaced by KSG in OXA-24, -25, and -26. OXA-25 and -26 enzymes were strongly active against oxacillin, but unusually for an OXA-type beta-lactamase, OXA-27 had apparently weak activity, although measurement was complicated by biphasic kinetics. None of the new enzymes was transmissible to Escherichia coli recipients. Many Acinetobacter isolates are multiresistant to other antibiotics, and the emergence of class D enzymes with carbapenem-hydrolyzing activity is a disturbing development for antimicrobial chemotherapy.

Biochemical characteristics of a carbapenemase from an Acinetobacter baumannii isolate collected in Buenos Aires, Argentina.

Three carbapenem-resistant Acinetobacter baumannii isolates were collected at a hospital in Buenos Aires, Argentina. Isoelectric focusing revealed multiple beta-lactamases, with two of the isolates showing identical profiles. A pI 6.9 carbapenemase with a molecular weight of 30 kDa was purified from one of these two isolates. The enzyme was predominantly a penicillinase, with its highest Vmax for oxacillin but highest Vmax/Km for benzylpenicillin. First-generation cephalosporins and imipenem were weaker substrates than penicillins, and oxyimino-aminothiazolyl cephalosporins were essentially stable. Meropenem-hydrolysing activity was not detected, despite resistance. The carbapenemase was inhibited by clavulanic acid and tazobactam, but not by EDTA. These kinetics place the enzyme into functional group 2; as an oxacillinase it could be placed in sub-group 2d or, as a zinc-independent carbapenemase, in sub-group 2f.