Characterisation of Staphylococcus aureus and Enterococcus faecalis mutants with reduced susceptibility to the investigational oxazolidinone RWJ-416457.

RWJ-416457 is a novel investigational oxazolidinone with minimum inhibitory concentrations (MICs) to staphylococci and enterococci that are two- to four-fold lower than those of linezolid. Single-step and serial passage in vitro resistance selection experiments were performed for RWJ-416457 and linezolid with Staphylococcus aureus and Enterococcus faecalis laboratory and clinical isolates. RWJ-416457 selected for resistant mutants in single-step selections at a frequency of ≤1×10(-10), similar to that of linezolid. In serial passage selection experiments, a G2576T transversion in the domain V region of the 23S rRNA gene was the predominant mutation observed for both oxazolidinones, suggesting similar 23S rRNA binding sites. The associated development of increasing oxazolidinone resistance in E. faecalis (four 23S rRNA alleles) required fewer passages than with S. aureus isolates (six 23S rRNA alleles), and resistance was generally proportionate to the number of mutated (G2576T) 23S rRNA alleles. Fold changes in MICs were similar for both compounds, and MICs for RWJ-416457 remained two- to four-fold lower than those of linezolid for mutants selected by either compound. Serial passage of linezolid with S. aureus OC 2878 yielded a novel A2572T 23S rRNA mutation, whilst the final passages of S. aureus OC 10517 with RWJ-416457 resulted in the apparent loss of a mutated (G2576T) allele 6.

In vivo activity of ceftobiprole in murine skin infections due to Staphylococcus aureus and Pseudomonas aeruginosa.

Ceftobiprole, a broad-spectrum cephalosporin with activity against methicillin-resistant Staphylococcus aureus (MRSA) (P. Hebeisen et al., Antimicrob. Agents Chemother. 45:825-836, 2001), was evaluated in a subcutaneous skin infection model with Staphylococcus aureus Smith OC 4172 (methicillin-susceptible S. aureus [MSSA]), S. aureus OC 8525 (MRSA), Pseudomonas aeruginosa OC 4351 (having an inducible AmpC beta-lactamase), and P. aeruginosa OC 4354 (overproducing AmpC beta-lactamase). In the MSSA and MRSA infection models, ceftobiprole, administered as the prodrug ceftobiprole medocaril, was more effective in reducing CFU/g skin (P < 0.001) than were cefazolin, vancomycin, or linezolid based on the dose-response profiles. Skin lesion volumes in MSSA-infected animals treated with ceftobiprole were 19 to 29% lower than those for cefazolin-, vancomycin-, or linezolid-treated animals (P < 0.001). In MRSA infections, lesion size in ceftobiprole-treated mice was 34% less than that with cefazolin or linezolid treatment (P < 0.001). Against P. aeruginosa, ceftobiprole at similar doses was as effective as meropenem-cilastatin in reductions of CFU/g skin, despite 8- and 32-fold-lower MICs for meropenem; both treatments were more effective than was cefepime (P < 0.001) against the inducible and overproducing AmpC beta-lactamase strains of P. aeruginosa. Ceftobiprole was similar to meropenem-cilastatin and 47 to 54% more effective than cefepime (P < 0.01) in reducing the size of the lesion caused by either strain of P. aeruginosa in this study. These studies indicate that ceftobiprole is effective in reducing both bacterial load and lesion volume associated with infections due to MSSA, MRSA, and P. aeruginosa in this murine model of skin and soft tissue infection.

Antimicrobial efficacy and ocular cell toxicity from silver nanoparticles.

Silver in various forms has long been recognized for antimicrobial properties, both in biomedical devices and in eyes. However, soluble drugs used on the ocular surface are rapidly cleared through tear ducts and eventually ingested, resulting in decreased efficacy of the drug on its target tissue and potential concern for systemic side effects. Silver nanoparticles were studied as a source of anti-microbial silver for possible controlled-release contact lens controlled delivery formulations. Silver ion release over a period of several weeks from nanoparticle sources of various sizes and doses in vitro was evaluated in vitro against Pseudomonas aeruginosa strain PA01. Mammalian cell viability and cytokine expression in response to silver nanoparticle exposure is evaluated using corneal epithelial cells and eye-associated macrophages cultured in vitro in serum-free media. Minimal microcidal and cell toxic effects were observed for several silver nanoparticle suspensions and aqueous extraction times for bulk total silver concentrations commensurate with comparative silver ion (e.g., Ag(+) ((aq))) toxicity. This indicates that (1) silver particles themselves are not microcidal under conditions tested, and (2) insufficient silver ion is generated from these particles at these loadings to produce observable biological effects in these in vitro assays. If dosing allows substantially increased silver particle loading in the lens, the bactericidal efficacy of silver nanoparticles in vitro is one possible approach to limiting bacterial colonization problems associated with extended-wear contact lenses.