Next-Generation Sequence Analysis Reveals Transfer of Methicillin Resistance to a Methicillin-Susceptible Staphylococcus aureus Strain That Subsequently Caused a Methicillin-Resistant Staphylococcus aureus Outbreak: a Descriptive Study

ABSTRACT Resistance to methicillin in Staphylococcus aureus is caused primarily by the mecA gene, which is carried on a mobile genetic element, the staphylococcal cassette chromosome mec (SCC mec ). Horizontal transfer of this element is supposed to be an important factor in the emergence of new clones of methicillin-resistant Staphylococcus aureus (MRSA) but has been rarely observed in real time. In 2012, an outbreak occurred involving a health care worker (HCW) and three patients, all carrying a fusidic acid-resistant MRSA strain. The husband of the HCW was screened for MRSA carriage, but only a methicillin-susceptible S. aureus (MSSA) strain, which was also resistant to fusidic acid, was detected. Multiple-locus variable-number tandem-repeat analysis (MLVA) typing showed that both the MSSA and MRSA isolates were MT4053-MC0005. This finding led to the hypothesis that the MSSA strain acquired the SCC mec and subsequently caused an outbreak. To support this hypothesis, next-generation sequencing of the MSSA and MRSA isolates was performed. This study showed that the MSSA isolate clustered closely with the outbreak isolates based on whole-genome multilocus sequence typing and single-nucleotide polymorphism (SNP) analysis, with a genetic distance of 17 genes and 44 SNPs, respectively. Remarkably, there were relatively large differences in the mobile genetic elements in strains within and between individuals. The limited genetic distance between the MSSA and MRSA isolates in combination with a clear epidemiologic link supports the hypothesis that the MSSA isolate acquired a SCC mec and that the resulting MRSA strain caused an outbreak.

In VivoEfficacy of Humanized Ceftaroline Fosamil-Avibactam Exposures in a Polymicrobial Infection Model

ABSTRACTAlthough Gram-positive cocci are the most common pathogens in diabetic foot infections, these infections often are polymicrobial. The objective of this study was to assess the efficacy of a simulated human dose of 600 mg ceftaroline fosamil–600 mg avibactam every 8 h as a 1-h infusion in a polymicrobialin vivomurine model. Seven isolates were used (3 methicillin-resistantStaphylococcus aureus[MRSA] isolates, 1 methicillin-susceptibleS. aureus[MSSA] isolate, 1Escherichia coliisolate, 1Enterobacter cloacaeisolate, and 1Bacteroides fragilisisolate) in various combinations in an immunocompromised polymicrobial tissue infection to assess the efficacy of the simulated regimen. Each infection was comprised of at least oneS. aureusisolate with a MIC of 0.25 to 1 μg/ml and oneEnterobacteriaceaeisolate with a MIC of 1 or 4 μg/ml. Eight of 16 infections also includedB. fragilis, with a MIC of 0.5 μg/ml, as a third organism. Efficacy was evaluated after 24 h as the change in log10CFU from the level of 0-h controls. Efficacy was seen against all isolate combinations, with at least a 1-log kill againstEnterobacteriaceaeand a minimum of a 2-log kill againstS. aureusandB. fragilisisolates. These bacterial reductions correlate with free drug concentration above the MIC (fT>MIC) produced by the humanized regimen of 100, 86, and 56% at MICs of 1, 2, and 4 μg/ml, respectively. The humanized regimen of 600 mg ceftaroline fosamil–600 mg avibactam every 8 h as a 1-h infusion showed predictable efficacy against all infections tested in this model. These data support further clinical investigation of ceftaroline fosamil-avibactam for the treatment of polymicrobial tissue infections.

Daptomycin Activity against Staphylococcus aureus following Vancomycin Exposure in an In Vitro Pharmacodynamic Model with Simulated Endocardial Vegetations

ABSTRACT Recently, the emergence of reduced susceptibility to daptomycin has been linked to the reduced vancomycin susceptibility that occurs after vancomycin exposure in Staphylococcus aureus in vivo and in vitro. This study evaluated this propensity in clinical isolates of S. aureus using an in vitro pharmacokinetic/pharmacodynamic model with simulated endocardial vegetations over 8 days. Five clinical isolates (four methicillin-resistant S. aureus isolates and one methicillin-susceptible S. aureus [MSSA] isolate), all of which were reported to have become nonsusceptible to daptomycin, were evaluated. The following regimens were evaluated: vancomycin 1 g every 12 h for 4 days followed by daptomycin 6 mg/kg of body weight daily for 4 days and daptomycin 6 mg/kg daily for 8 days. If nonsusceptibility was detected, the following regimens were evaluated: no treatment for 4 days followed by daptomycin 6 mg/kg daily for 4 days, vancomycin 1 g every 12 h for 4 days followed by daptomycin 10 mg/kg daily for 4 days, and daptomycin 10 mg/kg daily for 8 days. The emergence of daptomycin nonsusceptibility (12- to 16-fold MIC increase) was detected only with the MSSA isolate with daptomycin 6 mg/kg daily for 4 days after vancomycin exposure. However, the bactericidal activity of daptomycin was maintained and the MIC increases of these isolates, which had no mprF or yycG mutations, were unstable to serial passage on antibiotic-free agar. Subsequent regimens did not demonstrate nonsusceptibility to daptomycin. These findings suggest that reduced daptomycin susceptibility can be a strain-specific and unstable event. Further evaluation of the susceptibility relationship between daptomycin and vancomycin is necessary to understand the factors involved and their clinical significance.