In vitro activity of ceftolozane/tazobactam alone and in combination with amikacin against MDR/XDR Pseudomonas aeruginosa isolates from Greece.

OBJECTIVES: We evaluated the in vitro activity of ceftolozane/tazobactam and comparator agents against MDR non-MBL Pseudomonas aeruginosa isolates collected from nine Greek hospitals and we assessed the potential synergistic interaction between ceftolozane/tazobactam and amikacin. METHODS: A total of 160 non-MBL P. aeruginosa isolates collected in 2016 were tested for susceptibility to ceftolozane/tazobactam and seven comparator agents including ceftazidime/avibactam. Time-kill assays were performed for synergy testing using ceftolozane/tazobactam 60 or 7.5 mg/L, corresponding to the peak and trough concentrations of a 1.5 g q8h dose, respectively, in combination with 69 mg/L amikacin, corresponding to the free peak plasma concentration. Synergy was defined as a ≥2 log10 cfu/mL reduction compared with the most active agent. RESULTS: Overall, ceftolozane/tazobactam inhibited 64.4% of the P. aeruginosa strains at ≤4 mg/L. Colistin was the most active agent (MIC50/90, 0.5/2 mg/L; 96.3% susceptible) followed by ceftazidime/avibactam (MIC50/90, 4/16 mg/L; 80.6% susceptible). GES-type enzymes were predominantly responsible for ceftolozane/tazobactam resistance; 81.6% of the non-producers were susceptible. MICs for the P. aeruginosa isolates selected for synergy testing were 2-32 mg/L ceftolozane/tazobactam and 2-128 mg/L amikacin. The combination of ceftolozane/tazobactam with amikacin was synergistic against 85.0% of all the isolates tested and against 75.0% of the GES producers. No antagonistic interactions were observed. CONCLUSIONS: Ceftolozane/tazobactam demonstrated good in vitro activity against MDR/XDR P. aeruginosa clinical isolates, including strains with co-resistance to other antipseudomonal drugs. In combination with amikacin, a synergistic interaction at 24 h was observed against 85.0% of P. aeruginosa strains tested, including isolates with ceftolozane/tazobactam MICs of 32 mg/L or GES producers.

Evaluation of in vitro methods for testing tigecycline combinations against carbapenemase-producing Klebsiella pneumoniae isolates.

OBJECTIVES: Treatment of infections caused by carbapenemase-producing Klebsiella pneumoniae (CPKP) frequently involves combination therapy with various antimicrobial agents in the hope of achieving synergistic effects. Routine laboratory antimicrobial synergy testing is a service that is currently unavailable owing to the laborious nature of the reference time-kill assay (TKA) as well as the widely used chequerboard method. In this study, we explored whether easier methods, based on the Etest technique, might offer a suitable alternative. METHODS: In vitro interactions of tigecycline combination with colistin, gentamicin, fosfomycin or meropenem against 26 CPKP isolates were evaluated employing the TKA, chequerboard method and three Etest methodologies (the MIC/MIC ratio, the cross formation and the agar/Etest method). Rates of consequent synergy and concordance of the studied methods were determined. RESULTS: All antimicrobial combinations demonstrated some degree of synergy against the CPKP isolates tested. No antagonism was observed for any of the combinations. All methods showed poor synergy concordance with the TKA, producing non-significant kappa (κ) results. Etest methods (MIC/MIC ratio and agar/Etest) exhibited fair agreement (κ=0.29 and 0.38, respectively) with the chequerboard method. CONCLUSION: There is a poor correlation between synergy testing methods of tigecycline combinations, which may be associated with their different endpoints. To elucidate method comparability and reliability, their correlation with clinical outcomes appears important.

Nationwide surveillance of resistance rates of Staphylococcus aureus clinical isolates from Greek hospitals, 2012-2013.

Purpose To evaluate the in vitro efficacy of several anti-staphylococcal agents against a nationwide collection of contemporary Staphylococcus aureus clinical isolates from several healthcare centres in Greece. Methods Thirty hospitals throughout Greece (18 in Attica) provided all clinical isolates of S.aureus from April 2012 to May 2013 to a central lab to be re-submitted to susceptibility testing. The MICs were evaluated by Vitek® 2 with the exception of ceftaroline (OXOID M.I.C. Evaluator™). Vancomycin and daptomycin MICs were also evaluated by Etest®. Heterogeneously vancomycin-intermediate strains (hVISA) were detected by the Etest® GRD. VISA phenotype was confirmed by PAP-AUC. Results A total of 1005 isolates (39% MRSA) were studied. Susceptibility rates were: erythromycin 66.5%, clindamycin 79.2%, SXT 98.9%, rifampicin 97.3%, fusidic acid 67%, moxifloxacin 78.8%, vancomycin 99.9%, ceftaroline 92.9% and linezolid, tigecycline and daptomycin 100%. For mupirocin, high level resistance could be excluded for 98.9% of isolates. Vancomycin Etest® MIC50/90 were 1.5/1.5 mg/L, 58.5% of isolates exhibited a MIC > 1 and 8.7% a MIC of 2 mg/L, while Vitek® MIC50/90 were 1/1 and 3.1% showed MIC > 1 mg/L. One VISA strain was detected. Among the selected 175 isolates that were screened for hVISA phenotype, six (3.4%) were positive. In 315 bloodstream isolates, 64.1% had a vancomycin Etest® MIC > 1 mg/L. Conclusions This multi-centre surveillance study revealed that a significant percentage of contemporary S.aureus isolates from Greek patients have a vancomycin MIC (> 1 mg/L) that may compromise the clinical efficacy of the drug for the treatment of serious infections. The in vitro activity of SXT, rifampicin, mupirocin, linezolid, tigecycline, daptomycin and ceftaroline remains excellent.