Multicenter Clinical Evaluation of Etest Meropenem-Vaborbactam (bioMérieux) for Susceptibility Testing of Enterobacterales (Enterobacteriaceae) and Pseudomonas aeruginosa.

Meropenem-vaborbactam (MEV) is a novel carbapenem-beta-lactamase inhibitor combination antibiotic approved by the U.S. Food and Drug Administration (FDA) for treatment of complicated urinary tract infections, including pyelonephritis, in adults. In this study, we evaluated the performance of Etest MEV (bioMérieux, Marcy l'Etoile, France) compared to that of broth microdilution for 629 Enterobacterales and 163 Pseudomonas aeruginosa isolates. According to CLSI/FDA breakpoints, 13 Enterobacterales isolates (12 clinical and 1 challenge) were resistant to MEV. Overall, Etest MEV demonstrated 92.4% essential agreement (EA), 99.2% category agreement (CA), 0% very major errors (VME), 0% major errors (ME), and 0.8% minor errors (mE) with clinical and challenge isolates of Enterobacterales Individual species demonstrated EA rates of ≥80%, with the exception of Proteus mirabilis, for which clinical and challenge isolates demonstrated 34.3% EA, 97.1% CA, 0% ME, and 2.9% mE, precluding the use of Etest MEV with this species. Excluding P. mirabilis, MEV Etest MEV demonstrated 95.8% EA, 99.3% CA, 0% VME, 0% ME, and 0.7% mE with Enterobacterales isolates. When evaluated using European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints, Etest MEV performance with clinical (16 MEV resistant) and challenge (12 MEV resistant) isolates of Enterobacterales (excluding P. mirabilis) and P. aeruginosa demonstrated an unacceptably high VME rate of 7.1% despite 95.2% EA, 99.2% CA, and 0.5% ME compared to the reference method. In conclusion, we report that Etest MEV is accurate and reproducible for MEV susceptibility testing for P. aeruginosa and Enterobacterales, with the exception of P. mirabilis, using CLSI/FDA breakpoints. Etest MEV should not be used with P. mirabilis due to unacceptable analytical performance.

Gut microbiology of UK care home residents: a cross-sectional analysis from a randomised controlled trial.

OBJECTIVE: To describe the prevalence of potentially clinically relevant gut pathogens and associations with the carriage of resistant organisms in UK care home residents. METHODS: Stool samples were collected pre-randomisation from care home residents participating in a randomised placebo-controlled trial. Cultivable clinically relevant bacteria were analysed. Antimicrobial susceptibility testing was performed by agar dilution (amoxicillin, co-amoxiclav, gentamicin, trimethoprim, nitrofurantoin, and ciprofloxacin). We also aimed to detect resistance to third-generation cephalosporins, carbapenems, and vancomycin. RESULTS: Stool samples were available for 159/310 residents participating in the trial (51%) from 23 care homes between 2016 and 2018. In total, 402 bacterial isolates were cultured from 158 stool samples and 29 different species were cultured. The five most common species were Escherichia coli (155/158, 98%), Pseudomonas aeruginosa (40/158, 25%), Enterococcus faecalis (35/158, 22%), Enterococcus faecium (30/158, 19%), and Proteus mirabilis (25/158, 16%). Enterobacterales isolates were cultured from 157 samples (99%), and resistance to at least one of the tested antimicrobials was found in 119 of these (76%). There were high levels of variation in outcomes by care home. DISCUSSION: We demonstrated that care home residents harbour significant levels of antimicrobial-resistant organisms in their stool. This work emphasises the importance of both enhanced infection control practices and antimicrobial stewardship programmes to support the appropriate use of antimicrobials in this setting.

Evaluation of susceptibility testing methods for Burkholderia cepacia complex: a comparison of broth microdilution, agar dilution, gradient strip and EUCAST disc diffusion.

OBJECTIVES: To evaluate the accuracy and reproducibility of antimicrobial susceptibility testing methods in Burkholderia cepacia complex (BCC). METHODS: Minocycline, ciprofloxacin, trimethoprim/sulphamethoxazole, meropenem, ceftazidime and chloramphenicol were tested against 155 BCC strains using broth microdilution at 35 ± 1°C (BMD35) in triplicate, then BMD at 30 ± 1°C (BMD30), agar dilution at 30°C and 35°C (AD30 and AD35), gradient strip (GS) and EUCAST standardized disc diffusion (DD) testing methods once. RESULTS: BMD35 reproducibility ranged from 70% to 84.5% for all agents. Correlations of MICs from BMD35 with BMD30 ranged from 63% to 85%, with AD35 from 32.9% to 87% and with GS methods from 36% to 83.9%. Essential agreement (EA) of MICs by GS with BMD35 ranged from 62.6% (trimethoprim-sulphamethoxazole) to 83.9% (minocycline). EA of EUCAST DD zone diameters using CLSI breakpoint criteria was between 85.8% and 97.4%, however Very Major Errors (VME) for trimethoprim/sulphamethoxazole were 31%. CONCLUSIONS: BMD at 35 ± 1°C was poorly reproducible for most agents and no method showed acceptable performance. Of particular concern were the GS results. Although this is the most commonly used method for determining MICs in laboratories, there was poor correlation with BMD35 for meropenem and trimethoprim/sulphamethoxazole. EUCAST DD correlated poorly with BMD35 MICs. This study confirms that no susceptibility method is capable of providing reproducible and accurate MICs when testing BCC.

Gold standard susceptibility testing of fosfomycin in Staphylococcus aureus and Enterobacterales using a new agar dilution panel®.

OBJECTIVES: Many clinical laboratories have difficulty in routinely performing in vitro fosfomycin susceptibility testing using the agar dilution (AD) method, considered to be the gold standard method. The objective of our work was to evaluate a rapid commercial fosfomycin agar dilution panel against clinical Staphylococcus aureus and Enterobacterales strains, in two different centres located in Italy and in the UK. METHODS: A total of 99 Enterobacterales (mostly Escherichia coli and Klebsiella pneumoniae) and 80 S. aureus clinical isolates was used to evaluate the commercial device, a 12-well panel containing fosfomycin incorporated into CA-MH agar supplemented with 25mg/L of glucose-6-phosphate (Liofilchem S.r.l., Roseto degli Abruzzi, Italy). Testing was performed in two centres (Italy and UK) and kit results were compared against the gold standard in-house AD MIC method. RESULTS: According to the EUCAST breakpoints, fosfomycin inhibited 61% of the S. aureus strains, and 76% of the Enterobacterales isolates tested by the AD reference method. There was a Categorical Agreement (CA) of 100% and an Essential Agreement (EA) of 91.25% for S. aureus; while the Enterobacterales strains showed a CA of 94% and an EA of 97%. No evaluation errors were observed among S. aureus, while 5% Major Error and 1% Very Major Error were observed for the Enterobacterales. CONCLUSIONS: Our results confirmed the feasibility of determining fosfomycin susceptibility using a commercial AD panel as a routine substitution for the AD test. The few differences observed were only in strains with MICs around the breakpoint used.

Molecular epidemiology of methicillin-resistant Staphylococcus aureus from bacteraemia in northern Italy.

BACKGROUND: Vancomycin is frequently used in the treatment of methicillin-resistant Staphylococcus aureus (MRSA) bacteraemia; reduced susceptibility to vancomycin is therefore disturbing. METHODS: molecular epidemiological analysis of 81 MRSA bacteraemia isolates collected during 2002-10 in the province of Bolzano, northern Italy was performed. MICs of a range of antimicrobials were determined by agar microdilution, screening for hGISA was by Macro-Etest and Etest GRD and confirmed by PAP-AUC with vancomycin and teicoplanin. All isolates were characterised by toxin gene profiling, agr, spa, and SCCmec-typing; MLST and PFGE were carried out on representative strains. RESULTS: The dominant clones identified were ST8-MRSA-IVc (55%) and ST228- and ST111-MRSA-I (25%); most of the latter two lineages (19/20; 95%) were GISA or PAP-AUC confirmed hGISA. One ST8-MRSA-IVc isolate harboured ccrA2B2 together with ccrA4B4. The remainder were diverse genotypically and belonged to MLST clonal complexes 1, 22, 45 and 398. CONCLUSIONS: Diverse lineages of MRSA were identified as causing bacteraemia in a province in northern Italy. The association of a specific genotype with the hGISA and GISA phenotypes among representatives of the second most common lineage identified is of clinical concern.