Antimicrobial susceptibility testing of Kingella kingae with broth microdilution and disk diffusion using EUCAST recommended media

ObjectivesIncreasing use of improved culture techniques and sensitive nucleic acid amplification assays have resulted in recognition of Kingella kingae as an important cause of invasive infections in young children, especially in septic arthritis, osteomyelitis, bacteraemia, and endocarditis. In 2016, EUCAST established clinical MIC breakpoints for K. kingae (published in EUCAST Clinical Breakpoint Tables v 7.0, 2017). The present study was carried out to produce MIC-zone diameter correlations for K. kingae on an international collection of isolates, with the aim of suggesting zone diameter breakpoints corresponding to the clinical MIC breakpoints.MethodsAntimicrobial susceptibility testing was performed for 18 clinically relevant agents on a collection of 159 clinical isolates of K. kingae. Broth microdilution MIC determination and disk diffusion were performed according to EUCAST recommendations for fastidious organisms.ResultsThe correlation between MICs and zone diameters was good for all agents with EUCAST breakpoints for K. kingae. β-lactamase was detected in 41 isolates (26%) and these isolates were resistant to aminopenicillins. These isolates were also resistant to trimethoprim-sulfamethoxazole. Resistance to tetracyclines was detected in 8% of all isolates. All resistant isolates were correctly categorized for these agents with the proposed zone diameter breakpoints. One isolate, resistant to erythromycin but susceptible to other macrolides, was categorized as susceptible with erythromycin disk diffusion. No resistance was detected for the cephalosporins, carbapenems, and fluoroquinolones tested.ConclusionBased on the results in this study, zone diameter breakpoints for K. kingae calibrated to EUCAST clinical MIC breakpoints were proposed and approved by EUCAST.     

Evaluation of Etest and disk diffusion methods compared with broth microdilution antifungal susceptibility testing of clinical isolates of Candida spp. against posaconazole

We performed Etest, disk diffusion, and broth microdilution susceptibility testing of 2,171 clinical isolates of Candida spp. against posaconazole. By using provisional breakpoints for comparison purposes only, the categorical agreement between the agar-based methods and broth microdilution results ranged from 93 to 98%, with <1% very major errors. The essential agreement (within 2 well dilutions) between the Etest and broth microdilution methods was 94%. These agar-based methods hold promise as simple and reliable methods for determination of the posaconzole susceptibilities of Candida spp.     

Evaluation of disk diffusion and Etest compared to broth microdilution for antifungal susceptibility testing of posaconazole against clinical isolates of filamentous fungi

We performed Etest, disk diffusion, and broth microdilution susceptibility testing of posaconazole against 146 clinical isolates of filamentous fungi. By using provisional breakpoints for comparison purposes only, categorical agreement between the results of the agar-based methods and those of broth microdilution were 96 to 98%, with no very major errors. These agar-based methods hold promise as simple and reliable methods for determining the posaconazole susceptibilities of filamentous fungi.     

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

ObjectivesTo evaluate the accuracy and reproducibility of antimicrobial susceptibility testing methods in Burkholderia cepacia complex (BCC).MethodsMinocycline, 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.ResultsBMD₃₅ 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%.ConclusionsBMD 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.     

Comparison of a highly automated 5-h susceptibility testing system, the Cobas-Bact, with two reference methods: Kirby-Bauer disk diffusion and broth microdilution.

The results of susceptibility tests performed with the Cobas-Bact system were compared with those of the Kirby-Bauer disk diffusion and the broth microdilution methods. The evaluation included tests with 24 antibiotics against 250 isolates of the family Enterobacteriaceae and 13 antibiotics against 100 gram-positive cocci. Complete agreements between the Cobas-Bact and Kirby-Bauer methods were 82.8 and 84.5% for gram-positive cocci and gram-negative bacilli, respectively. Agreements between the Cobas-Bact and broth microdilution methods were 76.7% for gram-positive cocci and 84.8% for gram-negative bacilli. Complete agreements between the Kirby-Bauer and broth microdilution methods were 87.0% for gram-positive cocci and 92.2% for gram-negative bacilli. Despite generally satisfactory results with most organism-antibiotic combinations tested, additional modifications of the Cobas-Bact system are required to reduce the number of major and very major discrepancies, as well as to permit testing of Pseudomonas spp. and other gram-negative nonfermentative bacilli.