Aim: To compare minimum inhibitory concentrations (MICs) obtained by two antimicrobial susceptibility testing (AST) methods; Etest® and Sensititre® broth microdilution for Granulicatella and Abiotrophia spp.
Background: Granulicatella adiacens, Granulicatella elegans and Abiotrophia defectiva are nutritionally fastidious organisms that require pyridoxal or L-cysteine supplementation of culture media for growth. They cause bacteremia and infective endocarditis which is associated with significant morbidity and mortality. Efficient and reliable AST methods are needed to guide treatment decisions.
Methods: Twenty-seven isolates of Granulicatella adiacens (n=13), Granulicatella elegans (n=1) and Abiotrophia defectiva (n=13) submitted to MDU PHL for identification during 2019 to 2021 were included in the study. Identification of the isolates was confirmed by 16S rRNA sequencing and/or Vitek-MS (MALDI-TOF). Isolates underwent antimicrobial susceptibility testing to ceftriaxone, clindamycin, erythromycin, meropenem, penicillin and vancomycin by Etest® on Mueller Hinton agar with 5% sheep blood with 0.001% pyridoxal hydrochloride, and Sensititre® broth microdilution using the Thermo Scientific™ Sensititre™ STP6F panel. The manufacturer supplied cation adjusted Mueller Hinton broth with 4% lysed blood was supplemented in-house with 0.001% pyridoxal hydrochloride. MICs were interpreted by the Clinical and Laboratory Standards Institute (CLSI) M45 breakpoints.
Results: When comparing MIC values and their interpretative categorisation for the above-mentioned antimicrobials by Etest® and Sensititre® broth microdilution, 100% categorical agreement (CA) was observed for erythromycin and vancomycin, 96.3% CA for meropenem and clindamycin, 91.3% for ceftriaxone and 77.8% for penicillin. 86.3% of MICs were in essential agreement (EA) overall. Determining EA was restricted for some isolates due to the limited availability of antimicrobial dilutions in the STP6F panel.
Conclusion: This study demonstrates that both Etest® and Sensititre® broth microdilution methods provide highly concordant categorical agreement for MIC values for Granulicatella and Abiotrophia spp.