Identification and Antimicrobial Susceptibility Testing

Identification and AST were performed at the Erasmus MC for all isolates using subcultures, except for the Acinetobacter spp. provided by the CRBIP of which identification data were provided. Identification was performed using the MALDI biotyper (Bruker Daltonics, Bremen, Germany). AST was performed using the VITEK 2 system (BioMérieux, Marcy-l’Étoile, France). An overview of all isolates used for assay validation categorized by their (non-) susceptibility to ampicillin, amoxicillin-clavulanic acid, cefotaxime, ceftazidime, imipenem, meropenem, gentamicin, tobramycin, and ciprofloxacin is shown in Table 1. EUCAST-breakpoints (v 11.0, January 2021) were applied for category interpretation.

Categorization of 248 organisms by antimicrobial susceptibility testing results obtained for different classes of antibiotics.

The selected isolates that were previously collected from blood cultures and the selected isolates with less prevalent AMR mechanisms were inoculated in negative blood cultures and subsequently treated in the same way as the prospectively collected blood cultures. Antimicrobial susceptibility testing was performed for all isolates using the VITEK 2. EUCAST breakpoints (v 11.0, January 2021) were applied to categorize isolates as either susceptible (S) or non-susceptible (NS) to ampicillin (AMP), amoxicillin-clavulanic acid (AMC), cefotaxime (CTX), ceftazidime (CAZ), imipenem (IPM), meropenem (MEM), gentamicin (GEN), tobramycin (TOB), and ciprofloxacin (CIP).*Antimicrobial susceptibility testing was not performed for the A. baumannii isolates as the aim was not to predict resistance or susceptibility in A. baumannii.