Usefulness of EUCAST rapid antibiotic susceptibility breakpoints and screening cut-off values directly from blood cultures for the inference of β-lactam resistance mechanisms in Enterobacterales

Background Reducing the turnaround time for reporting antimicrobial susceptibility testing (AST) results is important for adjusting empirical treatments and may impact clinical outcomes of septic patients, particularly in settings with high antimicrobial resistance. Disc diffusion could be useful for inferring beta-lactam resistance mechanisms. Objectives To evaluate the usefulness of EUCAST rapid AST (RAST) disc diffusion breakpoints for the screening of resistance mechanisms (sRAST) and interpretive reading of resistance phenotypes to infer ESBL and carbapenemases production in Enterobacterales. Methods Blood cultures were artificially spiked with Enterobacterales clinical isolates with well-characterized beta-lactam resistance mechanisms (n = 93), WT phenotypes (n = 26) and ATCC strains (n = 8). AST was performed by disc diffusion directly from blood cultures and inhibition zones were manually measured at 4, 6 and 8 h. To infer the presence of resistance mechanisms, EUCAST RAST breakpoints and screening cut-off values (sRAST) combined with the double-disc synergy test (DDS) for ESBLs or aztreonam susceptibility for carbapenemases detection were used. Results DDS together with sRAST detected all ESBL producers as early as at 4 h incubation. Cefotaxime was the antibiotic with the highest discriminatory power. The suspicion of carbapenemase production by sRAST at 8 h was possible in 73% of Klebsiella pneumoniae and in 100% of Escherichia coli carbapenemase-producing isolates. Phenotypic analysis improves the detection of some low hydrolytic carbapenemases (OXA-48 or KPC-3 mutants). Conclusions Early detection of beta-lactam resistance mechanisms directly from positive blood cultures was possible using sRAST together with the interpretive reading of antibiotic resistance phenotypes. Some carbapenemase types such as OXA-48 might be difficult to infer. Screening-positive isolates should be confirmed using an alternative technique.