Fully automated disc diffusion for rapid antibiotic susceptibility test results: a proof-of-principle study

Background: Antibiotic resistance poses a significant threat to patients suffering from infectious diseases. Early readings of antibiotic susceptibility test (AST) results could be of critical importance to ensure adequate treatment. Disc diffusion is a well-standardized, established and cost-efficient AST procedure; however, its use in the clinical laboratory is hampered by the many manual steps involved, and an incubation time of 16-18 h, which is required to achieve reliable test results. Methods: We have evaluated a fully automated system for its potential for early reading of disc diffusion diameters after 6-12 h of incubation. We assessed availability of results, methodological precision, categorical agreement and interpretation errors as compared with an 18 h standard. In total, 1028 clinical strains (291 Escherichia coli, 272 Klebsiella pneumoniae, 176 Staphylococcus aureus and 289 Staphylococcus epidermidis) were included in this study. Disc diffusion plates were streaked, incubated and imaged using the WASPLab (TM) automation system. Results and conclusions: Our results demonstrate that: (i) early AST reading is possible for important pathogens; (ii) methodological precision is not hampered at early timepoints; and (iii) species-specific reading times must be selected. As inhibition zone diameters change over time and are phenotype/drug combination dependent, specific cutoffs and expert rules will be essential to ensure reliable interpretation and reporting of early susceptibility testing results.