Evaluation of the Scan® 1200 as a rapid tool for reading antibiotic susceptibility testing by the disc diffusion technique.

Abstract

In clinical microbiology, some instruments are able to automatically read inhibition zone diameters for antibiotic susceptibility testing (AST) performed by the disc diffusion (DD) method. The actual resolution of commercial reader systems is low and high-resolution scanners have been developed for microbiology. Here, we evaluated and compared the reading and interpretation of AST by the DD method using the Scan® 1200 instrument as compared with the Sirscan® system on 211 clinical strains and the possibility to read AST after 6 or 8 h of incubation as compared with 24 h on 121 additional Gram-negative strains and 76 non-fermenter Gram-negative and Gram-positive strains. Validation of the technique was assessed on three reference strains as requested by EUCAST for analysis of the repeatability and reproducibility of the method. Correlation between the two methods, assessed using 211 clinical isolates (n = 2439 zones of growth inhibition measured), was excellent with a correlation coefficient of 0.97. For the earlier reading experiments, preliminary results demonstrate the possibility of reading AST for drug-species combinations after 6 and 8 h for Gram-negative bacteria with rapid growth (correlation coefficient at 6 h = 0.96 and at 8 h = 0.98) and at 8 or 10 h for Gram-positive bacteria. The Scan® 1200 has many advantages thanks to its small size, rapidity of use and high-resolution imaging allowing the possibility to improve AST results after only 6-8 h of incubation. This AST reader system represents a robust alternative tool for routine use in clinical microbiology laboratories.