Abstract
BackgroundStandardisation of disk diffusion readings could improve reproducibility and accuracy of antibiotic susceptibility testing (AST). This study evaluated accuracy, reproducibility, and precision of automated inhibition zone reading using the “Sirscan automatic” zone reader (i2a, Perols Cedex, France).ResultsIn a first step we compared Sirscan results with manual calliper measurements for comparability and accuracy. Sirscan readings were checked and adjusted on-screen as recommended by the manufacturer. One hundred clinical bacterial isolates representing a broad spectrum of organisms routinely isolated in a clinical laboratory were tested, and zone diameter values and interpretation according to EUCAST guidelines were compared. In a second step we analysed, whether fully automated zone reading can decrease standard deviation of diameter measurements and, thus, improve reproducibility and precision of the disk diffusion method. Standard deviations of manual measurements, on-screen adjusted Sirscan measurements, and fully automated Sirscan readings were compared for 19 repeat independent measurements of inhibition zones of S. aureus ATCC 29213, E. coli ATCC 25922, and P. aeruginosa ATCC 27853 (EUCAST quality control strains).On-screen adjusted Sirscan and calliper measurements displayed high comparability. No significant differences were detected comparing the results of both reading methods. Standard deviations of inhibition zone diameters were significantly lower for fully automated Sirscan measurements compared with both adjusted Sirscan readings and the manual method, resulting in better reproducibility and precision of the automated readings.ConclusionsOur results indicate that fully automated zone reading can further improve standardisation of AST by decreasing standard deviation and, thus, improve precision of inhibition zone diameter results.
Highlights
Standardisation of disk diffusion readings could improve reproducibility and accuracy of antibiotic susceptibility testing (AST)
The main advantages of commercial microdilution systems including automated reading and rapidity are compromised by the still lower sensitivities in the detection of important resistance mechanisms compared with the disk diffusion method, e.g. inducible macrolide-lincosamidestreptogramin resistance (MLSB-Type), extended spectrum beta-lactamases (ESBL), and AmpC beta-lactamases [2,3,4,5]
The following non-duplicate clinical isolates with confirmed resistance mechanisms were included to test for adequate detection of individual resistance mechanisms by the Sirscan instrument: 117 Extended-spectrum beta-lactamase (ESBL) producing Enterobacteriaceae isolates (105 CTX-M type, 10 SHVESBL-type, and 2 TEM-ESBL type), 38 AmpC producing Enterobacteriaceae isolates (24 plasmid-encoded CIT-type AmpC, 2 plasmid-encoded DHA-type AmpC, and E. coli isolates harboring ampC promoter mutations leading to overexpression of AmpC), carbapenemase producing Enterobacteriaceae isolates (6 KPC type, 3 VIM type, 2 OXA-48 type, 1 NDM-1 type, 1 GIM-1 type), 17 vancomycin-resistant enterococci (VRE) isolates, and 50 methicillin-resistent S. aureus (MRSA) isolates [5,9]
Summary
Standardisation of disk diffusion readings could improve reproducibility and accuracy of antibiotic susceptibility testing (AST). The main advantages of commercial microdilution systems including automated reading and rapidity are compromised by the still lower sensitivities in the detection of important resistance mechanisms compared with the disk diffusion method, e.g. inducible macrolide-lincosamidestreptogramin resistance (MLSB-Type), extended spectrum beta-lactamases (ESBL), and AmpC beta-lactamases [2,3,4,5]. The sensitivity for detection of resistance mechanisms largely depends on the composition of the antibiotic drug panel used in the automated microdilution systems, which cannot be changed or modified by the user [2,7]. The disk diffusion method readily permits detection of inducible phenotypes and most combinations of resistance mechanisms including ESBL and AmpC co-production. Disadvantages of the disk diffusion method are its labour cost due to manual measurements and manual data documentation, and the investigator dependence and variation of results [10]
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