Laboratory evaluation of an automated antimicrobial susceptibility system

A newly introduced automated method for antibiotic susceptibility testing, AUTOBAC 1, has been evaluated by comparison with the disk agar diffusion method (Bauer-Kirby). A total of 2,518 strains of gram-positive (540) and gram-negative (1,978) organisms isolated from clinical specimens was examined by both methods with eight or ten antibiotics, including Tobramycin. An overall agreement of 97.4% was obtained when results were compared by individual antibiotic. However, many discrepancies were observed when individual genera or species were analyzed. Of 2,518 strains examined, 651 (26%) showed discrepancies in response to one or more antibiotics. Strains, showing discrepancies were re-examined by the broth dilution susceptibility method. The results obtained favored the disk agar diffusion method. Reproducibility experiments revealed a greater inconsistency in the AUTOBAC 1 system than in the agar diffusion test. It is concluded that although a rapid automated system for antibiotic sensitivity testing is desirable, the conventional disk agar diffusion method is easier to perform, more reliable, and a less expensive procedure for antibiotic sensitivity determination.