Evaluation of the Vitek 2 system for susceptibility testing of Streptococcus pneumoniae isolates

Vitek 2 (bioMérieux, France) is a new commercial system that allows rapid identification and rapid determination of the minimum inhibitory concentration (MIC) of Streptococcus pneumoniae by monitoring the growth kinetics of the organisms in microwells. The accuracy of the Vitek 2 system in susceptibility testing was evaluated by determining the MICs of 50 penicillin-susceptible and 150 intermediate or penicillin-resistant Streptococcus pneumoniae isolates and comparing the results with those obtained using the agar dilution method. The essential agreement between the Vitek 2 system and the reference method was 91% for penicillin, 93% for cefotaxime and ceftriaxone, and more than 94% for amoxicillin, erythromycin, ofloxacin, co-trimoxazole, tetracycline, and imipenem. One very major error (1.1%) and one major error (0.9%) were obtained for tetracycline. The minor error rate for penicillin of 19.3% was mainly due to intermediate category isolates (n = 29) being identified as resistant and susceptible isolates (n = 6) being identified as intermediate by the commercial system. The minor error rates for amoxicillin, cefotaxime, ceftriaxone, imipenem, and ofloxacin were 25.4%, 25.4%, 29.4%, 19.2%, and 31.5%, respectively. Vancomycin, tetracycline, co-trimoxazole, and erythromycin showed minor error rates of 0-6.1%. In conclusion, Vitek 2 shows good agreement with the reference method, as demonstrated by the low numbers of major errors, but it has a tendency to overestimate MICs, resulting in minor errors.