[Evaluation of the Spiral method to determine the MIC of various antibiotics]

The determination of minimal inhibitory concentrations (MICs), which is sometimes necessary in routine, is cumbersome. We tried to compare the Spiral gradient endpoint method, using the Spiral plater in order to prepare continual antibiotic gradient plates, vs. the reference agar dilution MIC determination. In an attempt to evaluate this method, 6 different antibiotics and 10 to 20 various strains with each antibiotic were used: imipenem and P. cepacia, imipenem, amikacin, ciprofloxacin and P. aeruginosa, vancomycin, teicoplanin and Streptococcus sp., Enterococcus sp., penicillin G and S. pneumoniae. The MICs obtained with the Spiral method were shown by regression analysis to be similar to the MICs obtained with the reference agar dilution method. Rates of agreement (+/- 1 dilution) between the two methods were > or = 90 p. cent for all six antibiotics tested and minor discrepancies between the categories S, I, R were only found for imipenem and penicillin. The Spiral gradient endpoint method for MIC determination seems to be interesting in routine because it is a reproducible and sensitive method, which reduces materials and personnel time.