Application of a spectrophotometric method for the determination of post-antibiotic effect and comparison with viable counts in agar

The post-antibiotic effects of gentamicin and ciprofloxacin at 1 x, 2x and 4x MIC on Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853 and Staphylococcus aureus ATCC 29213 were studied using a spectrophotometric method and the classic method of viable counts on agar as a reference. Monitoring of the growth kinetics was carried out by viability counting on the plate every hour and by means of the optical density of the cultures measured by spectrophotometry at a wavelength of 450 nm. No statistically significant differences were found between the results obtained with the spectrophotometric method and the reference method. The former method was much quicker, much easier to use and to replicate.