Evaluation of the susceptibility of Aeromonas salmonicida to oxytetracycline and tetracycline using antimicrobial disk diffusion and dilution susceptibility tests

The development of resistance to antibiotics commonly used in animal production is an increasing concern for agriculture industries and public health. This study examined the utilisation of proposed standard methods for antimicrobial susceptibility testing as described by Alderman and Smith [Alderman, D.J., Smith, P. 2001. Development of draft protocols of standard reference methods for antimicrobial agent susceptibility testing of bacteria associated with fish diseases. Aquaculture. 196, 211-243.] for oxytetracycline (OTC) and tetracycline (TET). Fifty strains of Aeromonas salmonicida subsp. salmonicida were tested using disk diffusion and MIC techniques, with comparison of incubation periods of 24 and 48 h. The relationship between the MIC and the disk diffusion data was also used to evaluate possible breakpoints for the isolates utilised in this experiment. Susceptible and non-susceptible MIC breakpoints of ≤ 1 μg/ml and > 16 μg/ml, and ≤ 1 μg/ml and > 4 μg/ml fit the datasets for OTC and TET respectively. The calculation of the inhibition zone breakpoints using best-fit line equations yielded values of 15 and 26 mm for OTC and 21 and 28 mm for TET, for the non-susceptible and susceptible breakpoints respectively. The apparent differences in the inhibition zone distributions of the two tetracycline derivatives presented in this study argue for further investigation of breakpoints, with the inclusion of more bacterial strains of intermediate resistance as well as methodology development of susceptibility testing for both oxytetracycline and tetracycline.