Evolution of drug resistance and minimum inhibitory concentration to enrofloxacin in Tenacibaculum maritimum strains isolated in fish farms

The in vitro susceptibility of 63 isolates of Tenacibaculum maritimum from four fish farms to eight chemotherapeutic agents used for the treatment of bacterial diseases in fish were assessed. The results indicated that all strains were resistant to oxolinic acid and susceptible to amoxicillin, nitrofurantoin, florfenicol, oxytetracycline and trimethoprim-sulphamethoxazole. However, some isolates presented resistance to enrofloxacin and flumequine, ranging from 10 to 30%, and from 25 to 60%, respectively, depending on the farm sampled. These data were used in an attempt to predict whether the resistance to enrofloxacin was static or evolved during the time of sampling from 2003 to 2004. A relationship between the use of enrofloxacin and levels of resistance was detected in the studied farm, increasing significantly from no resistant isolates in 2003 to 44.8% resistant strains in 2004, the year in which this drug was commonly employed. This result was accompanied by a marked decline of about 29.2% of the inhibition zone sizes for the T. maritimum strains in comparison to the initial values (average 21.5 mm). Minimum inhibitory concentration (MIC) of enrofloxacin for 100 T. maritimum strains was determined by the microdilution method. Twenty isolates were resistant to enrofloxacin (> 256 mu g ml(-1)), while the remaining strains showed a bimodal distribution, which ranged from 0.5 to 32 mu g ml(-1). Our interpretation of the enrofloxacin MIC data suggests that the breakpoint for T. maritimum should be 4 mu g ml(-1). However, similar studies in other laboratories are necessary to validate this breakpoint value.