The antimicrobial susceptibilities of Turkish F. psychrophilum isolates were investigated using a Clinical and Laboratory Standards Institute (CLSI) standard microdilution testing protocol. The significance of the minimum inhibitory concentrations was determined by applying empirically and statistically determined epidemiological cut-off values calculated by normalized resistance analysis. Twenty-five F. psychrophilum isolates made over four years from three different trout species in five different regions were examined. All isolates were fully susceptible to amoxicillin and florfenicol, but 88% of our isolates were categorized as having reduced susceptibility to the two quinolones, oxolinic acid, and enrofloxacin. Eighty percent of the isolates were categorized as showing reduced susceptibility to oxytetracycline.
Sequence analysis demonstrated that the reduced susceptibility to the quinolones was associated with chromosomal mutations in the gyrA and parE genes. PCR analysis demonstrated that the genes tetA, tetB, tetC, tetE, tetH, tetL, and tetM were not associated with the reduced susceptibility to oxytetracycline and the significance of this observation is discussed. PCR analysis also showed the presence of sul2 in six isolates and floR in one isolate. This study is the first report of these genes in F. psychrophilum. It was noted, however, that the floR was detected in an isolate categorized as fully susceptible to florfenicol, but the reasons for its lack of expression in the phenotype was not investigated.
It is argued that the Turkish trout industry, in common with those of many countries world-wide, is critically dependent on the continued susceptibility of F. psychrophilum to florfenicol.