Abstract

Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined to five antimicrobial agents against four isolates of Piscirickettsia salmonis. Each MIC was considered as the lowest antimicrobial agent concentration which inhibited the cytopathic effect (CPE) of P. salmonis completely using 24 well plates containing confluent monolayers of CHSE-214 cells. To determine MBCs, the medium of the MIC assays showing CPE inhibition was replaced with antimicrobial free medium and observed for CPE for additional 10 days. According the MIC cutoff, three out of four (3/4) isolates showed susceptibility to chloramphenicol and oxytetracycline. Only one isolate (1/4), although a different one, was susceptible to oxolinic acid and to flumequine, respectively. Finally, all of the isolates were resistant to gentamicin. Results showed that there is great variation in the antimicrobial sensitivity patterns among the different isolates, therefore the use of a particular drug regime for the treatment of any outbreak of piscirickettsiosis should not be recommended.