ANTIMICROBIAL SUSCEPTIBILITY OF FISHSTREPTOCOCCACEAEAND DRUG RESISTANCE DETERMINANTS OF ERYTHROMYCIN- AND TETRACYCLINE-RESISTANTLACTOCOCCUSISOLATED IN TAIWAN

Abstract

Infection of Streptococcaceae family in fish is common and associated with the increased water temperature. The outbreak of Streptococcaceae infection happens in summer and causes economical loss in fish industry in Taiwan. The objectives of the present study were to investigate the antimicrobial susceptibility of Streptococcaceae isolated from fish in Taiwan. The resistance determinants of erythromycin and tetracycline in Lactococcus isolates were also studied. In total, 61 Enterococcus, Lactococcus and Streptococcus isolates from different fish species and regions during 2001–2011 were collected and analyzed. Disc diffusion assay and minimum inhibitory concentration (MIC) were carried out to determine antimicrobial susceptibility. High resistance rate of tetracycline (76%) was noted in Enterococcus and Lactococcus isolates, in contrast to Streptococcus isolates with no resistance to tetracycline. A total of 26 isolates of Lactococcus sp. with high MIC value of tetracycline (≧ 256 μg/mL) possessed tet(S). Among them, one isolate of L. lactis subsp. lactis with resistance to both of erythromycin and tetracycline (MIC ≧ 256 μg/mL and ≧ 256 μg/mL, respectively) carried resistance determinants of tet(S) and erm(B) on plasmids and transferred the resistance determinants to E. faecalis JH2-2 by conjugation. Tetracycline-resistant L. garvieae isolates transferred the resistance determinant tet(S) to E. faecalis JH2-2 and tetracycline-susceptible L. garvieae. The present study suggested that under antimicrobial pressure, erythromycin- and tetracycline-resistance determinants in Lactococcus isolates could spread to susceptible Lactococcus and other bacterial pathogens.