Infectious diseases represent one of the main issue for sustainable and profitable aquaculture productions. The rapid emergence of antibiotic resistant bacteria endangers the efficacy of antibiotic and leads to the spread of resistance genes.
In the present study, phenotypic antibiotic resistance against 12 antibiotics (kanamycin 30 μg; ampicillin 10 μg or 25 μg; gentamycin 10 μg or 30 μg, florphenicol 30 μg; streptomycin 10 μg; erythromycin 15 μg; oxolinic acid 2 μg or 10 μg; flumequine 30 μg; penicillin 10 μg; amoxicillin 20 μg or 25 μg; neomycin 30 μg and oxytetracycline 30 μg) and the presence of 14 antibiotic resistance genes (florfenicol (floR), tetracycline (tetA, tetB, tetC, tetG), erythromycin (ermA, ermB, ermC), streptomycin (strB), β-lactamase ampicillin (blaSHV, blaOXA, blaTEM), quinolones oxalinic acid (gnrA), and quinolones flumequine (gyrA) were evaluated in archival Lactococcus garvieae and Lactococcus petauri strains isolated from small-scale rainbow trout farms in Turkey, Italy, Greece, and Spain.

Overall, antimicrobial susceptibility tests indicated that 93.8% of the isolates were resistant to oxolinic acid, followed by flumequine (91.3%), streptomycin (88.8%), kanamycin (72.5%), and neomycin (66.3%). Whereas, the highest susceptibilities were seen toward gentamycin (100%), oxytetracycline (97.5%), florfenicol (75%), ampicillin (73.8%), and amoxicillin (72.5%). Among all strains (n=80), 96.3% (n=77) had at least one resistance gene and 57.5% (n=46) had 2 or more antibiotic resistance genes. Among the screened tetracycline genes, tetC (57.5%) was the most prevalent gene followed by tetG (33.8%), ermC (23.8%) and tetB (21.3%), whereas floR (2.5%), ermA (2.5%), tetA (3.8%), and blaSHV (6.3%) were among the least detected genes.

Prevalence of resistance to commonly used antibiotics in aquaculture and detection of resistance genes in strains may lead to prolonged treatments and to the increase L. garvieae/L. petauri related mortalities. This study provides a starting point for comprehensive monitoring of antimicrobial resistance of L. garvieae/L. petauri strains in rainbow trout aquaculture in the Mediterranean Region.