Antibiotics are commonly used in aquaculture to prevent and treat bacterial diseases such as lactococcosis caused by Lactococcus garvieae. Antimicrobial resistance of bacteria may occur and spread as a consequence of inappropriate and extensive use of antibiotics. In this study, presence of 14 antibiotic genes and phenotypic antibiotic resistance to 11 commonly used antibiotics were assessed in 20 archival L. garvieae strains collected from 2016 to 2020, in 5 small-scale rainbow trout farms in Northern Italy. Antibiotic susceptibility tests were carried out by the disk diffusion method using kanamycin (K; 30 μg), ampicillin (AMP; 10 μg or 25 μg), florphenicol (FFC; 30 μg), streptomycin (S; 10 μg), erythromycin (E; 15 μg), oxolinic acid (OA; 2 μg or 10 μg), flumequine (FLM; 30 μg), penicillin (P; 10 μg), amoxicillin (AMX; 20 μg or 25 μg), neomycin (N;30 μg), and oxytetracycline (OT; 30 μg) discs. The presence of 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) resistance genes were tested by PCR. Resistance to oxacillin was found to be the highest (75%) among archival strains, followed by flumequine (65%) and streptomycin (25%). Whereas gentamicin (100%), oxytetracycline (95%), ampicillin (95%), amoxicillin (90%), and penicillin (80%) had the lowest susceptibilities to antimicrobials. MAR index was calculated for each strain and in particular it was reported >0.2 in one of the farms in each year of the sampling, indicating the presence of multidrug resistance. Resistant or inducible phenotypes for erythromycin were observed especially in the same farm. In terms of antibiotic resistance genes, tetG (23.75%) and tetB (21.25%) were the most common. The least detectable genes, on the other hand, were ermB (1.25 %), strB (1.25 %), blaOXA (1.25 %), and gyrA (1.25 %). tetA, ermA, floR, and blaSHV antibiotic resistance genes were not found in any of the strains.

The present investigation has been conducted under the SUPERTROUT European Project (https://mel.cgiar.org/projects/supertrout) funded by PRIMA.