# Comparative Molecular and Antimicrobial Analysis of Lactococcus garvieae and Lactococcus petauri from Marine and Freshwater Fish Farms in the Mediterranean

## TL;DR

This study examines bacterial strains from Mediterranean fish farms to track disease evolution and determine effective antibiotics for treating lactococcosis.

## Contribution

The study provides updated antimicrobial susceptibility profiles and identifies emerging bacterial lineages in Mediterranean aquaculture.

## Key findings

- Lactococcus petauri has become more prevalent in freshwater trout farms in recent years.
- Amoxicillin showed high susceptibility, while flumequine was ineffective against the isolates.
- Marine farms predominantly host Lactococcus garvieae lineage ST95, distinct from freshwater strains.

## Abstract

Fish farming in the Mediterranean is a major industry increasingly threatened by lactococcosis, a bacterial disease that causes substantial production losses and poses a potential zoonotic risk. We analyzed 39 bacterial isolates from farmed fish to determine the species involved, elucidate transmission pathways, compare intra-species strain diversity, and assess antimicrobial susceptibility to identify optimal therapeutic options. We used molecular assays to identify bacterial species and track clonal lineages, and we determined minimum inhibitory concentrations (MICs) for key antimicrobials. In freshwater trout farms, earlier isolates were predominantly Lactococcus garvieae, whereas in recent years Lactococcus petauri has become prevalent. In marine farms, only Lactococcus garvieae was detected, predominantly the same emergent lineage reported in marine European production and distinct from the freshwater lineages described to date. Across isolates, amoxicillin exhibited low MICs (suggesting high susceptibility), florfenicol showed intermediate activity, and flumequine was ineffective at the concentrations tested. Oxytetracycline and trimethoprim–sulfamethoxazole yielded variable results requiring prudent use. These region-specific, updated susceptibility profiles provide veterinarians with actionable baseline data to guide empirical antimicrobial therapy decisions while awaiting confirmatory laboratory results, supporting improved diagnostics, prudent stewardship, enhanced animal welfare, reduced farm losses, and public health protection.

Piscine lactococcosis is an emerging bacterial disease that threatens freshwater and marine aquaculture in the Mediterranean region. This study characterized isolates of Lactococcus garvieae and Lactococcus petauri from farmed fish through molecular identification, genomic typing and antimicrobial susceptibility testing. A total of 39 bacterial strains were analyzed using species-specific real-time PCR assays, multilocus sequence typing and broth microdilution to determine minimum inhibitory concentrations. Results suggest a temporal shift in freshwater systems, where L. garvieae predominated in earlier isolates (mainly ST13, CC4), while L. petauri (ST14, CC14) appears as the dominant species in recent years. In marine fish, only L. garvieae was detected, mainly ST95 (CC95), a lineage previously reported in Europe. Molecular variability was found in both species with lineages capable of infecting livestock and humans. Amoxicillin displayed promising results; florfenicol showed moderate activity, while flumequine exhibited no inhibitory effect. Oxytetracycline and trimethoprim–sulfamethoxazole showed variable results requiring prudent use. These region-specific susceptibility profiles provide updated baseline data to guide empirical antimicrobial therapy while awaiting laboratory confirmation, highlighting the evolution of lactococcosis in aquaculture and emphasizing the need for molecular surveillance, antimicrobial stewardship, and vaccine updates within a One Health framework to mitigate impacts on Mediterranean aquaculture and public health.