Antimicrobial Resistance in Lactococcus spp. Isolated from Native Brazilian Fish Species: A Growing Challenge for Aquaculture

<i>Lactococcus</i> spp. has emerged as a pathogen that is affecting global aquaculture, with <i>L. garvieae</i>, <i>L. petauri</i>, and <i>L. formosensis</i> causing piscine lactococcosis. While antimicrobials are commonly used to treat diseases in aquaculture, reports of antimicrobial resistance in fish isolates are increasing. However, little is known about the susceptibility patterns of <i>Lactococcus</i> spp. strains isolated from native fish species in Brazil. This study aimed to assess the antimicrobial susceptibility of these strains and establish a provisional epidemiological cutoff value for <i>L. garvieae</i> using the normalized resistance interpretation approach. A total of 47 isolates were tested: 17 <i>L. garvieae</i>, 24 <i>L. petauri</i>, and 6 <i>L. formosensis</i>. The isolates were classified as wild-type (WT) or non-wild-type (NWT) based on inhibition zone diameters. Isolates classified as NWT for three or more antimicrobial classes were considered multidrug-resistant, and the multiple antibiotic resistance (MAR) index was calculated. The results revealed heterogeneity in antimicrobial resistance profiles, with higher resistance to trimethoprim/sulfamethoxazole and norfloxacin. Resistance to other antimicrobials, including florfenicol and oxytetracycline (approved for use in Brazil), varied according to the bacterial species. <i>Lactococcus petauri</i> (87.5%) and <i>L. formosensis</i> (66.7%) showed the highest multidrug resistance, compared to <i>L. garvieae</i> (11.7%), along with higher MAR index values. These findings suggest that multidrug-resistant strains could pose future challenges in the production of native species, underscoring the need for ongoing monitoring of antimicrobial resistance and responsible use of antimicrobials in aquaculture.