Genomic Diversity, Predicted Serotype Distribution, and Antimicrobial Susceptibility of Tenacibaculum maritimum from Croatian Marine Aquaculture

Tenacibaculum maritimum is an important bacterial pathogen of marine fish, causing disease outbreaks in aquaculture worldwide and raising concerns about antimicrobial resistance. This study investigated the genomic diversity, predicted serotype distribution, and antimicrobial susceptibility of T. maritimum isolates recovered from Croatian marine fish farms. Eight isolates obtained from European seabass (Dicentrarchus labrax) fry and Atlantic salmon (Salmo salar) were characterized using whole-genome sequencing, multilocus sequence typing (MLST), phylogenetic and pangenome analyses, O-antigen gene cluster (O-AGC) typing, phenotypic antimicrobial susceptibility testing, and in silico screening for resistance and virulence-associated genes. The isolates showed high genomic similarity with average nucleotide identity (ANI) values exceeding >99%. MLST identified six sequence types, four of which were novel. O-AGC typing identified two predicted serotypes, with O3 predominating (6/8 isolates) and O2 detected in the remaining two. Phenotypic antimicrobial susceptibility testing revealed generally low minimum inhibitory concentrations (MICs) for most antimicrobials. However, all isolates exhibited reduced susceptibility to gentamicin, as indicated by elevated MICs and consistently small inhibition zones. Resistome analysis identified a limited repertoire of conserved resistance-associated homologs with no evidence of widespread acquired antimicrobial resistance determinants among the isolates examined. Virulence-associated homologs, including those encoding Type VI secretion system-related proteins, were highly conserved. These findings provide an initial genomic and phenotypic characterization of T. maritimum isolates from Croatian marine aquaculture and establish a basis for further investigations involving larger and more representative isolate collections.