Genomic investigation of Lactococcus formosensis , Lactococcus garvieae , and Lactococcus petauri reveals differences in species distribution by human and animal sources

is a fish pathogen that can cause diseases in humans and cows. Two genetically related species, and , may be misidentified as . It is unclear if these species differ in host specificity and virulence genes. This study analyzed the genomes of 120 . , 53 . , and 39 . isolates from various sources. The genetic diversity and virulence gene content of these isolates were compared. The results showed that 77 isolates previously reported as were actually or . The distribution of the three species varied across different collection sources, with being predominant in human infections, human fecal sources, and rainbow trout, while was more common in bovine isolates. The genetic diversity of isolates within each species was high and similar. Using a genomic clustering method, , , and were divided into 45, 22, and 13 clusters, respectively. Most rainbow trout and human isolates of belonged to different clusters, while isolates from bovine and human sources were also segregated into separate clusters. In , most human isolates were grouped into three clusters that also included isolates from food or other sources. Non-metric multidimensional scaling ordination revealed the differential association of 15 virulence genes, including 14 adherence genes and a bile salt hydrolase gene, with bacterial species and certain collection sources. In conclusion, this work provides evidence of host specificity among the three species. and are two newly discovered bacteria, which are closely related to garvieae, a pathogen that affects farmed rainbow trout, as well as causes cow mastitis and human infections. It is unclear whether the three bacteria differ in their host preference and the presence of genes that contribute to the development of disease. This study shows that and were commonly misidentified as . The three bacteria showed different distribution patterns across various sources. was predominantly found in human infections and rainbow trout, while was more commonly detected in cow mastitis. Fifteen genes displayed a differential distribution among the three bacteria from certain sources, indicating a genetic basis for the observed host preference. This work indicates the importance of differentiating the three bacteria in diagnostic laboratories for surveillance and outbreak investigation purposes.