The association of Lactococcus petauri with lactococcosis is older than expected

Lactococcosis is a globally prevalent infectious disease that has a significant economic and sanitary impact on the rainbow trout industry. Lactococcus garvieae has traditionally been considered the only species implicated in the etiology of this disease, but Lactococcus petauri, a new species, has recently been implicated as another etiological agent. Both species cannot be distinguished by routine methods commonly used in diagnostic laboratories, resulting in their misidentification. In the present study, the identification of 48 isolates initially identified as L. garvieae was studied by determining their in-silico DNA–DNA hybridization (dDDH) and average nucleotide identity (ANI) values using pairwise comparisons of their whole genome sequences and the genomes of the type strains of L. garvieae and L. petauri. The genome sequences of 37 isolates from countries in which lactococcosis can be considered endemic (Spain, Italy, Türkiye, and Greece) were obtained in this study, and the genomes of 11 isolates were retrieved from the GenBank database. Isolates from Italy, Singapore, Japan, South Korea, India, one Turkish isolate from 2013 and two Spanish isolates recovered in 1992 and 1996 were confirmed as L. garvieae. The remaining isolates from Spain and Türkiye, as well as those from Portugal, Israel, USA, and Greece were identified as L. petauri. Overall, 60.4% of isolates previously identified as L. garvieae were found to be L. petauri. These results confirm the implication of both species in the etiology of lactococcosis and suggest that L. petauri plays a significant role in the epidemiology of this disease. Some of the isolates identified as L. petauri in the present study were isolated three decades ago, indicating that its association with lactococcosis is older than might be expected from the recent descriptions. The commercial Rapid ID32 Strep system was unable to discriminate between L. garvieae and L. petauri. However, both species exhibited some biochemical differences that might serve as phenotypic markers for their presumptive recognition. Consequently, isolates that hydrolyze hippurate and produce acid from sucrose and tagatose could be presumptively recognized as L. petauri, while those that fail these tests could be identified as L. garvieae. The results of this work indicate that great attention should be given to L. petauri in the epidemiology of lactococcosis.