Characteristics and virulence gene profiles of a pathogenic Lactococcus garvieae isolated from diseased Macrobrachium rosenbergii.

Macrobrachium rosenbergii is a commercially important freshwater prawn cultured on a large scale mostly in south and south east Asian countries. Diseases are one of the bottlenecks for the successful culture and production of this important species. Lactococcus garvieae is a Gram-positive coccus commonly found in aquatic environments causing fish and shellfish diseases. In the present investigation, we have isolated and characterized L. garvieae, as etiological agent of white muscle disease in freshwater prawn juveniles. The infected prawn samples showing clinical signs of opaque and whitish muscles, sluggishness and mortality were collected and processed. The isolated bacterium was identified using biochemical methods and 16s rDNA sequencing and species-specific PCR. The sequence obtained revealed > 99% identity with L. garvieae reported elsewhere. Koch's postulate was experimentally established through intramuscular challenge and the infected prawn muscle revealed massive coagulative necrosis with the presence of cocci. The isolate was found to be resistant to antibiotics namely clindamycin, cefoxitin, amoxicillin and co-trimoxazole. The heavy metal tolerance assay revealed the isolate to be tolerant to Cu(2+) and Cr(6+) and less tolerant to Hg(2+). Class I integron was also identified in L. garvieae isolate. Further, the isolate was screened for several virulence genes and found to have hemolysins 1, 2, and 3, adhesin PsaA, adhesin Pav, enolase, LPxTG 1, 3, and 4, adhesin clusters 1 and 2, and adhesin in the PCR assay. It is the first report of L. garvieae infection in freshwater prawns in India and will pave the way for developing suitable preventive measures for future sustainable culture and production of this important aquaculture species.