Isolation and characterisation of virulent Serratia marcescens associated with a disease outbreak in farmed ornamental fish, Poecilia reticulata in Kerala, India

Pathogenic strain of Serratia marcescens (NPSM-1) with multiple drug resistance was isolated from guppy Poecilia reticulata with clinical signs of fin rot and was confirmed by biochemical tests and 16S rRNA gene sequencing. The extra cellular proteins (ECP) of the bacteria exhibited marked cytotoxic activity in vitro on Cyprinus carpio koi fin (CCKF) cell line. The in vivo challenge studies confirmed that the isolate was highly pathogenic to fish when the fishes were injected with 1 x 10(4) CFU/fish and the same bacterium was re-isolated from infected fish, post-challenge. S. marcescens produced large zones of haemolysis on 10% sheep blood agar. The bacteria was found to carry virulence genes; extracellular metalloprotease gene (Pr596) and AHL synthase gene (SpnI). The bacterial isolate was tested to determine sensitivity against 16 antibiotics and was sensitive to only 5 viz., cefixime, chloramphenicol, ciprofloxacin, gentamycin and erythromycin. The study indicates that S. marcescens can cause disease in ornamental fish and the bacterium being a known human pathogen, may also cause infections in humans having direct contact with infected fishes. This is the first report describing S. marcescens as a pathogen of freshwater ornamental fish in India.