Assessment of bio-accumulation of bacteria in oysters from shellfish growing waters in Ashtamudi Lake (Kerala, India): A RAMSAR wetland

•In India, this is the first-ever assessment, which offer an insight on the accumulation of bacteria in farmed oysters.•We investigated the dynamic process of microbial bio-accumulation and how it is influenced by rainfall.•Bio-accumulation was high during the pre-monsoon season when the Lake is brackish.•Salinity and temperature play a major role in the survival of coliform bacteria in the shellfish growing water.•We can reduce input costs in the laboratory by analysing faecal coliform as a quality indicator instead of using E. coli as an indicator.

Oysters are commercially cultured from the Ashtamudi Lake in India, and support 12,000 tonnes of bivalve fisheries/year. Oysters and oyster growing waters were sampled from July 2012 to June 2013 for analysis of total coliforms (TC), faecal coliforms (FC), Escherichia coli, faecal Streptococci (FS) and total plate counts (TPC). E. coli MPN values in oyster growing waters were below the threshold limits set by the USFDA and EU during the months of December to April. Seasonally, the highest MPN values for E. coli were obtained during the monsoon season (June-September), and this trend gradually decreased during the post-monsoon (October-January) and pre-monsoon (February-May) periods. E. coli displayed a significant (p<0.01) variation in accumulation during different seasons. A strong negative correlation (R2=−0.70, p<0.05) between temperature and E. coli numbers in oysters was observed, while rainfall and E. coli were positively correlated (R2=0.695, p<0.05). Hence, we strongly recommend depuration and proper cooking of oysters before consumption during the monsoon season.