Microbial contamination and foodborne pathogens in farmed and shucked retail oysters (Saccostrea cucullata) in eastern Thailand

This study investigated microbial contamination in oysters from eastern Thailand through two complementary assessments: an environmental survey of farmed oysters in Chon Buri and a post-harvest survey of retail-shucked oyster products collected from markets in Chon Buri and Trat provinces. Phase 1 evaluated environmental conditions and microbial contamination in surrounding waters from five stations in the Bang Pakong River and Chon Buri Bay, with farmed oysters sampled from two stations between October 2021 and March 2022. Phase 2 examined microbial quality in retail oysters collected during the dry and wet seasons in 2024. Samples were categorized as aseptically shucked oysters (ASO), freshly shucked oysters (FSO), and packed shucked oysters (PSO). Environmental conditions varied among stations, particularly salinity (0.2–32.8 psμ), dissolved oxygen (3.7–5.6 mg/L), total suspended solids (32.4–75.5 mg/L), and nutrient concentrations. Stations near river discharge and coastal communities showed elevated total coliform concentrations exceeding 1000 MPN/100 mL. Total bacterial counts (TBC) in farmed oysters ranged from 4.9 to 9.1 log CFU/g. TBC and Vibrio spp. Concentrations varied significantly with sampling period, season, station, and product type (P < 0.05). Retail oysters collected during the wet season showed significantly higher TBC values than those collected during the dry season, while ASO and FSO samples exhibited significantly higher Vibrio spp. Concentrations than PSO samples (P < 0.05). Salmonella spp. Were detected in 5% (6/120) of retail oyster samples, whereas Vibrio cholerae, V. parahaemolyticus, and V. vulnificus were not confirmed by PCR. These findings highlight the influence of environmental conditions and post-harvest handling on oyster microbial quality in eastern Thailand.