Depuration as a method to reduce Vibrio vulnificus populations in live Crassostrea virginica oysters

Vibrio vulnificus is a foodborne bacterial pathogen associated with raw oyster consumption. Shellfish depuration for 48 hours is a dynamic process where coliform bacteria are purged; however, this process is ineffective against V. vulnificus. The current study investigated the use of prolonged two-week depuration on V. vulnificus populations in Gulf Coast oysters. The study evaluated the impact of prolonged depuration on V. vulnificus fatty acid profile change and the ability to survive in simulated gastric fluid. Oyster depuration in seawater (10 or 22oC, 14 days) reduced V. vulnificus counts, but not to non-detectable level, indicating close ecological relationship between the pathogen and mollusk. Greatest V. vulnificus count reductions were seen in 12 ppt 10°C seawater (2.7 log10 CFU/g) and in 20 ppt 22°C seawater (2.8 logs). Mesophilic vibrios dominated the overall microflora of freshly harvested oysters, while refrigeration selected for psychrotrophic bacteria. Depuration at 22°C retained dominance of mesophilic vibrios, including pathogenic species. Although aerobic plate counts were lower in 22°C depurated oysters (5.0 logs vs 6.0 logs), depuration at 10°C had little to no advantage over 22°C in terms of vibrio population reduction. Use of prolonged depuration remains economically questionable since this method failed to completely eliminate V. vulnificus. Starved V. vulnificus behavior in artificial seawater showed that low temperature (4oC) and high seawater salinity (35 ppt) contributed to pathogen population reduction. Starved V. vulnificus did not adjust membrane fluidity to storage temperature within the investigated time frame. However, a significant fatty acid switch from C18:1w7c to C18:1w6c by double bond relocation was observed. The relocation was faster at ambient temperatures compared to refrigerated temperatures. The majority of V. vulnificus foodborne infections occur during warm summer months. Vibrio vulnificus ATCC 27562 was significantly less resistant (3.7 min D-value) to simulated gastric fluid (pH 4.0) after 7-day storage at 4oC compared to the control (7.8 min D-value). Therefore, greater gastric fluid sensitivity of the pathogen may occur in winter-harvested oysters and may partially explain the low number of winter outbreaks