By Charmain Tan Courcelle

MISSISSIPPI STATE -- That raw oyster appetizer might sound tempting, but you may want to consider eating it later in a meal.

Mississippi Agricultural and Forestry Experiment Station food microbiologist Douglas Marshall has found eating raw oysters on an empty stomach can increase the risk of food poisoning. And taking an antacid beforehand could make the situation even worse.

Marshall led a team that determined the effect of antacids on the survival of Vibrio vulnificus, a bacterial pathogen found naturally in oysters and other shellfish, in the stomach and intestines. The study was part of a larger effort to understand how this potentially deadly bacterium interacts with its environment.

"Infection with Vibrio vulnificus is very rare, but if you get it, you have about a 40 percent chance of dying," Marshall said. "So, why do some people get V. vulnificus and die, while for most it's no big deal, they don't get sick and they never know they were exposed?"

One reason most people never get food poisoning from V. vulnificus is the bacterium can't survive the trip through the acid environment of the stomach. Marshall said he wanted to know whether this would change if that acidic environment became altered by antacids.

"We know there's an increased risk of infection with other common food-borne pathogens when antacids are consumed because of reduced stomach acidity. What we didn't know when we started this study was how V. vulnificus would behave in a less aggressive stomach environment," Marshall said.

Unable to find a suitable model of the human gastrointestinal tract for their studies, Marshall and his team constructed their own system out of glass beakers, circulating pumps, a warm-water bath and solutions that mimic digestive fluids found in the stomach and intestines.

"Our model simulates the dynamics of the gastrointestinal system and allows us to follow food as it passes through the 'stomach' and 'intestine'," Marshall said.

Mississippi State University has applied for a patent for the group's invention.

The team added sterilized, raw oysters that had been "chewed" in a blender with a simulated saliva solution to the mechanical digestive system. They then added V. vulnificus at levels found in Gulf Coast oysters and the equivalent of two teaspoons of liquid antacid to the stomach.

Samples from the device's stomach and intestinal compartments were tested for living V. vulnificus cells at regular intervals after the "meal."

Results from the bacterial growth assays showed V. vulnificus was eliminated from the stomach within 30 minutes. But when antacid was used, Marshall's group could find surviving bacteria in the stomach for up to two hours -- the antacid-neutralized environment had little effect on the pathogen's numbers.

More surprising to the team was what they found in the intestinal compartment. V. vulnificus that survived the stomach's acid treatment were able to quickly multiply within the intestinal compartment. Up to 100 million V. vulnificus cells could be found in a milliliter of intestinal fluid. With antacid use, this number increased another tenfold.

"The liquid content of the stomach empties into the intestinal tract quickly in the first 10 to 20 minutes of eating," Marshall said. "Because oysters are semisolid and eaten as appetizers, they would most likely empty from the stomach easily."

Marshall explained this rapid stomach emptying rate would decrease the amount of time V. vulnificus was exposed to stomach acid and allow living bacteria to be delivered to the intestine where they could readily multiply.

"Even a meal of just one oyster could carry more than 100,000 V. vulnificus," he said.

Because the stomach's emptying rate slows as more food is ingested to allow the small intestine to complete its processing of the stomach's contents, V. vulnificus bacteria consumed after the first 30 minutes of eating would more likely be exposed to gastric acid.

"So individuals, and especially high-risk individuals, may want to eat oysters later in a meal to give the acid in their stomachs a chance to kill the bacterium," he said.

Marshall conducted this research with then-graduate research assistant Jaheon Koo and research scientist Angelo DePaola of the U.S. Food and Drug Administration's Gulf Coast Seafood Laboratory. Results from this research were published in Applied and Environmental Microbiology.

Contact: Dr. Doug Marshall, (662) 325-8722