Researchers study microbial threats to shellfish safety

Agricultural Research, March, 2005 by Jim Core

Shellfish can remind us of treasured times--going clam digging with the family, slurping down oysters on the half shell at a raw bar with friends, or shucking them yourself in the hopes of finding a pearl. Unfortunately, eating them is often discouraged today because of environmental contamination.

In hopes of reversing this trend, scientists at the Microbial Safety of Aquaculture Products Center of Excellence in Dover, Delaware, are conducting research to provide safer shellfish. A field location of the ARS Eastern Regional Research Center's Microbial Food Safety Research Unit in Wyndmoor, Pennsylvania, the Center of Excellence is a partnership between ARS and Delaware State University, one of the 1890's Historically Black Colleges and Universities. ARS's Centers of Excellence are intended to foster complementary research on problems of national and regional concerns and to enhance cooperative research. Established in 1999 on the university's campus, the Dover worksite is the only ARS laboratory studying bacterial and viral safety of shellfish.

According to Gary P. Richards, a microbiologist and the center's lead scientist, shellfish are filter feeders, so they concentrate pollutants, including human pathogens, from seawater.

"The laboratory focuses on developing rapid, cost-effective, and practical methods to detect microorganisms in oysters, clams, and mussels and is evaluating processing strategies to eliminate these potentially harmful pathogens," Richards says.

Oysters, clams, and mussels are considered aquaculture species because of the amount of management that goes into maintaining productive molluscan shellfish beds. These shellfish are a concern to Richards and his colleague, microbiologist David H. Kingsley, because bacterial and viral pathogens can become concentrated within edible shellfish tissues. Many types of viral and bacterial pathogens that grow in the gut of infected people may contaminate water and food. Shellfish live along the shore, where they are subject to contamination with pathogens from improperly treated municipal waste, leaking septic systems, floodwaters, runoff, or overboard discharge of boat wastes. They may also become contaminated by unsanitized hands and surfaces tit harvesting, processing, and preparation facilities.

From a food-safety standpoint, three groups of pathogens are of greatest concern to the molluscan shellfish trade: The noroviruses (formerly known as the Norwalk-like viruses), hepatitis A virus, and Vibrio bacteria.

Noroviruses, the leading cause of nonbacterial gastrointestinal illness in the United States, have gained recent notoriety because of several outbreaks on cruise ships. There are an estimated 9.2 million cases of norovirus infection caused by food in the United States annually. Norovirus illness comes on rapidly, lasts only a few days, and is rarely life threatening.

The second agent is hepatitis A virus. It is far less prevalent than norovirus--about 23,000 cases of hepatitis A are reported each year in the United States. But this virus infects the liver and can be more serious than norovirus infection. Although many illnesses are mild and asymptomatic, some require hospitalization or lead to death. Contamination of shellfish beds by direct exposure to human fecal wastes can readily lead to hepatitis A and norovirus infections.

The third group of human pathogens are bacteria of the genus Vibrio. Perhaps the best-known Vibrio infection is cholera, which sickens many people in underdeveloped countries through contaminated food and water. In the United States, V. cholerae is not a problem, but two other vibrios are of concern: V vulnificus and V. parahaemolyticus. These bacteria are naturally found in shellfish and seawater, particularly when water temperatures are warm.

"Consumption of V. vulnificus-contaminated oysters is not a problem for healthy individuals," says Richards, "but that's not the case for sick, elderly or immunocompromised people, especially those with liver disease or diabetes. These groups should avoid eating raw shellfish because of the widespread presence of V. vunificus in the marine environment."

Mortality rates for those who acquire a V. vulnificus infection exceed 50 percent, with rapid disease onset and death often within 3-4 days. This bacterium is also a flesh-eating organism, which can produce major disfigurement in those who survive infection.

The other Vibrio pathogen found in the United States is V. parahaemolyticus, which causes a gastrointestinal illness that is generally not life threatening. Illnesses from V. parahaemolyticus have resulted in the closure of shellfish beds on the Atlantic, Pacific, and Gulf coasts of the United States and have led to major economic hardships for the shellfish industry.

Bacterial Virulence Factors

Deaths from V. vulnificus continue to occur among immunocompromised oyster consumers. To better understand how these and related bacteria invade the human host, Richards focused on identifying Vibrio enzymes that may enhance bacterial invasiveness. Leading a research group involving scientists from Delaware State University and the National Institutes of Health, Richards recently discovered and characterized an enzyme in V. vulnificus and identified it as phosphoglucose isomerase with a novel lysyl aminopeptidase activity.