New Technology Tracks Down "Bad" Bacteria in Food

Technology allowing DNA fingerprinting of bacteria responsible for food-borne illness is turning the Houston Department of Health and Human Services into an even better disease detective.

The enhanced disease-surveillance is made possible with a technology known as pulse-field gel electrophoresis. The technique uses equipment that enables the Department of Health and Human Services' Bureaus of Epidemiology and Laboratory Services to identify illness outbreaks and track down the infectious sources more precisely and rapidly than ever.

"This technology determines the existence of cases that look alike," said Dr. Raouf Arafat, chief of the Bureau of Epidemiology. "Finding similar cases tells us that we need to go to the community, perform an immunological investigation to link them to a food source and declare them a cluster or an outbreak."

Previously, when cases of food-borne illnesses were discovered in different areas of the city, the Department of Health and Human Services could determine that they were related epidemiologically only after an investigation revealed the people affected ate at the same restaurant or participated in a common activity.

In contrast, pulse-field gel electrophoresis detects patterns in bacteria implicated in illnesses and stores the information in an electronic database of DNA fingerprints easily accessed by health and human services' computers. During outbreaks, the pulse-field gel electrophoresis software can sort through the patterns to establish which bacteria are part of the same strain.

Essentially, the technique permits health officials to study cases and clusters of food-borne illness at a genetic level.

"Bacteria that we isolate from one source may be quite different from what we isolate from another source," said Dr. Vern Juchau, chief of the Bureau of Laboratory Services. "They may be the same genus and species, but they are not the same organism. If we can say they appear to be the same strain of an organism, then we can say that they probably came from a common source."

Houston is only the third city in the country awarded the pulse-field gel electrophoresis technology through a grant from the Centers for Disease Control and Prevention, which is targeting the tool for state health departments. The CDC accepted and funded the Department of Health and Human Services' grant proposal, awarding Houston $281,000 to launch the Epidemiology and Laboratory Capacity for Infectious Disease Surveillance program. The grant helped the deparment secure the equipment and hire two additional staff members for the Bureau of Epidemiology and a third at the lab.

Houston's characteristics make it a prime candidate for the technology. Aside from being one of the largest cities in the country, Houston has more than 11,000 restaurants - 25 percent to 30 percent ethnic, serving millions of meals every week. The Port of Houston is the world's eighth busiest and in part earned that distinction because Houston is a big importer of food.

Also important is the fact that 40 percent of the investigations conducted every year by the Bureau of Epidemiology are food related.

The three most prevalent food-borne diseases in Houston are hepatitis A, shigella and Salmonella. A review of food-borne illnesses between 1995 and 1999 revealed that an average of 315 cases of hepatitis A are reported every year, followed by an average of 282 cases of shigella and 231 cases of Salmonella.

A distant fourth is campylobacter at an average of 53 cases per year. Other infectious diseases, each registering less than 10 cases in Houston, include vibrio, listeria, brucella and E. coli.

CDC reports 76 million cases of food-borne disease occur each year in the United States. The agency also estimates that there are 323,000 hospitalizations and 5,000 deaths related to food-borne illnesses. The most severe cases tend to occur in the very old, the very young, those who have an illness that reduces their immune system function and in healthy people exposed to a very high dose of an organism.

However, most cases are mild and cause symptoms for only a day or two. The symptoms produced depend greatly on the type of microbe. Numerous organisms cause similar symptoms, especially diarrhea, abdominal cramps and nausea. There is so much overlap in symptoms that it is rarely possible to say which organism is likely causing a given illness unless laboratory tests are conducted to identify the microbe, or unless the illness is part of a recognized outbreak. The CDC has identified more than 250 food-borne diseases.

Raw or undercooked meat and poultry, raw eggs, unpasteurized milk and raw shellfish are the most common culprits of food-borne disease.

Pulse-field gel electrophoresis technology not only permits health officials to compare newly collected stool sample bacteria to an archived database of past local cases, but also allows comparison with organisms isolated and identified by Texas Department of Health and CDC laboratories. If the bacteria's DNA pattern matches a strain identified at the local, state or national level, the Department of Health and Human Services can alert the public that an outbreak may be developing, and at the same time take appropriate steps to stop the spread of disease.

The CDC has already certified the Epidemiology and Laboratory Capacity for Infectious Disease Surveillance as a program able to isolate and identify bacteria implicated in cases of Salmonella, shigella and E. coli infection. The program will continue the certification process for microbes causing the most prevalent food-borne illnesses in the country until becoming a CDC-certified site.

Dr. Arafat said he expects the Department of Health and Human Services will earn comprehensive certification sometime near the end of the year. The process involves collecting and isolating samples of the disease-causing organisms and confirming results, first with the state's certified site and then the CDC.

The Epidemiology and Laboratory Capacity for Infectious Disease Surveillance Program helps ensure the Department of Health and Human Services' participation in PulseNet, the national molecular subtyping network for food-borne disease surveillance. The department will send the DNA fingerprints of organisms collected in Houston to PulseNet for electronic storage at the CDC. PulseNet is a component of the Food-Borne Diseases Active Surveillance Network, or FoodNet, a collaborative project begun in 1996 by the CDC, the Department of Agriculture and the Food and Drug Administration as a response to the increase in food-borne illnesses.

In the past, health officials could isolate and identify bacteria, which helped with treatment because physicians could determine the best antibiotic to use. Now, pulse-field gel electrophoresis technology greatly enhances the ability to track down infectious food sources to stop the spread of disease, resulting in a happier and healthier Houston.

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©2006 Texas Medical Center

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