Salmonella DNA test promises fast detection of harmful strain

By Paul Elias The Associated Press
Wednesday October 31, 2001

SAN FRANCISCO — Salmonella-contaminated eggs may be identified within hours, rather than days or weeks, using a rapid-detection technique developed by germ warfare researchers at Lawrence Livermore National Laboratory. 

The DNA-based detection system distinguishes a deadly salmonella strain from the many benign forms of the bacteria, according to a paper to be published Thursday in a scientific journal. 

Most large processors spray eggs with chlorinated water heated to around 110 degrees, which is hot enough to kill salmonella. Still, an estimated one in every 10,000 eggs on grocery store shelves is infected with salmonella enteritidis, a significant source of food poisoning that can cause diarrhea, fever, abdominal cramps, headache, nausea and vomiting when undercooked eggs are eaten. 

Approximately 1.4 million people nationwide fall ill each year due to salmonella, 300,000 of which are affected by the enteritidis strain, according to the Centers for Disease Control and Prevention. 

Healthy people usually recover, but the disease can be life-threatening for children, the elderly and for people with weakened immune systems. The government estimates that a consumer eats undercooked eggs 20 times a year. 

Federal officials hope to cut salmonella food poisoning from eggs in half by 2005 and eliminate it totally by 2010 through the Egg Safety Action Plan, a joint effort of the Food and Drug Administration and the U.S. Department of Agriculture. The plan includes safe handling warnings and new refrigeration requirements. 

Bacteria can be on an egg’s shell, since the egg leaves a hen’s body through the same passageway as feces. Many benign bacteria closely resemble the pathogen, including many strains of salmonella. Because of this, it currently takes at least two tests and several days for inspectors to determine if suspect chickens and eggs are truly infected with the pathogen. 

By comparing the genomes of the benign salmonella with the bad salmonella, lead researcher Gary Andersen and his team were able to pinpoint a tiny fragment of DNA unique to the pathogen. The scientists then dropped that unique DNA strand — a “DNA signature” — into a culture of suspected salmonella enteritidis to see if they would bind. Binding indicates the presence of the pathogen. 

“We’re making Caesar salads safe to eat,” joked Andersen, who is using the same comparative genomic methods to develop a similar test for anthrax, plague and other pathogens thought to be used in biological weapons. 

Lawrence Livermore is developing a handheld detector fueled by Andersen’s technology. The lab also has licensed the DNA signature technology to biotechnology company Cephied, which is developing its own germ warfare detector. 

Using DNA signatures, scientists are able to determine with one test and within hours if suspect eggs are contaminated. 

“It seems to work very well,” said Richard Walker, an inspector with the California Animal Health and Food Safety Laboratory. Walker said he’s been using Lawrence Livermore’s test alongside traditional tests in the field the last six months. 

Still, Walker said the detection technology would need to be evaluated and approved as an alternative to conventional testing by the FDA and the California Department of Food and Agriculture. 

The lab’s research is to be published in the journal Applied and Environmental Microbiology.