Lab researchers develop rapid-detection method for identifying salmonella

Salmonella may soon be identified within hours, rather than as long as a couple weeks, thanks to a rapid-detection technique developed by Laboratory researchers.
A paper by biomedical scientists Peter Agron and Gary Andersen about their development of a DNA-based detection system will appear in Thursday’s edition of the journal Applied & Environmental Microbiology.

The scientists’ technique is expected to cut the time for the detection of salmonella from days, or even as long as two weeks in some cases, to possibly as little as two hours.

While people who eat raw eggs in Caesar salads or egg nog are at increased risk for contracting salmonella food poisoning, that risk may become much less in the near future because of this technology.

Co-authors on the paper are Jessica Wollard of Livermore, along with Richard Walker, Sherilyn Sawyer and Dawn Hayes of the California Animal Health & Food Safety Laboratory in Davis, and Hailu Kinde of the California Animal Health & Food Safety Laboratory in San Bernardino.

In a yearlong effort, Agron and Andersen identified several potential unique strands of DNA in a strain of salmonella known as salmonella enteritidis. Those pieces of DNA were then compared with the genomes from other strains of salmonella closely related to salmonella enteritidis to ensure they were unique to the enteritidis strain.
The most common pathogen of eggs, salmonella enteritidis is estimated to be present in only about one in 10,000 or more eggs. This bacteria only becomes a problem, causing food poisoning for humans, when disease-carrying eggs have not been thoroughly cooked or are left unrefrigerated.

"It is difficult to distinguish this pathogenic salmonella from all of the other salmonella strains that do not cause disease and are not a problem," Andersen said, adding that neither the chickens or eggs with this bacteria show any symptoms of the disease.

Richard Walker, a professor of clinical microbiology at UC Davis and the California Animal Health & Food Safety Laboratory, has been using the Lab’s diagnostic DNA signatures (or tests) for about six months.

Walker has been validating the new DNA-based detection system in a clinical setting, running the new tests in parallel with the classical, slower microbiological approach.

"So far, the Laboratory’s DNA signatures look very good; they’re promising," Walker said. "The primers (DNA copies) are very specific and provide tools that can speed up testing. They’re very useful in screening out negative samples."

If the validation process continues to proceed well, the Livermore DNA-based detection system would likely be used by the California Animal Health & Food Safety Laboratory for future testing, Walker said.

The Livermore-developed detection technology would need to be evaluated and approved as an alternative to conventional testing by the federal Food & Drug Administration and the California Department of Food & Agriculture, according to Walker.

Current approach can take two weeks
In the classical microbiology approach to detect salmonella enteritidis or other pathogens, something that can take four or five days to a couple weeks, a sample is incubated in a broth over night. The sample is then grown in an agar plate for another night before suspicious colonies are picked out for several days of more refined testing.

Currently, the California Animal Health & Food Safety Laboratory tests with the Livermore system take about two days, although in the future tests could take as little as two hours.

As a part of their work, Agron and Andersen designed primers, or copies of short unique DNA regions of salmonella enteritidis that could be used in a polymerase chain reaction (PCR) machine.

Once a sample is inserted into a PCR instrument, the primers seek out unique sequences of DNA, such as salmonella enteritidis.

If present, the primers attach themselves to the DNA and make billions of copies, indicating a positive reaction. If not, the primers have nothing to which to attach themselves, indicating absence of the pathogen.

With the Laboratory’s DNA signatures for salmonella enteritidis, it may be possible to eliminate or drastically reduce the level of these bacterial infections, Andersen believes.

It may also be possible to track the source of the bacterial infections, whether they stem from feed, water, a manure pile or even processing equipment, according to Andersen.

"This technology could be used widely in the future. I think the day will come when virtually every poultry farm is analyzed for the presence of salmonella enteritidis."
The California Egg Commission provided about $50,000 for the Laboratory’s research and its development of the DNA signatures for salmonella enteritidis.

In May, unique DNA signatures for plague discovered by Andersen and fellow researchers were used in Northern Arizona to confirm the presence of a naturally occurring outbreak of plague within four hours. The work was done in collaboration with Northern Arizona University. Normally, plague detection tests have required about seven to 10 days.