New assay detects Salmonella subtypes in minutes

UNSW researchers have developed a sensitive and specific assay to detect different serotypes of Salmonella, paving the way for rapid serotyping directly from specimens. Described in The Journal of Molecular Diagnostics, this apparent improvement on current testing methods could play a critical role in quickly tracing the origin of the infection.

Salmonella bacteria typically live in animal and human intestines and are shed through faeces. Humans may become infected by ingesting contaminated water or food such as raw or undercooked meat, poultry or egg products, with signs and symptoms including nausea, vomiting, abdominal cramps, diarrhoea, fever and headache.

“It is essential for public health investigators to have a fast, simple way of tracking down the source of Salmonella outbreaks — so the ability to test for different types of Salmonella is important,” said Professor Ruiting Lan, from the School of Biotechnology and Biomolecular Sciences at UNSW.

“Multiple cross-displacement amplification (MCDA) is a method that can detect very small amounts of DNA rapidly and is also performed at a single constant temperature, in contrast to the cycling of temperatures required in other methods such as PCR. This makes it a good fit for a simple, rapid and sensitive bacterial detection test. Although an MCDA test for any Salmonella already exists, it does not distinguish between different serotypes.

“It’s important to be able to detect the different serotypes because some are more likely to be associated with local infections while others are more likely to be associated with imported cases,” said UNSW PhD candidate Xiaomei Zhang.

With this in mind, the investigators developed an MCDA assay for each of the seven serovar (subtype)-specific targets of the five most common Salmonella serotypes in Australia: S. Typhimurium, S. Enteritidis, S. Virchow, S. Saintpaul and S. Infantis. Each of the new assays can accurately detect as few as 10 copies of DNA and can produce results in approximately 8 min. They also do not require specialised detection equipment.

“The assays developed in this study are unique because the gene markers used were selected based on analysing thousands of genomes,” Prof Lan said. “Thus, these markers futureproof Salmonella serotyping in the era of culture-independent diagnostic testing.”

Thus, by combining the seven serovar-specific assays with the existing species assay, Salmonella detection should be simplified and accelerated in future. And because at least two of the five serovars studied are also the top Salmonella serovars throughout the world, the researchers believe the results are applicable to other geographic regions.

“It’s difficult to know when our tests would become available, but they are part of the global trend towards culture-independent diagnostic tests which can identify the bacteria causing a foodborne illness without the need to culture the bacteria in a lab,” Prof Lan said.

“Down the track, our tests could be used in clinical diagnosis in determining the common serotypes that cause Salmonella disease, to analyse trends of serotypes for public health surveillance, and they could also have application in the food industry.”

Image caption: Salmonella is one of the most common causes of foodborne disease worldwide, including in Australia. Image credit: Public Health Image Library, US Centers for Disease Control and Prevention, James Archer (2019).

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