New tests detect Salmonella subtypes in minutes

BABS researchers have created new DNA tests to accelerate Salmonella detection and, in the future, pinpoint the source of food poisoning outbreaks.

CAROLINE TANG | 01 May 2020

UNSW Sydney researchers have developed a series of accurate and highly sensitive DNA tests which can identify the five most common Salmonella subtypes in Australia. Salmonella is one of the most common causes of foodborne disease worldwide, including in Australia. These new tests are fast – detecting DNA in as little as eight minutes – and work at a constant temperature – unlike other methods that require specialised equipment for temperature cycling.

salmonella
Image: Public Health Image Library, US Centers for Disease Control and Prevention, James Archer (2019).

Pending further research, the scientists say their tests have the potential to help public health laboratories and industry curb the spread of Salmonella outbreaks in the future.

The research, published in The Journal of Molecular Diagnostics today, overcomes the expensive and laborious traditional bacteria culture method and paves the way for fast-track testing directly from specimens.

Senior author Professor Ruiting Lan, of the UNSW School of Biotechnology and Biomolecular Sciences, said the new tests could play a critical role in quickly and accurately tracing the origin of future Salmonella infections.

“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,” Prof Lan said.

Salmonella, whether it’s in a clinical or food sample – even in faecal matter – may exist in minute amounts and requires highly sensitive methods to detect.

“Our enhanced Multiple Cross Displacement Amplification (MCDA) method can detect tiny amounts of DNA rapidly and at a constant temperature, which makes it an excellent fit for a simple, rapid and sensitive bacterial detection test.

“It’s a clear improvement upon the existing MCDA test for Salmonella that does not distinguish between different subtypes of Salmonella.”

Common Salmonella subtypes

In 2017, more than 16,000 cases of Salmonella poisoning were reported in Australia – a 30 per cent increase on the previous 10-year average – while the rate is estimated at 185 cases per 100,000 people, per year.

Five Salmonella subtypes, known as serotypes, caused more than 85 per cent of the infections in 2017.

First author Xiaomei Zhang, UNSW Science PhD candidate, said the new detection method can identify the five most common Salmonella serotypes and will be crucial to help control the spread of infection during outbreaks.

“Thousands of Salmonella serotypes exist, but we developed and evaluated seven MCDA tests for the rapid detection and differentiation of the five most common Salmonella serotypes in Australia: S. Typhimurium, S. Enteritidis, S. Virchow, S. Saintpaul and S. Infantis,” Ms Zhang said.

“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.

“In addition, at least two of these serotypes – S. Typhimurium and S. Enteritidis – are also the most prevalent Salmonella serotypes throughout the world. So, our research is applicable to other regions and countries.

“Another advantage is the fact that our tests do not require specialised detection equipment, thereby simplifying future application in clinical or industrial settings.”

Future-proofed testing

Prof Lan said the new DNA tests developed in the study were unique because the gene markers used were selected from analysing thousands of Salmonella genomes.

“These markers are specific to the given serotypes and thus future-proof our new tests as traditional culture-based serotyping is being phased out,” he said.

Prof Lan said the team looked forward to continuing their research through field testing.

“The performance of the MCDA tests warrants further validation – so, more work needs to be done,” he said.

“The test has been developed using pure cultures. It will need to be validated using samples in health, environment and food industry settings. 

“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.

“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.”

What is salmonella poisoning?

According to the World Health OrganisationSalmonellosis is the disease caused by the bacteria Salmonella.

It is usually characterised by acute onset of fever, abdominal pain, diarrhoea, nausea and sometimes vomiting.

People generally catch Salmonellosis through the consumption of contaminated food of animal origin (mainly eggs, meat, poultry, and milk), although other foods, including vegetables contaminated by manure, have been implicated in its transmission.

Person-to-person transmission can also occur through the faecal-oral route.

Human cases also occur where individuals have contact with infected animals, including pets. Infected animals often show no signs of disease.