Laser tool speeds up detection of Salmonella in food

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A team of researchers from Purdue University in Indiana, USA has developed a new technique to detect Salmonella in food products. The ‘Bacterial Rapid Detection Using Optical Scattering Technology’ (BARDOT) system uses a laser to scan bacteria colonies and identify harmful pathogens three times faster than existing techniques. In the future, the researchers anticipate that the machine could be used as a screening tool for food processing companies and provide safer food to consumers.

Salmonella is a major foodborne pathogen that causes salmonellosis, a type of food poisoning that can be fatal in young children, the elderly and those with compromised immune systems.

The US Food and Drug Administration has a zero-tolerance policy for Salmonella in food products. If the bacteria is detected, the resulting product recalls can lead to significant financial loss and possible charges of criminal liability for the companies involved.

Current Salmonella detection methods can take 72 hours to yield results and often require artificial alteration of the bacteria colonies. But the BARDOT machine developed at Purdue identifies bacteria colonies by using light to illuminate their natural characteristics, therefore preserving the colonies for later study.

For the system, the researchers developed a laser sensor that can identify Salmonella bacteria grown from food samples approximately three times faster than conventional detection methods. The machine scans bacteria colonies and generates a distinct black and white fingerprint by which they can be identified. BARDOT takes less than 24 hours to pinpoint Salmonella.

‘BARDOT allows us to detect Salmonella much earlier and more easily than current methods,’ said Arun Bhunia, a professor of food science who collaborated with then-Purdue engineer Daniel Hirleman to create the machine. ‘This could ultimately help provide safer food to consumers.’

The BARDOT system uses a red diode laser to scan bacteria colonies on an agar plate. When the light penetrates a colony it produces a scatter pattern, a unique arrangement of rings and spokes that resembles the iris of an eye. The pattern is matched against a library of images to identify the type of bacteria.

To test BARDOT's ability to identify Salmonella, Bhunia and his fellow researchers grew bacteria from rinses of contaminated chicken, spinach and peanut butter on agar plates for approximately 16 hours. After the plates were covered with tiny spherical colonies of bacteria, they placed each plate inside BARDOT - which is about the size of a large microwave oven - and scanned the colonies.

BARDOT identified Salmonella bacteria with an accuracy of 95.9 per cent. It also individually distinguished eight of the most prevalent Salmonella serovars - distinct variations within a species of bacteria. Identifying a particular serovar helps trace bacteria to the original source of contamination. In addition to Salmonella, BARDOT can identify Escherichia coli, Vibrio, Listeria, Bacillus and many more foodborne pathogens.

According to Atul Singh, postdoctoral research associate and first author of the study, BARDOT could be an effective preliminary screening tool, especially for food processors testing a large number of samples: ‘BARDOT screens quickly and inexpensively. If you get a positive result for Salmonella, you can do a follow-up test. This can help food processors make more informed decisions.’

The research was published in the American Society for Microbiology’s open-access journal, mBio, on 4 February 2014.