Identifying Salmonella Bacteria With Laser Fingerprints
By Liisa Vexler
A major cause of food poisoning is found by contamination of food by the genus Salmonella bacteria. There are 40,000 reported cases of Salmonella in the US annually, however according to the Centers for Disease Control and Prevention the actual figure is 1 million cases of infection. The primary method for detecting salmonella is a range of polymerase chain reaction tests. These involve either testing bacteria taken from food rinses, water extracted from food shaken in a sterile bag containing sterile water, cultures on agar plates, or a layer of nutrients for bacteria on a glass plate. The bacteria, which form in colonies, are then subjected to biochemical tests.
However, researchers at Purdue University in West Lafayette, Indiana, headed by Arun Bhunia, found that when a laser is directed at the colonies symmetric patterns appear, differing for each type of bacteria. The discoveries, published in mBio (Jan/Feb 2014) which can be interpreted like fingerprints, are said to be a result of nutrients, which when processed by the bacteria cause the varying diffraction patterns. Researchers worked to create a system called BARDOT (bacterial rapid detection using optical scatter technology), consisting of an incubator with laser scanner, able to scan an agar plate in one minute. Observations are subsequently projected onto a screen. This system however does not replace current methods used by the U.S. Food and Drug Administration and others globally.
Patrick Fach, from the French Agency for Food, Environmental and Occupational Health (Anses) in France states that polymerase chain reactions give more detailed results, however the benefit of BARDOT is that it does not kill the colony and therefore further tests can be carried out. Bhunia recommends BARDOT as a good initial test and polymerase as a more in-depth study. The system is considered useful for checking other samples, not just food, such as blood, water or anything else that can grow on a plate.