Food adulteration

Detecting bacterial contamination before the consumer: a disposable biosensor for ATP bioluminescence

The need for rapid and reliable monitoring of bacteria is of increasing importance for food manufacturers. Chinese scientists have recently described the development of a new, disposable biosensor for detecting bacteria in food samples within five minutes - based on the measurement of ATP bioluminescence.

Bacterial contamination is perhaps the most-feared incident in the food industry. As underscored by the recent Salmonella peanut butter scandal and various E. coli cases in the US and Europe, the need for effective monitoring of bacterial counts at numerous points in the food chain is of utmost importance - both for consumer safety and for commercial viability. The use of the hazard analysis and critical control point (HACCP) system by food manufacturers has rendered obsolete most conventional bacterial detection methods based on culturing, as many of these require incubation periods of more than 24 hours. Methods for the rapid determination of bacterial counts in food are therefore essential.

Several such methods have been developed in recent years, including impedimetry, cell counting, automated immunoassays, and ATP bioluminescence. The latter technique is particularly attractive for on-line monitoring within the context of the HACCP system because it does not require culturing steps or large equipment. The assay is based on the fact that ATP (adenosine 5'-triphosphate) is the primary energy source in many different bacteria, with a relatively constant content between species of around 0.5 fg/cell. Using firefly luciferase, intracellular ATP released from bacterial cells is allowed to react with luciferin to generate bioluminescence - the intensity of which is directly proportional to the number of bacteria present.

Since it was first used in the 1970's to determine bacterial counts in food, ATP bioluminescence has been increasingly used by the food industry. Several commercial ATP bioluminescence detection systems are available, however these require costly reagent kits and/or involve complex procedures to avoid contamination from non-bacterial ATP. As such, these methods are not attractive for many companies for standard use in HACCP monitoring.

Now, a team at the State Key Laboratory of Transducer Technology in Beijing, has developed a disposable biosensor for the measurement of the ATP bioluminescence in food samples. The device consists of a sampler, a cartridge that chemically extracts ATP from bacterial cells, and a microtube, where the extracted ATP reacts with luciferase and luciferin to generate bioluminescence. The bioluminescence is then transformed into a quantifiable electrical signal by a detector element that contains a photomultiplier tube. In an initial test, the bioluminescence was further measured by a custom-built luminometer.

As described in an advance online publication in Analytical Biochemistry, the Chinese investigators first optimised parameters affecting ATP extraction, including the choice and concentration of ATP extractants and neutralizing reagents. Under optimal experimental conditions, the biosensor demonstrated a linear response to bacteria in the range between 10³ and 10⁸ CFU/ml, with a correlation coefficient of 0.925 (n=22). Importantly, the assay was performed in less than five minutes. In addition, the bacterial counts determined by this new biosensor from various food samples were similar to those obtained using the conventional plate count method.

The advantages of this disposable biosensor - including low cost, ease of operation, and rapid generation of results - make it an attractive option for the determination of bacterial counts using ATP bioluminescence.

Article by Xinxia Cai and colleagues on a disposable ATP bioluminescence sensor for bacterial monitoring, Analytical Biochemistry, 20 May 2009 (advance online publication)

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