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Spectroscopy classifies live, dead bacterial cells.

The ability to discriminate between live and dead bacterial cells is a necessity for food safety applications. Various processing treatments that cause injury and death to bacteria also influence the bacteria's cell wall surface composition. This, in turn, will likely influence the detectability of bacteria when using Fourier transform infrared (FT-IR) spectroscopy.

It is critically important to characterize how processing conditions influence the FT-IR spectra, and how they impact the ability to differentiate bacteria that have been subjected to different types of inactivation treatments. These treatments include, for example, heat, exposure to UV light, osmotic injury and alcohol treatment.

In tests, E. coli K-12 was cultured by Purdue University scientists at 37 C for 17 hours to a concentration of ~109 CFU per mL. The cultures were then subjected to: a sterilization treatment at 121 C for 15 minutes for heat injury; UV light for 2 hours; and alcohol (70% v/v) for up to 30 minutes. Or they were suspended in 0.9% sodium chloride for up to 30 minutes. Researchers then filtered the samples through a mixed cellulose filter (0.45 [micro]m).

The spectra of the bacteria suspended on the filter surface were collected using a flat plate multibounce ATR accessory on the FT-IR main bench. The scientists analyzed the spectra using discriminant analysis software in the amide II (1589-1493 cm-1) and fatty acid (3000-2800 cm-1) region. Statistics-based Mahalanobis distances were measured to quantify the spectral differences between the cells subjected to the different treatments.

Discriminant analysis was able to successfully classify and differentiate between the various processing treatments at a performance index of 85.6%. A graphical representation showed a clear separation between clusters of bacteria exposed to the various treatments. So it appears that an FT-IR-based classification of dead and live cells has potential for such applications as detecting viable bacteria in food that has been subjected to various processing treatments.

Further information. Richard Linton, Center for Food Safety Engineering, Food Science Building, Room 3171, Purdue University, 745 Agriculture Mall Dr., West Lafayette, IN 47907; phone: 765-494-6481; fax: 765-494-7953; email:
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Publication:Microbial Update International
Date:Feb 1, 2007
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