Discrimination of Stressed and Unstressed Non-O157 Shiga Toxin-producing Escherichia coli Serotypes by Fourier Tansform Infrared (FT-IR) Spectroscopy

Introduction:  Fourier transform infrared (FT-IR) spectroscopy represents a rapid, accurate and selective method to identify bacteria based on their biochemical cellular composition. Recently this technique has also been used to examine the effects of antimicrobial treatments on pathogens by detecting and discriminating injured cells. In 2013 non-O157 Shiga toxin-producing Escherichia coli (nSTEC) have been associated with 50% of the total STEC foodborne infections in the United States. Little information is available on the effect of sanitizers and stress conditions on nSTEC growth and the use of FT-IR as a discriminant technique between treatment technologies.

Purpose: The objectives of this research were to evaluate FT-IR as a suitable method to discriminate stressed and unstressed nSTEC cells and to develop a system to classify them based on spectral relatedness and different toxins produced.

Methods: Overnight cell cultures were harvested by centrifugation. Pellets suspended in PBS were treated with 100 ppm peroxyacetic acid solutions. After a 5 min exposure time, bacteria were neutralized and harvested by a series of washing steps. An aliquot was placed on a ZnSe window, dried and analyzed with the FT-IR spectroscope. Untreated nSTEC cultures were used as controls. Linear Discriminant Analysis (LDA) was used to process spectra.

Results: LDA was applied to differentiate nSTEC strains based on the spectra obtained. Two district groups were observed between control and treated bacteria. This indicated that FT-IR was able to discriminate stressed and unstressed cells of E. coli. Another LDA model was used to differentiate between Shiga toxin 1 and 2 producers and a relatively promising clustering based on the toxin gene present was observed.

Significance: FT-IR may represent a suitable, rapid and economical tool to discriminate nSTEC serotypes and gain an understanding of physiological stresses that occur in cells following antimicrobial treatments.