FTIR spectroscopy detects drinking water pathogens
Testing the safety of drinking water using conventional microbiological techniques is a time-consuming and labor-intensive task, especially because traditional methods require further identification and verification tests. There is a need for a rapid and accurate screening method for waterborne pathogens, and Fourier transform infrared spectroscopy (FTIR) could be the answer, according to researchers from Washington State University in Pullman, the University of Jordan in Amman, Hashemite University in Zarqa, Jordan, and Eastern Oregon University in LaGrande.
Two-dimensional principal component analysis clustering results from FTIR spectra show clear segregation between pure and mixed cultures of E. coli ATCC 25922 and P. aeruginosa resulting from variations in quantity and distribution of major bacterial cellular constituents, such as nucleic acids and proteins. The graph shows the control (A), E. coli ATCC 25922 (B), P. aeruginosa (C) and mixed culture (D). Reprinted with permission of the Journal of Agricultural and Food Chemistry.
They have used FTIR and multivariated analysis to successfully screen bottled drinking water for two important species of bacteria: Pseudomonas aeruginosa, which can reproduce in nutrient-restricted conditions such as distilled water and is an indicator of general water quality; and E. coli, which indicates the presence of mammalian feces.
“Using FTIR on mixed cultures is challenging because it is difficult to distinguish the spectral information from different strains of bacteria,” said Mengshi Lin from Washington State University’s department of food science and human nutrition. “However, we have shown that FTIR spectroscopy can not only determine whether a pure culture is present but also can discriminate between closely related bacteria based upon differences in biochemical and phenotypic characteristics represented in their spectra.”
The researchers bought bottled water from a local grocery store and tested it using conventional biological tests before inoculating the water samples with P. aeruginosa and E. coli. After incubation, the samples were filtered through an aluminum oxide membrane and dried to create biofilms. These filters were placed in direct contact with an infrared attenuated total reflection diamond crystal, and FTIR spectra from 4000 to 500 cm–1 at a resolution of 2 cm–1 were recorded with a Thermo Electron spectrometer.
Because some important FTIR spectral features overlapped, the researchers used second-derivative transformations to reduce baseline shift and to separate overlapping peaks. They were able to detect and identify both types of bacteria and found that the FTIR spectra of the mixed cultures were distinct from those of the pure cultures.
“With the latest developments in multivariate statistical analysis — such as principal component analysis — and improved signal-to-noise ratio, FTIR technology has become an increasingly powerful tool, especially for this application,” Lin said. “FTIR now has the sensitivity, specificity and reproducibility needed to develop accurate, rapid, safe, cheap and nondestructive methods for food and water analysis.”
The group plans to establish a spectral library of major food and waterborne pathogens and to evaluate other statistical analysis methods such as artificial neural networks for data analysis. The researchers are getting closer to commercialization of a test method for drinking water, but there is still a lot of work that needs to be done, Lin said.
Journal of Agricultural and Food Chemistry, Aug. 9, 2006, pp. 5749-5754.
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