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Harness Fourier-transform infrared spectroscopy for rapid detection, differentiation of Salmonella enterica serovars in juice.

Techniques for identifying Salmonella are time-consuming, require a pre-enrichment procedure or special worker training. However, Fourier-transform infrared (FT-IR) spectroscopy makes it possible to identify bacteria based on unique, reproducible, biochemical fingerprints of major cellular components.

Scientists at The Ohio State University wanted to develop a simple protocol that could be used to detect and differentiate selected S. enterica serovars in spiked apple juice using FT-IR and multivariate analysis. They determined that FT-IR could be used for rapid, accurate identification of Salmonella at the serovar level.

The researchers streaked selected Salmonella enterica serovars--anatum, heidelberg, kentucky, typhimurium, muenchen and enteritidis--onto Miller-Mallinson (MM) agar. These were incubated at 42 C for 24 hours. Single isolated colonies of about 108 CFU each were suspended in 10 [mu]l of 50% acetonitrile.

The suspensions were placed onto a multiple-bounce zinc-selenide crystal plate for attenuated total reflectance (ATR) analysis. All samples were vacuum-dried. The scientists created and validated soft independent class analogy models from derivatized spectra. These models are statistical methods used to collect data. Then the investigators filtered 100 ml of sterile apple juice, individually spiked in duplicate with Salmonella serovars muenchen, heidelberg and typhimurium, through hydrophobic-grid membranes (HGMs).

The membranes were placed onto MM agar and incubated at 42 C for 24 hours. Isolated colonies treated with acetonitrile (50%) and dried on the ATR crystal were analyzed for microbial identification. The development of a simple protocol combining bacterial growth in selective medium and the unique mid-infrared signature profiles made it possible to chemically classify intact microbial cells.

In the spectral region from 1300 to 900 [cm.sup.-1], multivariate modeling showed well-separated clusters that discriminated among Salmonella serovars, presumably caused by cell lipopolysaccharides. The use of HGMs made it possible to correctly identify Salmonella "unknowns" in the spiked juice, including if any were in the presence of mixed bacterial cultures. The use of HGM and a selective medium could permit recovery and identification of Salmonella serovars at low numbers in apple juice.

Further information. Ahmed E. Yousef, Center for Microbial Interface Biology, The Ohio State University, Tenth Floor, Biomedical Research Tower, 460 W. 12th Ave., Columbus, OH 43210; phone: 614-292-0918; fax: 614.292.9616; email:
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Publication:Microbial Update International
Date:Apr 1, 2008
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