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Use spectroscopy to detect, differentiate Salmonella enterica serovars in apple juice.

Many Salmonella identification approaches are time-consuming. It is often necessary to pre-enrich the sample, which can be tedious. Fourier-transform infrared (FTIR) spectroscopy makes it possible to identify bacteria based on unique, reproducible, biochemical fingerprints from major cellular components. FTIR could be used to quickly and accurately identify Salmonella at the serovar level.

Scientists at The Ohio State University have developed a simple protocol that could be used to detect and differentiate selected Salmonella enterica serovars in spiked apple juice using FTIR and multivariate analysis. The researchers streaked various Salmonella enterica serovars--Anatum, Heidelberg, Kentucky, Typhimurium, Muenchen and Enteritidis--onto Miller-Mallinson (MM) agar and incubated them at 42 C for 24 hours.

Single, isolated colonies of about 108 CFUs were suspended in 50% acetonitrile (10 [micro]l). The suspensions were placed on multiple-bounce zinc selenide crystal plates for attenuated total reflectance (ATR) analyses. All of the samples were vacuum-dried.

Sterile apple juice samples, which had been individually spiked in duplicate with Salmonella serovars Muenchen, Heidelberg and Typhimurium, were filtered through hydrophobic-grid membranes (HGMs). Researchers placed the membranes onto the MM agar. They were incubated at 42 C for 24 hours. Isolated colonies treated with acetonitrile (50%) and dried on the ATR crystal were analyzed for microbial identification.

A protocol that combined bacterial growth in selective media with unique mid-infrared signature profiles made it possible to chemically classify intact microbial cells. Using the spectral region from 1300-900 [cm.sup.-1], multivariate modeling showed well-separated clusters that discriminated among Salmonella serovars, which were presumably attributed to the cells' lipopolysaccharides.

Salmonella serovar recovery from apple juice using HGMs facilitated the correct identification of all Salmonella "unknowns" in spiked apple juice, including in the presence of mixed bacterial cultures. The use of HGMs and a selective medium could facilitate the recovery and identification of Salmonella serovars at low numbers in apple juice.

Further information. Luis E. Rodriguez-Saona, Department of Food Science and Technology, The Ohio State University, 110 Parker Food Science and Technology Building, Columbus, OH 43210; phone: 614-292-3339; fax: 614 292 0218; email:
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Publication:Microbial Update International
Date:Dec 1, 2006
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