Fluorescent Microspheres as Surrogates for Salmonella enterica serotype Typhimurium in Recovery Studies from Stainless Steel

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Virginia Tech


To compare the optimum recoveries of an inoculation of Salmonella enterica serotype Typhimurium, fluorescent microspheres (1.0 μm diameter, carboxylate-modified, crimson FluoSpheres®, Molecular Probes, Eugene, OR), or a combination of both from stainless steel, three recovery methods, including a standard rinse, a one-ply composite tissue (Kimwipe®) or a sonicating brush were used. Findings were used to assess the effectiveness of fluorescent microspheres as surrogates for S. Typhimurium. For each method, ten coupons (304 grade, 2.5 x 8 cm) were inoculated with either 100 μl of a S. Typhimurium culture, or a solution of fluorescent microspheres, or both, at approximate concentrations of 106. After drying for one hour, coupons were sampled using either a rinse of 100 ml of phosphate buffered saline solution (PBS) for one min, a Kimwipe® tissue method, or submerged in PBS and subjected to a sonicating brush for one min. After treatments, PBS solutions were analyzed using duplicate plate counting (Salmonella) or hemacytometry (microspheres). For microspheres and Salmonella, recovery by sonicating brush > rinse > Kimwipe® method. Additionally, the retention of microspheres on the steel ranged from 16 to 25% (mean from five coupons each recovery method). Microspheres yielded a significantly higher recovery rate (11 – 60%) than Salmonella (~1%) for each recovery method, therefore the microspheres used in this study, are not appropriate surrogates for S. Typhimurium for future recovery studies on stainless steel. However, due to their low standard deviations for their mean percent recovery, they hold the opportunity to provide better accuracy and reproducibility.



Salmonella Typhimurium, stainless steel, microspheres, bacterial recovery