Quantitative Evaluation of Recovery Methods for Listeria monocytogenes Applied to Stainless Steel
Kang, Suk-Kee David
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The ability of Listeria monocytogenes, to attach to various food contact surfaces such as stainless steel, polypropylene, and rubber compounds is well documented. The retention of these or other pathogenic bacteria on food contact surfaces increases the risk of transmission to food products. The objective of this study was to compare several methods for quantitative recovery of Listeria monocytogenes from stainless steel surfaces. A cocktail of four serotypes of Listeria monocytogenes (Scott A (4b)), 1/2b, 3b, and 4b) were mixed in equivalent concentrations and inoculated onto type 304 stainless steel coupons in a 2cm x 2cm area. After a one hour exposure, coupons were sampled by one of the following methods: 1) swabbing using a pre-moistened Dacron swab, 2) rinsing with phosphate buffered saline, 3) direct contact onto a Tryptic Soy Agar containing 0.6% yeast extract (TSA+YE) plate surface for 10 seconds, 4) sonication in an ultrasonic water bath (40 kHz), 5) contact with the bristles of a sonicating brush head for 1 min, and 6) indirect contact (2-4 mm) with the bristles of a sonicating brush head for 1 min. Coupon rinses were plated onto TSA containing 0.6% yeast extract and incubated for 24 hours at 35Â°C. The three sonication methods yielded higher recovery than the other three methods (p < 0.05). Brushing the coupons with the sonicating brush head yielded a recovery level of 58% and indirect exposure to the sonicating brush head permitted a recovery level of 65% from the initial microbial load. The lowest cell recovery (~20%) was observed with the swab and direct agar contact methods.
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