Hassan Ashraf N, Birt Dawn M, Frank Joseph F
Center for Food Safety, Department of Food Science and Technology, University of Georgia, Athens, Georgia 30602, USA.
J Food Prot. 2004 Feb;67(2):322-7. doi: 10.4315/0362-028x-67.2.322.
Listeria monocytogenes has been isolated from condensate-forming surfaces in food processing plants. The objective of this research was to observe the behavior of L. monocytogenes on condensate-covered stainless steel with a Pseudomonas putida biofilm. L. monocytogenes-containing biofilms, either with or without added chicken protein, were incubated in a high humidity chamber at 12 degrees C to allow formation of condensate. Samples were analyzed for attached and unattached L. monocytogenes and total plate count periodically for 35 days. Samples were also taken for microscopic observation of Listeria and bacterial extracellular polymeric substances (EPS). L. monocytogenes attached in significantly greater numbers (> 3-log difference) to surfaces with preexisting P. putida biofilms than to Pseudomonas-free surfaces. L. monocytogenes survived in the presence or absence of P. putida with no added nutrients for 35 days, with numbers of survivors in the range of 3 to 4 log CFU/cm2 in the presence of P. putida and less than 2.9 log CFU/cm2 in pure culture. Attached and unattached L. monocytogenes were at similar levels throughout the incubation under all conditions studied. The addition of protein to the biofilms allowed growth of L. monocytogenes in pure culture during the first 7 days of incubation. Numbers of L. monocytogenes were not affected by the presence of P. putida when protein was present. Unattached L. monocytogenes were at levels of 3.6 to 6.7 log CFU/cm2 on the protein-containing surfaces. Microscopic observation of the condensate-covered biofilms indicated that L. monocytogenes formed microcolonies embedded within an EPS matrix over a 28-day period. This research demonstrates that L. monocytogenes can survive on condensate-forming stainless steel in low and high nutrient conditions, with or without the presence of Pseudomonas biofilm. The Listeria can detach and, therefore, have the potential to contaminate product.
在食品加工厂的冷凝水形成表面分离出了单核细胞增生李斯特菌。本研究的目的是观察单核细胞增生李斯特菌在覆盖有恶臭假单胞菌生物膜的冷凝水覆盖的不锈钢表面上的行为。将含有单核细胞增生李斯特菌的生物膜，添加或不添加鸡肉蛋白，在12℃的高湿度培养箱中孵育以形成冷凝水。定期分析样品中附着和未附着的单核细胞增生李斯特菌以及总平板计数，持续35天。还采集样品用于单核细胞增生李斯特菌和细菌胞外聚合物（EPS）的显微镜观察。与无假单胞菌的表面相比，单核细胞增生李斯特菌附着在预先存在恶臭假单胞菌生物膜的表面上的数量显著更多（差异>3个对数）。在添加或不添加营养物质的情况下，单核细胞增生李斯特菌在有或没有恶臭假单胞菌存在的情况下都能存活35天，在有恶臭假单胞菌存在时存活菌数在3至4 log CFU/cm2范围内，在纯培养中少于2.9 log CFU/cm2。在所有研究条件下孵育过程中，附着和未附着的单核细胞增生李斯特菌水平相似。在生物膜中添加蛋白质可使单核细胞增生李斯特菌在纯培养中孵育的前7天内生长。当存在蛋白质时，恶臭假单胞菌的存在不影响单核细胞增生李斯特菌的数量。在含蛋白质的表面上，未附着的单核细胞增生李斯特菌水平为3.6至6.7 log CFU/cm2。对覆盖有冷凝水的生物膜的显微镜观察表明，单核细胞增生李斯特菌在28天内形成了嵌入EPS基质中的微菌落。本研究表明，单核细胞增生李斯特菌在低营养和高营养条件下，无论有无假单胞菌生物膜存在，都能在形成冷凝水的不锈钢表面存活。李斯特菌可以脱离，因此有可能污染产品。
