Lin Chia-Min, Zhang Lei, Doyle Michael P, Swaminathan Bala
Center for Food Safety, University of Georgia, 1109 Experiment Street, Griffin, Georgia 30223-1797, USA.
J Food Prot. 2006 Sep;69(9):2151-6. doi: 10.4315/0362-028x-69.9.2151.
Listeriosis associated with Hispanic-style soft cheese is an ongoing public health concern. Although rapid detection methods based on molecular and immunological technologies have been applied successfully for detecting Listeria monocytogenes in foods, obtaining isolates of the pathogen is a critical procedure for epidemiologic studies and regulatory analysis. Oxford agar, a medium recommended by the U.S. Food and Drug Administration Bacteriological Analytical Manual (BAM) to isolate L. monocytogenes from cheese, is unable to differentiate L. monocytogenes from other Listeria species. Hence, two selective isolation media, L. monocytogenes blood agar (LMBA) and Rapid 'L. mono agar (RLMA), were compared with Oxford agar for isolating L. monocytogenes from cheese. Queso fresco cheese was inoculated at 10(0) or 10(1) CFU/g with a five-strain mixture of L. monocytogenes or with the five-strain L. monocytogenes mixture and Listeria innocua. Cheese samples were stored at 21, 12, and 4 degrees C and Listeria counts were determined at 3, 7, and 10 days; 7, 10, 14, 21 days; and 2, 4, 8, and 12 weeks postinoculation, respectively. Surface and interior cheese samples as well as liquid exudate produced during storage were assayed individually to determine differences in Listeria contamination at different sampling locations. L. monocytogenes was more easily differentiated from L. innocua on RLMA than LMBA and Oxford agar. Similar L. monocytogenes counts (ca. 10(4) CFU/g) were obtained on the last sampling day on the surface and interior of cheese samples (P > 0.05) for all storage temperatures and both initial inoculation levels, but smaller cell numbers were detected in the exudate produced during storage. In addition, simultaneous inoculation of L. innocua with L. monocytogenes did not affect the final L. monocytogenes counts in the cheese. The amount of exudate released from the cheese and decrease of pH correlated with storage temperature. More exudate was produced and a greater decrease of pH occurred at 21 degrees C than at 12 or 4 degrees C. Our results indicate that RLMA is a suitable medium for isolating L. monocytogenes from queso fresco cheese. Higher counts of L. monocytogenes were obtained from surface and interior samples of cheese than from the exudate of the cheese during storage. In addition, pH may be a useful indicator of improperly stored queso fresco cheese.
与西班牙风味软奶酪相关的李斯特菌病一直是公共卫生关注的问题。尽管基于分子和免疫技术的快速检测方法已成功应用于食品中单核细胞增生李斯特菌的检测，但获得该病原体的分离株是流行病学研究和监管分析的关键步骤。美国食品药品监督管理局细菌分析手册（BAM）推荐用于从奶酪中分离单核细胞增生李斯特菌的牛津琼脂，无法区分单核细胞增生李斯特菌与其他李斯特菌属物种。因此，将两种选择性分离培养基，单核细胞增生李斯特菌血琼脂（LMBA）和快速单核细胞增生李斯特菌琼脂（RLMA），与牛津琼脂进行比较，以从奶酪中分离单核细胞增生李斯特菌。用单核细胞增生李斯特菌的五菌株混合物或单核细胞增生李斯特菌五菌株混合物与无害李斯特菌以10⁰或10¹CFU/g的量接种新鲜奶酪。奶酪样品分别在21、12和4℃下储存，并在接种后3、7和10天；7、10、14、21天；以及2、4、8和12周时测定李斯特菌数量。分别对奶酪的表面和内部样品以及储存期间产生的液体渗出物进行检测，以确定不同采样位置的李斯特菌污染差异。在RLMA上比在LMBA和牛津琼脂上更容易将单核细胞增生李斯特菌与无害李斯特菌区分开来。对于所有储存温度和两种初始接种水平，在奶酪样品表面和内部的最后采样日获得了相似的单核细胞增生李斯特菌数量（约10⁴CFU/g）（P>0.05），但在储存期间产生的渗出物中检测到的细胞数量较少。此外，将无害李斯特菌与单核细胞增生李斯特菌同时接种不会影响奶酪中最终的单核细胞增生李斯特菌数量。奶酪释放的渗出物量和pH值下降与储存温度相关。与12℃或4℃相比，21℃时产生的渗出物更多，pH值下降更大。我们的结果表明，RLMA是从新鲜奶酪中分离单核细胞增生李斯特菌的合适培养基。在储存期间，从奶酪的表面和内部样品中获得的单核细胞增生李斯特菌数量高于从奶酪渗出物中获得的数量。此外，pH值可能是新鲜奶酪储存不当的一个有用指标。
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