Lekkas Charidimos, Kakouri Athanasia, Paleologos Evaggelos, Voutsinas Leandros P, Kontominas Michael G, Samelis John
National Agricultural Research Foundation, Dairy Research Institute, Katsikas, Ioannina 45221, Greece.
Food Microbiol. 2006 May;23(3):268-76. doi: 10.1016/j.fm.2005.03.008. Epub 2005 Jun 8.
Post-process contamination of fresh acid-curd cheeses with Escherichia coli O157:H7 may pose a risk considering the low infectious dose and the ability of the pathogen to survive in acidic foods. To evaluate its survival in Galotyri, a traditional Greek acid-curd cheese, portions (0.5 kg) of two commercial fresh products, one artisan (pH 3.9+/-0.1) and the other industrial (pH 3.7+/-0.1), were inoculated with approximately 3.0 or 6.5 log cfu g(-1) of a five-strain cocktail of E. coli O157:H7, including rifampicin-resistant derivatives of the strains ATCC 43895 and ATCC 51657, and stored aerobically at 4 and 12 degrees C. Survival was monitored for 28 days by plating cheese samples on tryptic soy agar with 100 mg l(-1) rifampicin (TSA+Rif), SMAC and Fluorocult E. coli O157:H7 agar media. The pathogen declined much faster (P<0.05) in the industrial as compared to the artisan cheeses at both temperatures. Thus, while E. coli O157:H7 became undetectable by culture enrichment after 14 days at 4 degrees C in industrial samples, irrespective of the inoculation level, populations of 1.4-1.9 and 4.2-5.1 log cfu g(-1) survived after 28 days in the corresponding artisan cheeses with the low and high inocula, respectively. Survival was longer and greater (P<0.05) on TSA+Rif than on SMAC and Fluorocult, indicating the presence of acid-injured cells. Interestingly, survival of E. coli O157:H7 after 14-28 days in cheeses was better at 12 degrees C than at 4 degrees C, probably due to yeasts which grew on the surface of temperature-abused cheeses. The large difference in the pathogen's inactivation between the industrial and artisan cheeses at 4 degrees C could not be associated with major differences in pH or type/concentration of organic acids, suggesting another anti-E. coli O157:H7 activity by the industrial starter. The high survival of the pathogen in artisan Galotyri under conditions simulating commercial storage should be of concern.
考虑到大肠杆菌O157:H7的低感染剂量以及该病原体在酸性食品中存活的能力，新鲜酸凝乳干酪的后处理污染可能会带来风险。为了评估其在希腊传统酸凝乳干酪Galotyri中的存活情况，将两种市售新鲜产品（一份0.5千克），一种是手工制作的（pH 3.9±0.1），另一种是工业化生产的（pH 3.7±0.1），接种约3.0或6.5 log cfu g⁻¹的大肠杆菌O157:H7五菌株混合物，其中包括菌株ATCC 43895和ATCC 51657的耐利福平衍生物，并在4℃和12℃有氧条件下储存。通过将奶酪样品接种在含有100 mg l⁻¹利福平的胰蛋白胨大豆琼脂（TSA+Rif）、SMAC和Fluorocult大肠杆菌O157:H7琼脂培养基上，监测存活情况28天。在两个温度下，与手工制作的奶酪相比，该病原体在工业化生产的奶酪中下降得更快（P<0.05）。因此，在4℃下，无论接种水平如何，工业化生产的样品在14天后通过培养富集法无法检测到大肠杆菌O157:H7，而在相应的手工制作奶酪中，低接种量和高接种量的样品在28天后分别存活1.4 - 1.9 log cfu g⁻¹和4.2 - 5.1 log cfu g⁻¹。在TSA+Rif上的存活时间比在SMAC和Fluorocult上更长且数量更多（P<0.05），表明存在酸损伤细胞。有趣的是，在14 - 28天内，大肠杆菌O157:H7在奶酪中的存活情况在12℃下比在4℃下更好，这可能是由于在温度受虐的奶酪表面生长的酵母。在4℃下，工业化生产的奶酪和手工制作的奶酪在病原体灭活方面的巨大差异与pH值或有机酸的类型/浓度的主要差异无关，这表明工业化发酵剂具有另一种抗大肠杆菌O157:H7的活性。在模拟商业储存条件下，病原体在手工制作的Galotyri奶酪中的高存活率应引起关注。
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