Fratamico Pina M, Bagi Lori K
US. Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, Pennsylvania 19038, USA.
J Food Prot. 2007 Jul;70(7):1663-9. doi: 10.4315/0362-028x-70.7.1663.
A comparison was made of the relative efficiencies of three enrichment media, RapidChek Escherichia coli O157:H7 enrichment broth (REB), R&F broth (RFB), and modified E. coli broth containing novobiocin (mEC+n), and four selective plating media for detection of cold- and freeze-stressed E. coli O157:H7 in raw ground beef. Ground beef (25 g) was inoculated with E. coli O157:H7 at < or =0.5 and < or =2 CFU/g, and samples were then enriched immediately or were stored at 4 degrees C for 72 h or at -20 degrees C for 2 weeks and then enriched. After 8 or 20 h of enrichment, the cultures were plated onto R&F E. coli O157: H7 chromogenic plating medium, cefixime-tellurite sorbitol MacConkey agar, CHROMagar O157, and Rainbow agar O157 and tested using the RapidChek E. coli O157 lateral flow immunoassay and a multiplex PCR assay targeting the E. coli O157: H7 eae, stx1, and stx2 genes. Recovery of E. coli O157:H7 on the four agar media was 4.0 to 7.9 log CFU/ml with the REB enrichment, 1.4 to 7.4 log CFU/ml with RFB, 1.7 to 6.7 log CFU/ml with mEC+n incubated at 42 degrees C, and 1.3 to 3.3 log CFU/ml from mEC+n incubated at 35 degrees C. The percentages of positive ground beef samples containing nonstressed, cold-stressed, and freeze-stressed E. coli O157:H7 as obtained by plating, the immunoassay, and the PCR assay were 97, 88, and 97%, respectively, with REB, 92, 81, and 78%, respectively, with RFB, 97, 58, and 53%, respectively, with mEC+n incubated at 42 degrees C, and 22, 31, and 25%, respectively, with mEC+n incubated at 35 degrees C. Logistic regression analyses of the data indicated significant main effects of treatment, type of medium, enrichment time, inoculum concentration, and detection method. In particular, a positive result was 1.1 times more likely to occur after 20 h of enrichment than after 8 h, 25 times more likely with RFB and REB than with mEC+n at 35 degrees C, 3.7 times more likely with an initial inoculum of < or = 2.0 CFU/g than with < or = 0.5 CFU/g, 2.5 to 3 times more likely using freeze-stressed or nonstressed bacteria than with cold-stressed bacteria, and 2.5 times more likely by plating than by the immunoassay or the PCR assay. REB had better overall performance for enrichment of cold- and freeze-stressed E. coli O157:H7 present in ground beef than did the other media examined.
比较了三种增菌培养基，即快速检测大肠杆菌O157:H7增菌肉汤（REB）、R&F肉汤（RFB）和含新生霉素的改良大肠杆菌肉汤（mEC+n），以及四种选择性平板培养基用于检测生碎牛肉中经冷应激和冻应激的大肠杆菌O157:H7的相对效率。将25克碎牛肉接种大肠杆菌O157:H7，接种量分别为≤0.5 CFU/g和≤2 CFU/g，然后立即进行增菌或在4℃保存72小时或在-20℃保存2周后再进行增菌。增菌8小时或20小时后，将培养物接种到R&F大肠杆菌O157:H7显色平板培养基、头孢克肟-亚碲酸盐山梨醇麦康凯琼脂、CHROMagar O157和Rainbow琼脂O157上，并使用快速检测大肠杆菌O157侧向流动免疫分析法和针对大肠杆菌O157:H7 eae、stx1和stx2基因的多重PCR分析法进行检测。用REB增菌时，在四种琼脂培养基上大肠杆菌O157:H7的回收率为4.0至7.9 log CFU/ml，用RFB时为1.4至7.4 log CFU/ml，在42℃培养的mEC+n为1.7至6.7 log CFU/ml，在35℃培养的mEC+n为1.3至3.3 log CFU/ml。通过平板接种、免疫分析和PCR分析获得的含有未受应激、冷应激和冻应激大肠杆菌O157:H7的阳性碎牛肉样品百分比，使用REB时分别为97%、88%和97%，使用RFB时分别为92%、81%和78%，在42℃培养的mEC+n分别为97%、58%和53%，在35℃培养的mEC+n分别为22%、31%和25%。对数据进行的逻辑回归分析表明，处理、培养基类型、增菌时间、接种浓度和检测方法具有显著的主要影响。特别是，增菌20小时后出现阳性结果的可能性比8小时后高1.1倍，使用RFB和REB时比在35℃培养的mEC+n高25倍，初始接种量为≤2.0 CFU/g时比≤0.5 CFU/g时高3.7倍，使用冻应激或未受应激细菌时比冷应激细菌高2.5至3倍，通过平板接种检测时比免疫分析或PCR分析高2.5倍。与所检测的其他培养基相比，REB在富集碎牛肉中经冷应激和冻应激的大肠杆菌O157:H7方面具有更好的总体性能。
