LeJeune Jeffrey T, Hancock Dale D, Besser Thomas E
Food Animal Health Research Program, Ohio Agricultural Research and Development Center, The Ohio State University, 1680 Madison Ave., Wooster, Ohio 44691, USA.
J Clin Microbiol. 2006 Mar;44(3):872-5. doi: 10.1128/JCM.44.3.872-875.2006.
In order to more precisely predict food safety risks, the fecal presence of food-borne pathogens among animals at slaughter must be correctly determined. Quantification of Escherichia coli O157 is also desirable. In two separate experiments, detection and enumeration of a nalidixic acid-resistant strain of E. coli O157 in bovine feces was assessed by culture on MacConkey agar supplemented with nalidixic acid (MACnal) and compared to overnight broth enrichment followed by immunomagnetic separation (IMS) and to direct plating of dilutions of bovine feces onto sorbitol MacConkey agar containing cefixime and tellurite (SMACct). The sensitivity of detection of E. coli O157 by both direct plating and IMS was highly dependent upon the initial concentration of the target organism in the sample. Sensitivity of detection by IMS was poor below 100 CFU/g but was better, and not affected by initial E. coli O157 numbers, above this concentration. Sensitivity of detection of E. coli O157 in bovine feces at low initial concentrations is very poor for both direct plating and IMS. Direct plating of dilutions of bovine feces on SMACct can be used to determine the magnitude of fecal E. coli excretion among cattle excreting greater than 100 CFU/g. Among positive samples identified by direct plating on SMACct, the direct counts of E. coli O157:H7 were highly correlated with the estimates obtained with the MACnal plates (r = 0.88, P < 0.001). Because the majority of cattle excrete less than 10(2) CFU E. coli O157/g feces, most studies, including those using IMS methods, probably grossly underestimate the prevalence of E. coli O157 in cattle.
为了更精确地预测食品安全风险，必须正确测定屠宰动物粪便中食源性病原体的存在情况。对大肠杆菌O157进行定量测定也是可取的。在两项独立实验中，通过在添加了萘啶酸的麦康凯琼脂（MACnal）上培养，评估牛粪便中耐萘啶酸的大肠杆菌O157菌株的检测和计数，并将其与过夜肉汤增菌后进行免疫磁珠分离（IMS）以及将牛粪便稀释液直接接种到含有头孢克肟和亚碲酸盐的山梨醇麦康凯琼脂（SMACct）上的方法进行比较。直接接种法和IMS法检测大肠杆菌O157的灵敏度高度依赖于样品中目标微生物的初始浓度。IMS法在低于100 CFU/g时检测灵敏度较差，但在此浓度以上则较好，且不受初始大肠杆菌O157数量的影响。对于直接接种法和IMS法而言，低初始浓度牛粪便中大肠杆菌O157的检测灵敏度都非常低。将牛粪便稀释液直接接种到SMACct上可用于确定每克粪便中大肠杆菌排泄量大于100 CFU的牛群中粪便大肠杆菌的排泄量。在通过直接接种到SMACct上鉴定出的阳性样本中，大肠杆菌O157:H7的直接计数与MACnal平板获得的估计值高度相关（r = 0.88，P < 0.001）。由于大多数牛每克粪便中排泄的大肠杆菌O157少于10² CFU，包括那些使用IMS方法的研究在内，大多数研究可能严重低估了牛群中大肠杆菌O157的流行率。
