U.S. Department of Agriculture, Agricultural Research Service, U.S. Meat Animal Research Center, P.O. Box 166, State Spur D, Clay Center, Nebraska 68933, USA.
ORCID: https://orcid.org/0000-0002-5266-1746 [M.G.].
J Food Prot. 2020 Aug 1;83(8):1438-1443. doi: 10.4315/JFP-20-103.
Prevalence of heat-resistant bacteria in beef poses a potential problem as thermal interventions are routinely used in beef processing to control contamination. Despite extreme heat-resistant (XHR) Escherichia coli having been isolated from a ground beef processing plant, there has not been a study to assess the prevalence of XHR E. coli among types of cattle. Therefore, this study used a screening assay for XHR gram-negative bacteria and its molecular determinant, the locus of heat resistance (LHR), on feces collected from U.S. cattle. Fecal samples were collected from fed (n = 538), cull dairy (n = 425), and cull beef (n = 475) cattle at nine regional beef processing plants located across the United States. Among the 1,438 cattle sampled from northern (n = 288), southern (n = 288), eastern (n = 287), western (n = 287), and central (n = 288) regions of the United States, 91 (6.3%) cattle showed presence of XHR bacteria, as evident by growth in MacConkey broth following heat treatment of 80°C for 15 min, in their feces. Heat-resistant bacteria (n = 140) were isolated from the 91 fecal samples. Prevalence of XHR bacteria was highest (11%) in cattle from the northern region. Ninety percent of the XHR isolates were identified as E. coli. Multiplex PCR of all 1,438 fecal samples showed that the LHR was absent in 40.7% of samples and intact in 18.7% of samples. Despite the higher prevalence of intact LHR from PCR analysis, only 11 samples (0.8%) were confirmed to contain bacteria with an intact LHR. The LHR was absent in 91% of XHR bacteria, and only 7.9% of XHR bacteria had intact LHR, suggesting a novel mechanism of heat resistance. By developing and using the screening assays, we established the prevalence of XHR bacteria (6.3%) and LHR+ bacteria (0.8%) in U.S. beef cattle.
耐热菌在牛肉中的存在是一个潜在的问题，因为在牛肉加工过程中，通常会采用热干预措施来控制污染。尽管已经从一家绞碎牛肉加工厂分离出了极端耐热（XHR）大肠杆菌，但尚未有研究评估 XHR 大肠杆菌在牛种中的流行情况。因此，本研究使用一种筛选试验来检测来自美国牛的粪便中的 XHR 革兰氏阴性菌及其分子决定因素耐热基因座（LHR）。从美国九个地区的牛肉加工厂采集了 fed（n = 538）、淘汰奶牛（n = 425）和淘汰牛肉（n = 475）牛的粪便样本。在来自美国北部（n = 288）、南部（n = 288）、东部（n = 287）、西部（n = 287）和中部（n = 288）地区的 1438 头采样牛中，有 91 头（6.3%）牛的粪便中存在 XHR 细菌，这是通过 80°C 热处理 15 分钟后在 MacConkey 肉汤中生长而明显的。从 91 份粪便样本中分离出耐热细菌（n = 140）。来自北部地区的牛中 XHR 细菌的流行率最高（11%）。90%的 XHR 分离株被鉴定为大肠杆菌。对所有 1438 份粪便样本进行多重 PCR 分析显示，LHR 缺失的样本占 40.7%，完整的样本占 18.7%。尽管从 PCR 分析中 LHR 完整的比例较高，但只有 11 个样本（0.8%）被确认为含有完整 LHR 的细菌。LHR 在 91%的 XHR 细菌中缺失，只有 7.9%的 XHR 细菌具有完整的 LHR，这表明存在一种新的耐热机制。通过开发和使用筛选试验，我们确定了美国牛肉牛中 XHR 细菌（6.3%）和 LHR+细菌（0.8%）的流行率。
