Meyer-Broseta Stéphanie, Diot Annabelle, Bastian Suzanne, Rivière Jacques, Cerf Olivier
National Veterinary School, Maisons-Alfort, France.
Int J Food Microbiol. 2003 Jan 15;80(1):1-15. doi: 10.1016/s0168-1605(02)00117-4.
A time-series bacteriological analysis has been carried out on milk collected on farms from 1997 to 2001 by a plant producing raw milk soft cheese, with the purpose of assessing the time course of the presence/absence of Listeria monocytogenes. A standard data collection procedure was used, in which farms were tested on a monthly or biweekly basis and 2-3 days after the detection of milk tanker contamination. This procedure yielded low figures for contamination frequencies. The average value and the median of the monthly prevalence of farms detected positive for L. monocytogenes were 2.4 and 0%, respectively. A seasonal effect (with peaks in winter) was observed. Between 1997 and 2001, there was no significant decrease of contamination rates, in spite of the efforts on the contaminated farms. Over the last year of the study (from March 2000 to February 2001), a new data collection procedure was implemented that allowed much better detection of sporadic occurrences. Milk samples were collected from the bulk tank of each participating farm just before pick-up, then stored and subsequently analysed whenever the milk tanker was found contaminated. The average value and the median of the monthly prevalence of positive farms were found equal to 7.7 and 0%, respectively (for a mean prevalence of L. monocytogenes in the milk tanker of 3.2%). These results confirm that farm milk contamination is, most often, a sporadic event In addition to this prevalence study, contamination levels were quantified by enumerating L. monocytogenes using direct plating of small volumes of farm milk previously tested positive. Most often, these levels were extremely low. A simple simulation model shows that, when milk tankers were found positive, contamination levels in the corresponding bulk-tank milk are themselves very low (typically, below 3 L. monocytogenes per millilitre with most probable concentration 0.1 Colony Forming Unit (CFU)/ml and median ranging from 5.10(-2) to 0.1 CFU/ml). Such low levels are very likely to be due to environmental contamination.
一家生产生鲜乳软奶酪的工厂对1997年至2001年期间从农场采集的牛奶进行了时间序列细菌学分析，目的是评估产单核细胞李斯特菌存在与否的时间进程。采用了标准的数据收集程序，即每月或每两周对农场进行检测，并在检测到奶罐污染后2至3天进行检测。该程序得出的污染频率较低。检测出产单核细胞李斯特菌呈阳性的农场每月患病率的平均值和中位数分别为2.4%和0%。观察到季节性影响（冬季出现峰值）。在1997年至2001年期间，尽管对受污染农场采取了措施，但污染率并未显著下降。在研究的最后一年（2000年3月至2001年2月），实施了一种新的数据收集程序，该程序能够更好地检测散发病例。在每个参与农场的奶罐即将被取走之前采集牛奶样本，然后储存起来，并在发现奶罐受污染时随后进行分析。发现呈阳性的农场每月患病率的平均值和中位数分别为7.7%和0%（奶罐中产单核细胞李斯特菌的平均患病率为3.2%）。这些结果证实，农场牛奶污染通常是一个散发性事件。除了这项患病率研究之外，还通过对先前检测呈阳性的少量农场牛奶进行直接平板计数来对产单核细胞李斯特菌进行计数，从而对污染水平进行了量化。大多数情况下，这些水平极低。一个简单的模拟模型表明，当发现奶罐呈阳性时，相应奶罐牛奶中的污染水平本身也非常低（通常，每毫升低于3个产单核细胞李斯特菌，最可能浓度为0.1菌落形成单位（CFU）/毫升，中位数范围为5×10⁻²至0.1 CFU/毫升）。如此低的水平很可能是由于环境污染所致。
