Dineen S S, Takeuchi K, Soudah J E, Boor K J
Food Science Department, Cornell University, Ithaca, New York 14853, USA.
J Food Prot. 1998 Dec;61(12):1602-8. doi: 10.4315/0362-028x-61.12.1602.
We examined (i) the persistence of Escherichia coli O157:H7 as a postpasteurization contaminant in fermented dairy products; (ii) the ability of E. coli O157:H7 strains with and without the general stress regulatory protein, RpoS, to compete with commercial starter cultures in fermentation systems; and (iii) the survival of E. coli O157:H7 in the yogurt production process. In commercial products inoculated with 10(3) CFU/ml, E. coli O157:H7 was recovered for up to 12 days in yogurt (pH 4.0), 28 days in sour cream (pH 4.3), and at levels > 10(2) CFU/ml at 35 days in buttermilk (pH 4.1). For the starter culture competition trials, the relative inhibition of E. coli O157:H7 in the experimental fermentation systems was, in decreasing order, thermophilic culture mixture, Lactobacillus delbrueckii subsp. bulgaricus R110 alone, Lactococcus lactis subsp. lactis D280 alone, Lactococcus lactis subsp. cremoris D62 alone, and Streptococcus thermophilus C90 alone showing the least inhibition. Recovery of the rpoS mutant was lower than recovery of its wild-type parent by 72 h or earlier in the presence of individual starter cultures. No E. coli O157:H7 were recovered after the curd formation step in yogurt manufactured with milk inoculated with 10(5) CFU/ml. Our results show that (i) postprocessing entry of E. coli O157:H7 into fermented dairy products represents a potential health hazard; (ii) commercial starter cultures differ in their ability to reduce E. coli O157:H7 CFU numbers in fermentation systems; and (iii) the RpoS protein appears to most effectively contribute to bacterial survival in the presence of conditions that are moderately lethal to the cell.
（i）大肠杆菌O157:H7作为发酵乳制品巴氏杀菌后污染物的持续性；（ii）有和没有一般应激调节蛋白RpoS的大肠杆菌O157:H7菌株在发酵系统中与商业发酵剂培养物竞争的能力；以及（iii）大肠杆菌O157:H7在酸奶生产过程中的存活情况。在接种了10³CFU/ml的商业产品中，大肠杆菌O157:H7在酸奶（pH 4.0）中可回收长达12天，在酸奶油（pH 4.3）中可回收28天，在酪乳（pH 4.1）中35天时回收水平>10²CFU/ml。对于发酵剂培养物竞争试验，在实验发酵系统中，大肠杆菌O157:H7的相对抑制程度由高到低依次为嗜热培养物混合物、单独的德氏乳杆菌保加利亚亚种R110、单独的乳酸乳球菌乳酸亚种D280、单独的乳酸乳球菌乳脂亚种D62以及单独的嗜热链球菌C90，后者显示出的抑制作用最小。在存在单个发酵剂培养物的情况下，rpoS突变体的回收率在72小时或更早时就低于其野生型亲本。在用接种了10⁵CFU/ml牛奶生产的酸奶中，凝乳形成步骤后未回收到大肠杆菌O157:H7。我们的结果表明：（i）大肠杆菌O157:H7在加工后进入发酵乳制品代表了一种潜在的健康危害；（ii）商业发酵剂培养物在降低发酵系统中大肠杆菌O157:H7 CFU数量的能力方面存在差异；以及（iii）在对细胞有中度致死作用的条件下，RpoS蛋白似乎最有效地促进了细菌的存活。
