McCarthy J, Holbrook R, Stephens P J
Unilever Research, Bedford, UK.
J Food Prot. 1998 Sep;61(9):1093-7. doi: 10.4315/0362-028x-61.9.1093.
The use of sorbitol MacConkey agar (SMAC) performed poorly in supporting growth of stressed Escherichia coli O157:H7 cells. Up to a 3-log difference was observed between counts on SMAC and tryptone soy agar (TSA). It is critical in the risk assessment of certain foods to be able to enumerate stressed and healthy E. coli O157:H7 in a background of potentially healthy competing bacteria. Investigations carried out to overcome the inhibitory effect of SMAC included the reduction of the selective agent concentration, inclusion of a recovery stage in broth prior to plating out, addition of recovery agents, and delayed exposure to the selective agent. The only successful approach was delayed exposure to the selective agent. This was achieved by resuscitating the stressed cells on a membrane placed on the surface of a TSA plate and, after a defined time period sufficient for full resuscitation, transferring the membrane to the surface of a SMAC plate. The choice of membrane material was critical for maintaining the positive sorbitol color change used to identify wild-type E. coli. Track-etched polycarbonate membranes allowed the typical color reactions to be visualized, whereas cellulose acetate did not. The method was validated with E. coli O157:H7 cells stressed by low pH and high salt conditions, whereby all cells that would previously be undetectable on direct inoculation of SMAC were countable.
山梨醇麦康凯琼脂（SMAC）在支持应激的大肠杆菌O157:H7细胞生长方面表现不佳。在SMAC和胰蛋白胨大豆琼脂（TSA）上的计数之间观察到高达3个对数的差异。在某些食品的风险评估中，能够在潜在健康的竞争性细菌背景下对应激和健康的大肠杆菌O157:H7进行计数至关重要。为克服SMAC的抑制作用而进行的研究包括降低选择剂浓度、在平板接种前在肉汤中设置复苏阶段、添加复苏剂以及延迟接触选择剂。唯一成功的方法是延迟接触选择剂。这是通过将应激细胞在放置于TSA平板表面的膜上复苏，并在足以完全复苏的规定时间段后，将膜转移至SMAC平板表面来实现的。膜材料的选择对于维持用于鉴定野生型大肠杆菌的山梨醇阳性颜色变化至关重要。径迹蚀刻聚碳酸酯膜能使典型的颜色反应可视化，而醋酸纤维素膜则不能。该方法用经低pH和高盐条件应激的大肠杆菌O157:H7细胞进行了验证，由此所有先前直接接种到SMAC上无法检测到的细胞都可计数。
