Verheul A, Wouters J A, Rombouts F M, Abee T
Department of Food Technology and Nutritional Sciences, Wageningen University and Research Centre, The Netherlands.
J Appl Microbiol. 1998 Dec;85(6):1036-46. doi: 10.1111/j.1365-2672.1998.tb05269.x.
Exogenously provided carnitine (beta-hydroxy-L-tau-N-trimethyl aminobutyrate) was found to stimulate aerobic growth of enterohaemorrhagic Escherichia coli O157:H7 in a medium of inhibitory osmotic strength. Its osmoprotective ability is comparable with that of betaine. As carnitine is an important compound in mammalian tissues, it is suggested that it might play a role in the growth of the pathogen on low water activity (aw) meat products. Using specific uptake mutants of E. coli K-12, it was established that, under osmotic stress, carnitine accumulates in the cytoplasm following import through the ProP and ProU transport systems. Betaine and carnitine also protect E. coli cells while growing anaerobically at inhibitory osmolarity. Under these conditions, an E. coli K-12 strain with lesions in both proP and proU accumulates low levels of L-carnitine but fails to accumulate betaine when these compounds are supplied in the external medium. This is probably a result of uptake of L-carnitine by the secondary transporter CaiT. The caiT gene forms part of the caiTABCDE operon which encodes the carnitine pathway, and is transcribed during anaerobic growth in the presence of carnitine. However, further experiments revealed that the carnitine pathway, including CaiT, does not play a significant role in osmoregulation of E. coli during anaerobiosis. Together, the results indicate that ProP and ProU are the sole transport systems involved in carnitine influx, both in aerobically and anaerobically osmotically stressed E. coli cells.
研究发现，外源性提供的肉碱（β-羟基-L-τ-三甲基氨基丁酸）可刺激肠出血性大肠杆菌O157:H7在具有抑制性渗透压的培养基中进行需氧生长。其渗透保护能力与甜菜碱相当。由于肉碱是哺乳动物组织中的一种重要化合物，因此有人认为它可能在低水分活度（aw）肉制品上的病原体生长中发挥作用。利用大肠杆菌K-12的特异性摄取突变体，确定在渗透胁迫下，肉碱通过ProP和ProU转运系统导入后在细胞质中积累。甜菜碱和肉碱在抑制性渗透压下厌氧生长时也能保护大肠杆菌细胞。在这些条件下，proP和proU均有损伤的大肠杆菌K-12菌株积累的L-肉碱水平较低，但当在外部培养基中提供这些化合物时，该菌株无法积累甜菜碱。这可能是由于二级转运蛋白CaiT摄取L-肉碱的结果。caiT基因是caiTABCDE操纵子的一部分，该操纵子编码肉碱途径，并且在有肉碱存在的厌氧生长过程中被转录。然而，进一步的实验表明，包括CaiT在内的肉碱途径在大肠杆菌厌氧期间的渗透调节中并不起重要作用。总之，结果表明ProP和ProU是在需氧和厌氧渗透胁迫的大肠杆菌细胞中参与肉碱流入的唯一转运系统。
