Ko R, Smith L T, Smith G M
Department of Food Science and Technology, University of California, Davis 95616.
J Bacteriol. 1994 Jan;176(2):426-31. doi: 10.1128/jb.176.2.426-431.1994.
Listeria monocytogenes is a gram-positive food-borne pathogen that is notably resistant to osmotic stress and can grow at refrigerator temperatures. These two characteristics make it an insidious threat to public health. Like several other organisms, L. monocytogenes accumulates glycine betaine, a ubiquitous and effective osmolyte, intracellularly when grown under osmotic stress. However, it also accumulates glycine betaine when grown under chill stress at refrigerator temperatures. Exogenously added glycine betaine enhances the growth rate of stressed but not unstressed cells, i.e., it confers both osmotolerance and cryotolerance. Both salt-stimulated and cold-stimulated accumulation of glycine betaine occur by transport from the medium rather than by biosynthesis. Direct measurement of glycine betaine uptake shows that cells transport betaine 200-fold faster at high salt concentration (4% NaCl) than without added salt and 15-fold faster at 7 than at 30 degrees C. The kinetics of glycine betaine transport suggest that the two transport systems are indistinguishable in terms of affinity for betaine and may be the same. Hyperosmotic shock and cold shock experiments suggest the transport system(s) to be constitutive; activation was not blocked by chloramphenicol. A cold-activated transport system is a novel observation and has intriguing implications concerning the physical state of the cell membrane at low temperature.
单核细胞增生李斯特菌是一种革兰氏阳性食源性病原体，对渗透胁迫具有显著抗性，且能在冷藏温度下生长。这两个特性使其成为对公众健康的潜在威胁。与其他几种生物一样，单核细胞增生李斯特菌在渗透胁迫下生长时，会在细胞内积累甘氨酸甜菜碱，这是一种普遍存在且有效的渗透剂。然而，当它在冷藏温度的冷胁迫下生长时，也会积累甘氨酸甜菜碱。外源添加的甘氨酸甜菜碱可提高受胁迫而非未受胁迫细胞的生长速率，即它赋予细胞渗透耐受性和冷冻耐受性。甘氨酸甜菜碱的盐刺激积累和冷刺激积累都是通过从培养基中转运而非生物合成实现的。对甘氨酸甜菜碱摄取的直接测量表明，细胞在高盐浓度（4% NaCl）下转运甜菜碱的速度比不添加盐时快200倍，在7℃时比在30℃时快15倍。甘氨酸甜菜碱转运的动力学表明，这两种转运系统对甜菜碱的亲和力无法区分，可能是相同的。高渗休克和冷休克实验表明转运系统是组成型的；氯霉素并未阻断其激活。冷激活转运系统是一个新的发现，对于低温下细胞膜的物理状态具有有趣的启示。
