Institute of Food, Nutrition and Health, ETH Zurich, Schmelzbergstrasse 7, 8092 Zurich, Switzerland.
Environ Microbiol. 2013 Feb;15(2):433-46. doi: 10.1111/j.1462-2920.2012.02858.x. Epub 2012 Aug 23.
Despite its prominent role as an intracellular human pathogen, Listeria monocytogenes normally features a saprophytic lifestyle, and shares many environmental habitats with predatory protozoa. Earlier studies claimed that Acanthamoeba may act as environmental reservoirs for L. monocytogenes, whereas others failed to confirm this hypothesis. Our findings support the latter and provide clear evidence that L. monocytogenes is unable to persist in Acanthamoeba castellanii and A. polyphaga. Instead, external Listeria cells are rapidly immobilized on the surface of Acanthamoeba trophozoites, forming large aggregates of densely packed bacteria that we termed backpacks. While the assembly of backpacks is dependent on bacterial motility, flagellation alone is not sufficient. Electron micrographs showed that the aggregates are held together by filaments of likely amoebal origin. Time-lapse microscopy revealed that shortly after the bacteria are collected, the amoeba can change direction of movement, phagocytose the backpack and continue to repeat the process. The phenomenon was also observed with avirulent L. monocytogenes mutants, non-pathogenic Listeria, and other motile bacteria, indicating that formation of backpacks is not specific for L. monocytogenes, and independent of bacterial pathogenicity or virulence. Hence, backpacking appears to represent a unique and highly effective strategy of Acanthamoeba to trap and feed on motile bacteria.
尽管李斯特菌（Listeria monocytogenes）作为一种重要的人类细胞内病原体，但它通常以腐生生活方式为主，并与捕食性原生动物共享许多环境栖息地。早期的研究声称棘阿米巴（Acanthamoeba）可能是李斯特菌的环境储主，但其他研究未能证实这一假说。我们的研究结果支持后者，并提供了明确的证据表明李斯特菌无法在棘阿米巴属（Acanthamoeba）的两个种（即棘阿米巴双型变种（Acanthamoeba castellanii）和棘阿米巴多形变种（A. polyphaga）中存活。相反，外部李斯特菌细胞会迅速固定在棘阿米巴滋养体的表面，形成由大量紧密堆积的细菌组成的大聚集体，我们称之为背包。虽然背包的组装依赖于细菌的运动性，但鞭毛本身并不足以形成。电子显微镜显示，这些聚集体是由可能来源于阿米巴的丝状结构连接在一起的。延时显微镜观察表明，在细菌被收集后不久，阿米巴可以改变运动方向，吞噬背包并继续重复这个过程。这个现象也在无毒李斯特菌突变株、非致病性李斯特菌和其他运动性细菌中观察到，表明背包的形成并非特异性地针对李斯特菌，也不依赖于细菌的致病性或毒力。因此，背包现象似乎代表了棘阿米巴捕获和吞噬运动性细菌的独特而高效的策略。
