Abstract: 【Abstract】  Objective   To investigate the change of autophagy and morphological characteristics of the autophagic structures in free-living amoeba under nutritional stress.  Methods   Free-living amoebae were incubated on the agaric solid medium which had been covered with Escherichia coli in control group. In the experiment group， amoebae incubated on the agaric solid medium with E. coli were collected and moved to another solid medium without E. coli and incubated for 12 h. The morphological changes of free-living amoeba in the medium without E. coli were viewed with scanning electron microscope. The changes of autophagy and the structural features of the autophagosome precursors， autophagosomes and autophagolysosomes in amoeba were examined with transmission electron microscope， and the cross-section areas of the autophagic structures and cytoplasm were measured with an image analyzer. The autophagosomes in the organism were labeled with monodansylcadaverine （MDC） staining and quantitated using laser scanning confocal microscope.   Results   In the control group， free-living amoebae were all in the form of trophozoite. In the experiment group， trophozoites were induced to transform to cysts gradually. In control group， amoeba was full of fragment of E. coli. There was merely little autophagy with fewer autophagic structures in amoeba. When compared with the control group， the autophagic abilities of amoeba were enhanced significantly， number of autophagic structures increased in the experiment group. In addition， the ratio of the cross-sectional areas of the autophagic structures to that of the cytoplasm of amoeba was greater （P＜0.05 or 0.01）. There was fragment of E. coli that was not digested in some of the amoebae.  Conclusion   In the circumstance of nutritional stress， amoebic trophozoites were induced to transform to cysts gradually. The autophagic ability of free-living amoeba significantly enhanced.

Key words: Nutritional stress, Free-living amoeba, Trophozoite, Cyst, Autophagy