Interaction between two micro-organisms

 Experiments using Parameciu

 Biological reactions of a solitary Paramecium cell to mechanical stimula-tions were investigated by Naitoh and Sugino (1984). Avoiding reactions occur when a cell bumps against a solid object with its anterior end. The cell swims backward first, then gyrates about its posterior end, and finally resumes normal forward locomotion. Escape reactions occur when the cell's posterior end is mechanically agitated. The cell increases its forward swimming velocity for a moment, then resumes normal forward locomo-tion. The change in the swimming motion is regulated by membrane po-tential charges, because Paramecium cells, as well as other monads, have no nerves for transmitting stimulative information nor synapses to deter-mine transmission direction. Machemer (1974) clarified the directional re-sponses of cilia to membrane potential changes in Paramecium cells. There are many Ca^^ channels in the anterior end, whereas there are many

 K^ channels in the posterior end. The locality of ion channels is the essen-tial mechanism of controlling Paramecium!^ biological reaction. Interactions between two Paramecium were investigated by Ishikawa and Hota (2006). In this section, we briefly introduce the study using P. caudatum. Microscopic observation showed that the body length of an in-dividual cell is in the range of approximately 200-250|Lim and the width is in the range of approximately 40-50|um. The swimming speed of an indi-vidual cell was approximately Imm/s. The swimming motion of P. cauda-tum in a free space was not straight, but formed a left spiral. The pitch of the spiral was approximately 2mm and the width was 0.4mm. The experimental setup was designed to measure the displacements of cells in a still fluid between flat plates as shown in figure 1. The experi-mental setup consisted of a digital video (DV) camera with a x 24 mac-rolens, light sources, and inner and outer dishes. The test fluid was placed between the top of the outer dish and the bottom of the inner dish. The gap between the two dishes was about 70|Lim, so that cells could not overlap three-dimensionally. The test fluid was same as the culture fluid, but the volume fraction of cells was adjusted to be in the range of about 0.5-1% so that three-cell interactions rarely occurred. A typical hydrodynamic interaction, observed in the study is shown in figure 2. When one cell collides with the anterior end of the other cell, the two cells tend to swim side by side at first, then move away from each other with an acute angle, as shown in figure 2. When two cells are ini-tially facing, they come close to each other at first, then they change their