Difference in swimming speed and residence time

Swimming speed

 The difference in forward and backward swimming speed of V. alginolyticus cells was previously found (Magariyama et al. 2001). The swimming speed of YM4 cells was measured by a high-intensity dark-field microscopy. Figures 5(a) and (b) are histograms of the forward and backward swim-ming speeds, respectively. It is evident from the figures that the backward swimming speed is faster than the forward swimming speed.

Swimming characteristics of a V. alginolyticus strain, YM4. There were 150 measurements, (a) Histogram of forward swimming speeds, (b) Histogram of back-ward swimming speeds. From Magariyama et al. (2001) with permission.

 Residence time 

 Since the focal depth of a microscope is limited, a bacterial cell appears when the cell enters a certain thickness layer that is in focus, and disappears when the cell leaves that layer. Here, the residence time of a cell is defined as the period in which the cell is in focus. A suspension of cells was sealed in the simple chamber constructed from a glass slide, a cover slip and spacers. This sample was observed with an in-verted phase-contrast microscope. The residence time of three strains of V. alginolyticus, YM4, YM42, NMB102 were measured (Magariyama 2005). Figure 6 shows the frequency distributions of the residence time of the cells swimming the upper (a, b, c), middle (d, e ,f) and lower (g, h, i) layers. The YM4 cells tend to stay longer within the layer of focus when they are near either surface (a, g) than when they are in the middle layer. The NMB102 cells display the same tendency (c, f, i). On the other hand, the residence time of YM42 cells is irrespective of the distance from a surface. The residence time of YM4 and NMB102 swimming close to a surface (a, g, c, i) is longer than the residence time of YM42 (b, e, h). The residence time of YM4 and NMB102 swimming in the middle layer (d, f) is as same as the residence time of YM42 (b, e, h).