Figure 7.

Bacteria can adhere to the cell membrane for a long time but rarely infect. Bacterial positions from the microscopy video were analysed both by recording tracks of individual bacteria, and by analysing the time a bacterium stays attached to a macrophage. (a) Shows the time-integrated signal from bacteria, with the colour of a pixel representing the accumulated signal (the overall probability that bacteria were at that position). There is a large probability of finding bacteria on the macrophage exterior, and inside the cells, there are, in general, no bacteria. Most of the bacteria adhering to the macrophages will swim away during this movie. In the intercellular medium, the bacteria are swimming in more or less straight lines, with random directions. There is no discernible chemotaxis. (b) Shows the frequency distribution of the time that a bacterium spends apparently sticking to a macrophage's surface. One hundred and ninety-two events were recorded, and 10 of these were over 10 s.