Collaborative Surface Migration Behavior of Proteus mirabilis
After point inoculation of Proteus mirabilis on the surface of 0.5% nutrient agar medium, the short rod-shaped (1 to 2 µ m long) peritrichous organism (bacteria with randomly dispersed flagella over the cell surface) differentiated into filamentous (10 to 100 µ m long) multinucleated hyperflagellated cells having 50-fold more flagella (not visible in this optical microscopic image) per unit of cell surface and exerted flagellum-dependent surface spreading growth (swarming). However, at the spreading front, there were no bacteria that were moving singly. The elongated bacteria were gathering for collaborative migration to a new surface environment. They seemed to advance as one (e.g., 10 elongated cells were moving together in the first scene of this video). Occasionally some cells were left out from the migrating group; these single cells were unable to move independently. Thus, mutual sliding of elongated cells seemed to generate unified forward migration of the bacterial cluster on surface environments. Similar collaborative swarming has been observed with other bacterial species such as Serratia marcescens . The mechanism of such dynamic collaboration remains to be determined.
Proteus mirabilis cells were inoculated onto 0.5% nutrient agar medium and incubated at 37°C. Bacterial migration behavior was examined at 6 to 12 hours postinoculation. The real-time video was recorded by using a phase-contrast microscope DIAPHOTO-TMD (Nikon, Tokyo, Japan) and a CCD camera (Texas Instruments, USA) at 1,000x magnification. The movie was made from the microscopic video analog record using Quick Time Player.
Bacteria are known as unicellular organisms with ability to multiply and swim independently. Such a concept seems to elicit misunderstanding of bacterial life. In nature, most bacteria are living together on surface environments and developing various strategies for cooperative life. Dynamic collaboration shown in this video is a real example of cooperation in the bacterial world. On the other hand, many in-depth questions will arise from seeing this video. Why are single bacterial cells unable to move independently on the surface environments in contrast to randomly swimming single bacteria in liquid environments? How are neighboring cells communicating with each other for collaborative migration as one? How are they finding their collaborative colleagues in a heterogeneous microbial population? The bacterial world is full of mystery.
1. Harshey, R. M., and T. Matsuyama. 1994. Dimorphic transition in Escherichia coli and Salmonella typhimurium: surface-induced differentiation into hyperflagellate swarmer cells. Proc. Natl. Acad. Sci. USA 91:8631–8635.
2. Matsuyama , T., and M. Matsushita. 2001. Population morphogenesis by cooperative bacteria. Forma 16:307–326.
, T., Y. Takagi, Y. Nakagawa, H. Itoh, J. Wakita, and M. Matsushita.
2000. Dynamic aspects of the structured cell population in a swarming colony of Proteus mirabilis. J. Bacteriol. 182:385–393.