In addition to competition for limiting resources, some microorganisms may excrete substances that are toxic for other species. This chemical warfare is called allelopathy.
An interesting example of allelopathy has been named the rock-scissors-paper experiment after the popular children's game: rock crushes scissors, scissors cut paper, and paper covers rock. Stanford biologist Ben Kerr and colleagues showed that three naturally occurring strains of E. coli can play their own version of this game. One strain can produce a potent toxin called colicin. Colicin kills colicin-sensitive bacteria, but the colicin-producing strain is resistant to its own toxin. A second strain cannot produce colicin, but is nevertheless resistant to it. A third strain does not produce colicin either, but it is sensitive to colicin and hence killed by the toxin. However, this colicin-sensitive strain has the advantage of growing faster than the other two strains. The colicin-resistant strain, in turn, grows slower than the colicin-sensitive strain but faster than the colicin-producing strain. The colicin-producing strain has the lowest growth rate of all, because it needs to invest a lot of energy in producing the toxin. As a result, the colicin-sensitive strain displaces the colicin-resistant strain, the colicin-resistant strain displaces the colicin-producing strain, and the colicin-producing strain displaces the colicin-sensitive strain. In this way, the three strains can chase each other around on a petri dish in a rock-scissors-paper fashion.
Interestingly, when the three E. coli strains were mixed in liquid medium, the rock-scissor-paper dynamics rapidly crashed. In the homogeneous environment of a well-mixed flask, first the colicin-producing strain killed the colicin-sensitive strains, and then the colicin-resistant strain outgrew the colicin-producing strain, such that in the end only the colicin-resistant strain survived. Thus, the spatial playground provided by a petri dish seemed essential for the long-term maintenance of the rock-scissors-paper game. Spatial interactions may play a crucial role in many microbial ecosystems. Furthermore, this is an experimental example where the principle of competitive exclusion does not apply. On the petri dish, none of the three species goes extinct and, hence, the biodiversity of the ecosystem is preserved. The rock-scissors-paper system maintains its biodiversity, because the system never settles at equilibrium but remains in constant motion.
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