Santa Cruz
ONE person鈥檚 waste is another鈥檚 livelihood. And the same rule, it seems,
applies in the microbial world: slow-growing strains of a bacterium found in our
guts hold their own against faster-growing strains by feeding off their
competitors鈥� excreta.
As part of a series of experiments on bacterial evolution, researchers led by
Paul Turner of Michigan State University in East Lansing grew hybrids of
laboratory strains of Escherichia coli. According to classical
ecological theory, such an environment should soon become dominated by a single
strain. 鈥淲hat we expect is that the most fit competitor should take over,鈥� says
Turner.
Advertisement
Instead, two strains settled into an apparently stable coexistence. When
Turner looked more closely, he found that the two used very different strategies
for survival. Every 24 hours, he took a small sample of the culture and
transferred it into a fresh flask of broth rich in glucose. In this new growth
medium, one strain initially multiplied quickly, but stopped growing once it had
consumed all the glucose. But the other strain continued to grow, albeit
slowly.
Through further experiments, Turner showed that the slow-growing strain could
only keep growing if the more competitive strain was also present鈥攚hich
indicated that the sluggish strain was feeding off the by-products of its
companion鈥檚 metabolism (Ecology, vol 77, p 2119).
These studies show how easily natural systems can transform simple
environments into more complex ones, says Turner. He believes that biologists
may have overlooked other examples of 鈥渃ross-feeding鈥�, because most bacterial
strains are difficult to tell apart.