Farewell My Host

Once a strain of E. coli establishes itself in our guts, it can remain there for decades. But the bacteria also escape their hosts, by a route that's so obvious there's no need to dwell on it. Suffice it to say that every day the world's human population releases more than a billion trillion E. coli into the environment. Countless more escape from other mammals and from birds. They are swept down sewer pipes and streams, sowed upon the ground and sea. They must withstand summers and winters, droughts and floods. They must eke out an existence without a luxurious diet of half-digested sugar. For long stretches they may have to survive with no food at all. In soil and water there are many predators waiting to devour E. coli, including nematode worms and creeping amoebas. Some predators overpower E. coli by sheer size. Others, such as the bacteria Bdellovibrio, push their way into E. coli's periplasm and destroy it from within. The bacteria Myxococcus xanthus release molecules that smell to E. coli like the whiff of food. The unlucky microbe swims to its own destruction.

Leaving their hosts is probably a quick trip to death for most E. coli. But life can handle bad odds. Oaks shower the ground with acorns, almost none of which survive to become saplings. Our own bodies are made of trillions of cells, only a few of which may escape our own death by giving rise to children. Even if only a tiny fraction of E. coli in the wild survives and manages to find a new host, its life cycle will continue. And E. coli has several tricks for surviving on the outside. Its versatile metabolism lets it feed on many carbon-bearing molecules—even TNT. If a soil predator tries to eat it, the microbe can avoid being digested and instead thrive as a parasite. And if worse comes to worst, E. coli can fold down its DNA into a rugged crystal, slip into the stationary phase, and survive for years.

Or, just perhaps, E. coli can abandon hosts altogether. From time to time, scientists discover populations of E. coli that appear to be thriving as full-time outdoor microbes. In Australia, for example, researchers have discovered huge blooms of E. coli in lakes where none had been expected. The lakes are free of fecal matter, receiving no sewage or farm runoff. Yet on a warm day they are loaded with millions of billions of E. coli. The bacteria seem different from more familiar strains. For one thing they make an unusually tough capsule, which may act as a microbial wet suit, allowing them to survive year-round in the lakes. They no longer need hosts to avoid extinction. They have broken free.

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