Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [63]
Horizontal gene transfer not only spreads resistance genes around but also speeds up their evolution. Once a gene evolves some resistance to antibiotics, it can benefit not just its original host but other bacteria that take it up. And once in its new host, the gene can continue to undergo natural selection and become even more effective. Microbes can assemble arsenals to defend themselves against antibiotics, gathering weapons from the community of bacteria rather than just inheriting them from their ancestors.
Biologists were slow to recognize just how important the Shigella outbreak in Japan was. Horizontal gene transfer was helping to create a medical disaster, one that is continuing to unfold. At first biologists did not see much evidence of horizontal gene transfer beyond resistance to antibiotics. In the 1990s, scientists began to compare the entire genome of E. coli with that of other bacteria and make a careful search for traded genes. And when they did, our understanding of the history of life changed for good. Horizontal gene transfer, we now know, is no minor trickle of DNA. It is a flood. And it played a big part in making E. coli what it is today.
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OPEN SOURCE
A YOUNG SPECIES
E. COLI IS TRAILED BY thousands of personal historians. They chronicle the birth of sickening new strains in Omaha and Osaka. They trawl streams, lakes, and the guts of kangaroos. They carefully observe the peculiar ways of mutant strains. As the mutants are passed from lab to lab, frozen in stock centers and thawed for new experiments, scientists draw family trees to track their dynasties. Aside from ourselves, we have chronicled no other species so thoroughly.
The written history of E. coli is now far too big for any single person to read in a lifetime. But it is both vast and shallow. It begins only in 1885, with Theodor Escherich’s first sketches of bunches of rods. Archaeologists can offer a few clues to E. coli’s pre-Escherich existence. In 1983, English peat cutters discovered the body of a 2,200-year-old man preserved in a bog near Manchester. The man had been ritually killed: someone had clubbed him on the head, slit his throat, wrapped a cord tightly around his neck, and then pushed him into the bog. The acidic waters preserved his corpse and even its contents. In his stomach, scientists found barley and mistletoe. And in his intestines they found the DNA of E. coli.
There’s no reason to think that the bog man was the first human ever to carry E. coli. There is every reason to think that its history reaches much farther back. Bacteria have an ancient fossil record. Individual microbes left their marks on rocks as least 3.7 billion years ago. Ocean reefs built by bacteria 3 billion years ago still stretch for miles across Africa and Canada. E. coli does not do such a good job of forming fossils, because of its tenuous existence. But what E. coli lacks in fossils it more than makes up for in the historical record that it carries in its DNA. That genetic record rolls out before us like a carpet, back across millions of years to the origins of E. coli as a species, back farther to a time before life dwelled on dry land, back to the origins of cells, to the earliest days of life itself.
To read this record, it’s necessary to become a genealogist of bacteria. When a mutation arises in an E. coli, it will be passed down to its offspring. That mutation can sometimes serve as a genetic marker, revealing to scientists a group of bacteria that are closely related to one another. It’s these genetic markers that public health workers use when an outbreak of nasty E. coli occurs, in order to trace the pathogens to their source. Other scientists use these markers to draw branches on E. coli’s family tree. They have a long way to go before they finish drawing it, but they’ve already filled in enough branches to learn some profound things about the bacteria.
All living strains of E. coli descend