Expressing Our Individuality, the Way E. Coli Do
A colony of genetically identical E. coli is, in fact, a mob of individuals. Under identical conditions, they will behave in different ways. They have fingerprints of their own.
If two genetically identical E. coli are swimming side by side, for example, one may give up while the other keeps spinning its corkscrew-shaped tails. To gauge E. coli’s stamina, the late biologist Daniel Koshland once glued genetically identical bacteria to a glass cover slip. They floated in water, tethered by their tails. Dr. Koshland offered the bacteria a taste of aspartate, an amino acid that attracts them and motivates them to swim. Stuck to the slide, the bacteria could only pirouette. Dr. Koshland found that some E. coli clones twirled for twice as long as others.
E. coli expresses its individuality in many other ways, as well. Under identical conditions, some clones cover themselves in sticky hairs that let them stick to host cells, while others remain bald. Feed a colony of E. coli lactose (the sugar in milk), and some will respond by slurping it up through special channels and digesting it with special enzymes. Others will turn up their microbial noses.
These quirks of E. coli’s personality can mean the difference between life and death for the bacteria. In times of stress, some members of a colony respond by building thousands of toxin molecules and then burst open, killing off the unrelated E. coli around them. Their fellow clones survive, though, and thrive without the competition.
Certain viruses slip into E. coli through one of the many kinds of channels in its membrane. In a colony of genetically identical bacteria, some may be covered with these channels like pincushions. Others have none at all. The viruses will kill the vulnerable clones, while the other clones live on.
E. coli’s quirks can be a matter of life and death for us, as well. Some strains cause infections in the gut, the bladder, the blood and even the brain. In many cases, doctors try to kill the bacteria with antibiotics, which jam up the normal workings of their genes and proteins. In a susceptible colony of E. coli, a strong antibiotic will kill most of the bacteria, but not all of them. Some will survive.
The survivors escape death because they are trapped in a strange twilight existence called persistence. They make hardly any new proteins and grow barely, if at all. Antibiotics can’t kill persisters because there’s nothing in them to attack. The difference between normal cells and persisters cannot be found in their DNA. After persister cells survive an attack of antibiotics, some of their offspring switch back to normal growth and rebuild the colony. Most of their descendants will be normal E. coli. But some will be persisters. The colony remains the same motley crew of clones.
The key to understanding E. coli’s fingerprints is to recognize that the bacteria are not simple machines. Unlike wires and transistors, E. coli’s molecules are floppy, twitchy and unpredictable. In an electronic device, like a computer or a radio, electrons stream in a steady flow through the machine’s circuits, but the molecules in E. coli jostle and wander. When E. coli begins using a gene to make a protein, it does not produce a smoothly increasing supply. It spurts out the proteins in fits and starts. One clone may produce half a dozen copies of a protein in an hour, while a clone right next to it produces none.