Mutations (mew-tay-shuns) are misspellings or changes in the genetic code. They're pretty rare; only about one mutation occurs for every 1-10 million DNA bases.
But microbes reproduce very quickly. For example some bacteria can divide every nine minutes. And the entire DNA string or genome (gee-nome) of microbes is relatively small. For example, the average bacterium has only about five million DNA bases. So in each new generation of bacteria, there may be one or two mistakes in the genetic code.
Some mutations are random "mistakes." They can occur when a microbe is exposed to radiation or chemicals that cause changes in DNA. Microbes, like us, have DNA repair proteins that, like a mini construction crew, work hard to repair any such mistakes. They don't always catch every mistake, however.
Other mutations result from a change in a creature's circumstances. For example, if the temperature starts getting hotter and hotter, "jumping genes"—pieces of DNA that can move around—may pick up and move to a new place in the microbe's DNA. This switch may result in giving the microbe a new genetic trait—like greater heat tolerance—that helps it adapt and survive better. It's like a homebuilder who, upon realizing that he's building in a hurricane-prone area, decides that the heavy oak wood he planned to use for hardwood floors is more urgently needed to build the beams that support the house, so he changes it on the blueprint. Jumping genes cause genetic changes in all kinds of microbes.
Another way bacteria can undergo mutation is to take up bits of free-floating DNA in the environment. This is called transformation. Bits of DNA litter the environment when it's released from dying bacterial cells or bacteria burst open by a virus infection. DNA in the form of plasmids—circular bits of DNA that contain genes but exist outside the main chromosome—are also released by bacteria into the environment where they can be taken up by other bacteria.
Sometimes mutations are harmful and the microbes die off before they can reproduce and pass the mutations on to offspring. But other times a mutation gives rise to a new trait that helps the microbe better survive. Take antibiotic resistance among bacteria.
Antibiotics (an-tea-bye-ah-ticks) are life-saving drugs that kill bacteria. We discovered and began using them about 60 years ago. In that relatively short amount of time, bacteria have developed new traits through mutations that help protect them against antibiotics—sort of like a helmet protects you against injury.
Antibiotics attack and kill off bacteria
without the mutation...
The mutated ones survive after the
antibiotics are gone...
...and reproduce, passing along the mutation
to their offspring...
Eventually there are more antibiotic-resistant
bacteria than non-resistant.
Other mutations have helped microbes adapt to all sorts of environments from salty to icy to extremely hot and live off everything from decaying leaves to sunlight to bubbling sulfur.