The first genetic engineer was a scientist named Fred Griffith. In 1928, he was studying how Streptococcus pneumoniae, the bacteria that causes pneumonia, is able to evade the immune system.
Its trick is really quite simple. It has a gene for an enzyme that catalyzes the formation of a gooey capsule. Inside the capsule, the bacterium is protected.
Griffith worked with mice, animals which are extremely susceptible to the Streptococcus bacteria. The basic experimental technique was to inject an unfortunate mouse with bacteria. Within a day, it would be dead.
But Griffith had found a few naturally mutated Streptococci that lacked the protective capsule. Evidently they had lost the function of the relevant gene. He was able to reproduce those bacteria in great numbers and when mice were injected with the capsuleless strain, they did the mice no harm.
If the infectious type of bacteria were killed by high temperature and injected into mice, the mice were also unaffected.
But when Griffith injected mice with both the killed infectious Streptococci and the live capsuleless strain, the mice were killed, and furthermore, living infectious Streptococci were found in the dead mouse's blood.
Somehow a dead bacterium was able to pass a gene to a living bacterium. This didn't have to happen in a mouse. If the dead infectious bacteria were whipped up in a blender the resulting mixture of protoplasm was enough to transform a few of the capsuleless bacteria into the infectious type.
Griffith had no idea what he was doing. In 1928, nobody even suspected that DNA was the genetic material. But he was the first person to transfer a gene from one organism to another. (Of course, the bacteria had been doing it for eons with no help from us. In fact, bacteria can get DNA from unrelated species of bacteria.)
Griffith was killed during the bombing of London in 1941. Three years later Oswald T. Avery was able to show that the material necessary to transfer a gene was DNA. Many bacteria have a single chromosome containing most of their genes, but they may also have a very small chromosome called a plasmid, holding only a few genes. Bacteria can pass these tiny plasmids through their cell walls into the outside world, where other bacteria can take them in and use them to build working enzymes.