Essentially, transformation means a change. It could mean pretty much any kind of change: maybe a person changed their lifestyle for the better, or renovated a seriously damaged house. But did you know that it is possible for bacteria to change their genetic makeup? Horizontal gene transfer occurs when DNA is transferred to a cell, but not through cell division. One example of horizontal gene transfer is transformation: when a bacterial cell takes up raw, unpackaged DNA from the environment and incorporates it into its own DNA. Naturally competent bacteria are naturally capable of collecting new DNA from their environment. More hesitant bacteria can be encouraged by adding chemicals, heat shock, or electroporation (applying an electric current).

We can observe an example of transformation within Acinetobacter calcoaceticus. The wild-type strain of this species is able to create tryptophan (an amino acid it needs for nutrition) from raw ingredients. As an analogy, think of the wild-type strain as a talented cook; if it has the ingredients it can cook the meal. However, a certain mutant strain of Acinetobacter calcoaceticus cannot synthesize tryptophan from raw ingredients; it needs ready-made tryptophan molecules. Think of the mutant strain as not knowing how to cook, and as a huge fan of ready-made TV dinners. However, there is hope for our mutant strain- the bacteria can gain the genes necessary to synthesize tryptophan, or "learn how to cook". If we mix cells of the mutant strain with DNA from the wild-type strain, the mutant can take up the DNA, and become able to synthesize tryptophan. Want a quick history lesson? The first transformation was observed in 1928, by bacteriologist Frederick Griffith, as he considered making a vaccine against pneumonia. He worked with the species Streptococcus pneumoniae, which was able to form capsules (a protective coating around cells). In the process of his experiment, newly grown bacteria did not form capsules. A few years later in 1944, scientists Oswald T. Avery, Colin M. MacLeod, and Maclyn McCarty concluded that DNA was responsible for this change.

Transformation is only one type of horizontal gene transfer. Transfection is a subtype of transformation where the bacterial cell takes DNA from a virus. Conjugation happens when two cells physically touch each other, and DNA is passed along. Transduction occurs when a bacteriophage (virus that exclusively infects bacteria) transfers DNA between bacterial cells. Why do you think horizontal gene transfer might be useful for bacteria?