What is a Bacterial Artificial Chromosome?

A bacterial artificial chromosome (BAC) is one of a class of tools, called vectors, that microbiologists use to insert genes into a bacterium — usually e coli. Inserting genes alters the properties of the bacterium in a process called transformation. A scientist can alter a strain of bacteria using a BAC, then compare the altered bacteria to an unaltered strain to discover what role the inserted genes play in cell biology. While all vectors are used by scientists in a similar way, the BAC is noteworthy for being able to carry much more genetic material than competing tools.

Over the years, scientists have developed a number of different kinds of vectors for modifying the genetic makeup of bacteria. The bulk of these are created by modifying phages — viruses that infect only bacterial cells — or structures called plasmids. The bacterial artificial chromosome is one of a number of plasmid-based vectors. Plasmids are free-floating rings of DNA that many bacteria contain in addition to their chromosomal DNA. They are not considered a separate form of life, but nonetheless behave something like an organism within an organism: they can reproduce independently of the bacteria in which they "live."

Plasmids like the bacterial artificial chromosome are inserted into bacteria using a process called electroporation. Electroporation involves disturbing the cell membrane with an electric shock, which creates temporary openings through which molecules may be inserted. Forerunners to the BAC included modified plasmids with such exotic names as the cosmid and the fosmid. These often frustrated attempts at research because they could carry only a few tens of thousands of DNA base pairs, enough to insert only very small genes.

In 1992 the first bacterial artificial chromosome was created by Hiroaki Shizuya, a researcher at the California Institute of Technology, by modifying a plasmid called an F-factor. F-factor plasmids are used naturally by bacteria to transfer DNA from one cell to another during periods of environmental stress, in order to increase genetic variability and the likelihood of survival. Unlike its predecessors, the BAC could carry large genes with hundreds of thousands of DNA base pairs, or several genes at once.

A number of large BAC libraries are now maintained by university, private industry, and government groups. In addition to the genes under investigation, many BACs contain tools that allow for easier research. For instance, some BACs contain genes that turn bacteria blue or make them glow, for easier identification. Some contain genes that make the host resistant to certain antibodies. The cultures can be purified by flushing them with the antibody in question, killing all bacteria except the ones that carry the BAC.

Since bacteria reproduce rapidly, the bacterial artificial chromosome may also be used to clone large quantities of a particular genetic sequence for study. This has allowed for better study of the genomes of organisms that grow slowly or unpredictably in laboratory conditions. The ability to clone has sped up disease treatment research by allowing for more rapid identification of effective antiviral and antibacterial medications. It has also allowed for more effective production of sequences used in the genetic modification of other organisms, for research and industry.