Bacterial transformation is a natural process in which cells take up foreign DNA from the environment at a low frequency. After transformation, the cells may express the acquired genetic information, which may serve as a source of genetic diversity and potentially provide benefits to the host (e.g., antibiotic resistance). With the advent of molecular cloning in the 1970s, the process of transformation was exploited and enhanced to introduce recombinant plasmid DNA into bacterial strains that were made “competent,” or more permeable, for DNA uptake.
History behind discovery of bacterial transformation
The natural competency or transformability of bacteria was first reported by Frederick Griffith in 1928 . Griffith noted that mice died when injected with “smooth” pneumococcus (Streptococcus pneumoniae) (hence referred to as virulent) but did not die from the “rough” strain (nonvirulent). Virulence of the smooth strain could be abolished by heat-killing. However, when the heat-killed smooth strain was mixed with the nonvirulent rough strain, the rough strain acquired the smooth phenotype and became virulent (Figure 1). Griffith’s experiments suggested that a nonliving, heat-stable material derived from the smooth strain was responsible for transformation. It was not until 1944 that this transformative material was identified as DNA by Oswald Avery, Colin MacLeod, and Maclyn McCarty .
Figure 1. The Griffith experiments. The smooth appearance of pneumococcus is due to the presence of a polysaccharide coating that prevents recognition of the bacterial cells by the host’s immune system.
Overview of creating artificial cell competency
The first protocol for artificial transformation of E. coli was published by Mandel and Higa in 1970 . The procedure showed increased permeability of the bacterial cells to DNA after treatment with calcium (Ca2+) and brief exposure to an elevated temperature, known as heat shock. This method became the basis for chemical transformation. In 1983, Douglas Hanahan published an improved method to prepare competent cells, where optimal conditions and media for bacterial growth and transformation were identified for higher transformation efficiency .
As an alternate approach to chemical transformation, an electrical field can be applied to the cells to enhance the uptake of DNA, a method known as electroporation (Figure 2). In 1982, Neumann et al. reported introduction of foreign DNA into mouse cells by short pulses of high-voltage electric fields . Such electric fields are believed to increase the cell’s membrane potential, thereby inducing transient permeability to charged molecules like DNA. In 1988, transformation of E. coli cells by electroporation was published .
Figure 2. Chemical transformation vs. electroporation.
Progress in available competent cells
Since the development of artificial transformation of E. coli by chemicals and electroporation, preparations of competent cells, as well as other methods of transformation, are continually being devised to improve uptake of DNA [7–10]. In 1984, Gibco BRL (now part of Thermo Fisher Scientific) became the first company to offer competent cells commercially, with the introduction of the strains HB101 and RR1 (recA+ version of HB101). Today, a variety of competent cells—made for different transformation methods, transformation efficiencies, genotypes, and packaging—are available in ready-to-use formats to propagate cloned plasmids in molecular biology experiments.
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- Avery OT, Macleod CM, McCarty M (1944) Studies on the chemical nature of the substance inducing transformation of pneumococcal types: Induction of transformation by a desoxyribonucleic acid fraction isolated from Pneumococcus Type III. J Exp Med 79(2): 137–158.
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- Yoshida N, Sato M (2009) Plasmid uptake by bacteria: a comparison of methods and efficiencies. Appl Microbiol Biotechnol 83(5): 791–798.
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- Green R, Rogers EJ (2013) Transformation of chemically competent E. coli. Methods Enzymol 529: 329–336.
- Gonzales MF, Brooks T, Pukatzki SU et al. (2013) Rapid protocol for preparation of electrocompetent Escherichia coli and Vibrio cholerae. J Vis Exp 80: 50684.
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