The natural competency or transformability of bacteria was first reported by Frederick Griffith in 1928 [1]. 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 [2].

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.

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The first protocol for artificial transformation of E. coli was published by Mandel and Higa in 1970 [3]. 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 [4].

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 [5]. 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 [6].

Figure 2. Chemical transformation vs. electroporation.

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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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1. Griffith F (1928) The significance of pneumococcal types. J Hyg (Lond) 27(2): 113–159.
2. 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.
3. Mandel M and Higa A (1970) Calcium-dependent bacteriophage DNA infection. J Mol Biol 53(1): 159–162.
4. Hanahan D (1983) Studies on transformation of Escherichia coli with plasmids. J Mol Biol 166(4): 557–580.
5. Neumann E, Schaefer-Ridder M, Wang Y et al. (1982) Gene transfer into mouse lyoma cells by electroporation in high electric fields. EMBO J 1(7): 841–845.
6. Dower WJ, Miller JF, Ragsdale CW (1988) High efficiency transformation of E. coli by high voltage electroporation. Nucleic Acids Res 16(13): 6127–6145.
7. Yoshida N, Sato M (2009) Plasmid uptake by bacteria: a comparison of methods and efficiencies. Appl Microbiol Biotechnol 83(5): 791–798.
8. Aune TE, Aachmann FL (2010) Methodologies to increase the transformation efficiencies and the range of bacteria that can be transformed. Appl Microbiol Biotechnol 85(5): 1301–1313.
9. Green R, Rogers EJ (2013) Transformation of chemically competent E. coli. Methods Enzymol 529: 329–336.
10. 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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