transfer of genetic material

Prior to 1946

Scientists thought bacterial microorganisms were primitive and unsuitable for genetic research. An observation that Lederberg and Tatum set out to alter after being motivated by Oswald Avery's discovery of the presence of DNA molecules in 1944, Colin and Maclyn experiments (Bynum & Bynum, 2011). The two hypotheses are that biochemical features of animals, such as amino acid synthesis, are coded by genes in various portions of the genetic material.

Avery's results

On the other hand, indicated that the transition from non-virulent to virulent Pneumococcus bacterial cells occurred as a result of the uptake of DNA molecules from killed virulent Pneumococcus (Harper, 2016). However, Lederberg believes the organisms also transferred their genetic materials through mating (Harper, 2016). To investigate his hypothesis, he formulated his first model that failed to achieve his transformation objective. In the model, Lederberg used mutated nutritional Neurospora that could not support the synthesis of amino acid leucine and actively divided after adding the amino acid, restoring them to their wild-type. The organisms could now grow in a medium containing no leucine since they had regained the ability to synthesize their own (Srivastava, 2013). Although the transformation in Neurospora might have failed, he conceived the idea of using prototrophic recovery technique to test mating abilities in Escherichia coli, resulting in development of the second model.

Together with Edward Tatum

He studied two E.coli strains with a different nutritional requirement, where strain A needed methionine, leucine and thiamine supplements to grow, while strain B required threonine, leucine and thiamine. The strains were plated differently and another plate for the mixture of both strains (Wilson, 2013). After a period of incubation, progeny from the combination of the strains regained their wild-type, thus able to grow without any nutritional supplement. On the other hand, there were no colonies on plates containing strain A or strain B. The observation suggested recombination of genes between the two different strain genome. Lederberg called this new mechanism conjugation (Woods & Jason, 2016).

After controversies

Suggesting the two strains might have exchanged genetic materials through 'cross feeding,' where the cells leak substances that are absorbed by the other bacterial cells (Bynum & Bynum, 2011). However, this controversy was nullified by Bernard Davis when he constructed a U-tube equipment separated by fine spores not to allow bacteria to pass through, placing the different strains in each one. After incubation, Davis tested the growth of the various strains on minimal medium and found no bacterial colony (Harper, 2016). Therefore, he established that physical contact was necessary for conjugation to take place.

References

Bynum, W. F., & Bynum, H. (2011). Great discoveries in medicine (8th ed.). New York, NY: Thames & Hudson.

Harper, J. (2016). Diatom Transformation via Bacterial Conjugation v1. protocols.io, 4(14), 256-331.

Srivastava, S. (2013). Genetics of bacteria (6th ed.). New Delhi: Springer.

Wilson, J. W. (2013). Genetic Exchange in Bacteria and the Modular Structure of Mobile DNA Elements. Molecular Paradigms of Infectious Disease, 5(3), 34-77.

Woods, A., & Jason, G. (2016). Modified Bacterial Conjugation Protocol for Pseudonitzschia multiseries v1. protocols.io, 3(56), 791-890.