Transformation

Transformation is the simplest of these, and also the first to have been described. We have already referred to the classic experiment of Fred Griffith in 1928, the first demonstration

Bacteria Live smooth

Injectio^

Heat-killed smooth

Live rough

Mouse lives

Mouse lives

Heat-killed smooth

Mouse dies

Live rough

Dead mouse has live smooth bacteria

Mouse dies o

Figure 11.24 Transformation: the Griffith experiment. See the text for details. The result of the fourth experiment showed for the first time that genetic material could be passed from one bacterium to another. From Reece, RJ: Analysis of Genes and Genomes, John Wiley & Sons, 2003. Reproduced by permission of the publishers.

that genetic transfer can occur in bacteria. Griffith had previously demonstrated the existence of two strains of the bacterium Streptococcus pneumoniae, which is one of the causative agents of pneumonia in humans, and is also extremely virulent in mice. The S (smooth)-form produced a polysaccharide capsule, whilst the R (rough)-form did not. These formed recognisably different colonies when grown on a solid medium, but more importantly, differed in their ability to bring about disease in experimental animals. The R-form, lacking the protective capsule, was easily destroyed by the defence system of the host. Griffith observed the effects of injecting mice with bacterial cells of both forms; these are outlined below and in Figure 11.24. The results of experiments 1-3 were predictable, those of experiment 4 startlingly unexpected:

1 Mice injected with live cells of the R-form were unaffected.

2 Mice injected with live cells of the S-form died, and large numbers of S-form bacteria were recovered from their blood.

3 Mice injected with cells of the S-form that had been killed by heating at 60 °C were unharmed and no bacteria were recovered from their blood.

4 Mice injected with a mixture of living R-form and heat-killed S-form cells died, and living S-form bacteria were isolated from their blood.

The S-form bacteria recovered from the mice in the crucial fourth experiment possessed a polysaccharide capsule like other S-forms, and, critically, were able to pass on this characteristic to subsequent generations. This finding went against the prevailing view that bacteria simply underwent binary fission, a completely asexual process involving no genetic transfer. Griffith deduced that some as yet unknown substance had passed from the heat-killed S-form cells to some of the living R-forms and conferred on them the ability to make capsules (see Box 11.7). Not long afterwards it was shown that this process of transformation could happen in the test tube, without the involvement of a host animal, and, as we have seen, it was eventually shown that Griffith's 'transforming principle' was DNA.

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