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The NCERT account of Griffith's experiment takes one paragraph: heat-killed smooth bacteria plus live rough bacteria killed the mice, and living smooth bacteria came out. It is a good summary, but it leaves out the detail that made the result convincing, and it says nothing about the man who found it. This post goes back to Griffith's 1928 paper and to a memorial lecture about him, explains why the result surprised even him, and gives you a way to reason through the experiment rather than memorise it.
Who was Frederick Griffith?
A 1972 Griffith Memorial Lecture by A. W. Downie, published in the Journal of General Microbiology, gives the clearest picture. Fred Griffith was born in Cheshire and graduated in medicine at Liverpool in 1901. He joined the staff of the Local Government Board, whose laboratories were taken over by the Ministry of Health during the First World War. There, in Dudley House on Endell Street in London, he shared a single laboratory with his colleague Scott for most of their working lives.
Downie describes the laboratory as poorly equipped even for the time, and quotes a 1941 tribute by Hedley Wright saying the two men "could do more with a kerosene tin and a primus stove than most men could do with a Palace." Griffith was, in Downie's words, "a very shy person" who rarely went to meetings. Griffith and Scott died together in February 1941, when Griffith's London flat received a direct hit during an air raid.
What was Griffith actually studying?
Not genetics. Downie explains that his main interest was the epidemiology of infectious disease: to understand outbreaks, he wanted to identify exactly which types of bacteria caused them. Pneumococci, the bacteria behind lobar pneumonia, come in immunologically distinct types. Griffith noticed that patients sometimes carried several types, including weakly virulent ones, and wondered whether a virulent type could degrade into a harmless form, or a harmless form regain virulence.
To test this he worked with rough (R) pneumococci, which lack the capsule that makes smooth (S) forms virulent. The NCERT Class 12 chapter on molecular inheritance explains that S bacteria have a polysaccharide coat that R bacteria lack, and that S bacteria kill mice while R bacteria do not.
What happened in the mice?
The summary of Griffith's paper, The Significance of Pneumococcal Types (1928), sets out the key result. When he injected mice under the skin with a harmless R strain derived from one type, together with a large dose of virulent bacteria of another type killed by heating to 60 °C, he recovered a virulent S strain of the same type as the heated bacteria. For example, the S form of Type I was produced from the R form of Type II.
That detail is the point. If the bacteria recovered had been Type II, you could argue that the R strain had simply reverted to its original smooth form. Because they had the capsule type of the dead bacteria, something must have passed from the dead cells to the living ones. His paper also reports a control: heated R cultures, injected with small doses of living R bacteria, never caused a change of type.
Downie notes that the most surprising result came from what Griffith intended as a control, and that Griffith, who believed bacterial types were fixed, found it hard to believe. He ran many experiments with extensive controls before publishing.
Worked reasoning: predict each result
Cover the right-hand side and predict each outcome before reading it.
Live S (Type I) injected. The mouse dies; live Type I bacteria are recovered.
Live R (derived from Type II) injected. The mouse survives.
Heat-killed S (Type I) injected. The mouse survives; nothing alive is recovered.
Live R (from Type II) plus heat-killed S (Type I). The mouse can die, and live Type I S bacteria are recovered.
The common exam mistake is to say Griffith showed DNA is the genetic material. He did not. As NCERT puts it, the biochemical nature of the genetic material was not defined from his experiments. A second mistake is to say the heat-killed bacteria "came back to life". The living bacteria in case 4 are the R cells, changed.
What happened after Griffith?
Downie records that Neufeld in Berlin confirmed the result soon after. In Oswald Avery's laboratory in New York, Dawson showed the change could happen in a test tube, and Alloway showed a cell-free extract could do it. That line of work led to the 1944 paper identifying the active substance, told in our post on Avery, MacLeod and McCarty. For the full Class 12 chapter, with worked sums, see our guide to the molecular basis of inheritance.
What can a student take from Griffith's way of working?
Take your controls seriously. His most important finding came from a control.
Design so that only one explanation fits. Using two different types ruled out simple reversion.
Check a surprise many times. He repeated the experiment with extensive controls before publishing.
Good work does not need fine equipment. A crowded room and simple tools were enough.
If you find the chain from Griffith to DNA hard to follow, the Fermi Concept Library has every concept in biology with its own lesson, so you can revise transformation before the experiments that came after it.
Draw the four mouse experiments as a flow chart from memory, then check each outcome against the list above.
