Griffith's experiment (1928): transformation in bacteria

Biological background and material

The bacterium used in Griffith's work was Streptococcus pneumoniae, a species that can exist in different forms depending on surface structures. Griffith compared two variants: a smooth, encapsulated form that resists host defenses and causes disease, and a rough, non‑encapsulated form that is usually harmless. The capsule is made of polysaccharides and helps the smooth form evade the immune system.

Experimental design

Griffith combined living and killed bacteria in a series of controlled infections of laboratory mice to test whether the properties of one strain could be transferred to another. The key steps can be summarized as:

• Inject live encapsulated (virulent) bacteria into mice — typical result: disease and death.
• Inject live non‑encapsulated (non‑virulent) bacteria — mice survive.
• Inject heat‑killed encapsulated bacteria — bacteria are dead and mice survive.
• Inject a mixture of heat‑killed encapsulated bacteria and live non‑encapsulated bacteria — surprisingly, mice develop disease and virulent bacteria are recovered from their blood.

These procedures are described in Griffith's original report and related summaries of the experiment and its context (experiment summary, Frederick Griffith). The work involved laboratory mice as experimental hosts (mice) and relied on established bacteriological techniques of the time.

Observations and interpretation

Griffith observed that when heat‑killed virulent bacteria were mixed with live non‑virulent bacteria, the latter became virulent. He coined the term "transformation" to describe the change and proposed that a "transforming principle" from the dead virulent cells had been taken up by the live cells, conferring the ability to produce a protective capsule. At the time the chemical identity of the transforming substance was unknown; Griffith noted the phenomenon rather than identifying the molecule responsible.

Confirmation and molecular identification

Subsequent researchers tested the nature of the transforming principle. Work by Avery, MacLeod and McCarty showed that DNA removed from virulent cells could induce transformation when added to non‑virulent cells, providing strong evidence that DNA carried the genetic information needed for the capsule trait (Avery, MacLeod & McCarty). Later experiments, including those by Hershey and Chase, reinforced the conclusion that DNA is the material of heredity in many organisms (Hershey & Chase). These studies connected Griffith's phenomenological finding to molecular genetics and helped shift scientific opinion toward DNA as the genetic material (DNA).

Significance, examples and modern perspective

Griffith's experiment remains historically important because it provided the first clear example of horizontal transfer of heritable traits in bacteria and inspired experiments that identified DNA as the carrier of genetic information. The transformation process observed by Griffith is related to natural competence, a property of some bacterial species to take up free DNA from their environment. Understanding transformation has practical implications: it explains how bacteria can acquire virulence factors and antibiotic‑resistance genes, and it underlies molecular biology techniques used to introduce genetic material into cells.

Limitations and notable points

Griffith did not determine the chemical identity of the transforming agent; he interpreted his results cautiously and left open the nature of the substance. Later work established DNA as the active molecule, but Griffith's careful observation and experimental approach were crucial initial steps. The strains and structural features he compared — the rough versus smooth colony morphologies and the protective polysaccharide capsule — remain useful examples in teaching bacterial genetics and pathogenesis (strains, S. pneumoniae, polysaccharide capsule, immune system interaction). For additional background on bacteria and transformation processes see general bacteriology summaries (bacteria).