The Griffith mice experiment, also known as the Griffith transformation, is a classic experiment in the field of molecular biology. It was conducted in 1928 by Frederick Griffith, a British bacteriologist, and is considered a key milestone in the development of our understanding of DNA and its role in genetics.
In the experiment, Griffith used two strains of the bacterium Streptococcus pneumoniae: a virulent strain, which was able to cause disease in mice, and a non-virulent strain, which was not able to cause disease. He injected the mice with a mixture of the two strains and observed that the mice developed the disease and died. However, when he examined the bacteria in the mice's tissues, he found that the non-virulent strain had acquired the ability to cause disease.
Griffith's discovery was groundbreaking because it suggested that there was a way for genetic information to be transferred from one organism to another, a process we now know as transformation. This was a revolutionary concept at the time, as it challenged the prevailing belief that genes were fixed and could not be altered.
To understand how this transformation occurred, Griffith carried out a series of further experiments. He found that the virulent strain produced a substance called a transforming principle, which was able to convert the non-virulent strain into the virulent form. This transforming principle was later identified as DNA.
Griffith's experiment was a major step forward in our understanding of genetics and the role of DNA in the transmission of genetic information. It paved the way for further research on DNA and its role in heredity, and ultimately led to the discovery of the structure of DNA by James Watson and Francis Crick in the 1950s.
Today, the Griffith mice experiment is considered a cornerstone of molecular biology and is still taught in many biology courses as an example of the importance of experimentation in scientific discovery. It has had a profound impact on our understanding of genetics and has helped to shape the field of molecular biology as we know it today. Overall, the Griffith mice experiment remains a testament to the power of scientific inquiry and the importance of curiosity in driving scientific progress.
Frederick Griffith Experiment: Bacterial transformation
What happened when Griffith injected mice with the pneumonia causing strain of bacteria? It was first demonstrated that when purified RNA from tobacco mosaic virus was spread on tobacco leaves, the leaves showed lesions of viral infection. Initially, proteins were considered strong candidates as genetic material, considering their heterogeneity being made of 20 different amino acids, which form a wide variety of proteins and functional capabilities in living organisms. Griffith's experiment has also been important to how bacterial infections are currently treated. The effect on the activation of the LiaRS system via cell wall cleavage allows to hypothesize a complementary role of LiaRS to CiaRH and WalRK on pneumococcal survival and resistance to allolysis. The bacteria belonging to the R strains were thought to preserve the capacity to produce the polysaccharide capsule. Griffith observed that the mouse survived and didn't develop pneumonia.
In Griffith's experiment, while mice injected with either heat-killed encapsulated cells or live non-encapsulated cells survive infection, co-injection leads to infection due to transformation and expression of capsule synthesis genes by non-encapsulated pneumococci. Schematic diagram of Avery, MacLeod, and McCarty's experiment which demonstrates that DNA is the transforming principle. When Griffith injected mice with disease-causing bacteria, the mice developed pneumonia and died. In his experiment, Griffin injected two types of streptococcus pneumoniae, Type III-S and Type II-R, into mice. Griffith injected the mice with 4 different samples of bacteria. Neither type can cause death in mice when injected. The rapid increase in cell density leads to an extracellular accumulation of the quorum sensing signal, the competence-stimulating peptide CSP.
DNA as Genetic Material The Griffith experiment was a pivotal moment in the discovery of genetic material. Likewise, pneumococcal TCS03 or LiaRS has been shown to be involved in the competence process by responding to peptidoglycan PGN cleavage by LytA, CbpD and LytC murein hydrolases. After continued speculation regarding the molecular nature of the genetic material, certain groundbreaking experiments, including those by Fredrick Griffith 1928 , Averty, MacLeod, and McCarty 1944 , and Hershey and Chase 1952 laid conclusive evidence that helped establish that it is certainly the DNA, and not the RNA or proteins, which is the molecular basis of inheritance. Capsular transformation, also known as capsule switching, is very common in nature and is the source of the more than 90 different pneumococcal serotypes. Second, he injected the mice with dead encapsulated or smooth S bacteria. This implies that the polysaccharide capsule trait is heritable and permanent and not just "discarded" after infecting the host. Both DNA and protein fit these criteria, but only DNA is enriched inside the nucleus, whereas protein is enriched in the cytoplasm.
Encapsulated bacteria form smooth, shiny-surfaced colonies S when grown on an agar culture plate. Griffith's experiment discovered that bacteria can transfer genetic information through transformation. Reason: The dead Smooth type bacteria became alive and caused pneumonia. The second was Type II-R rough , which does not have a protective capsule; therefore, it can be killed by the host's immune system. Similarly, capsule-less R bacteria serotype II did not cause death and no bacteria were detected in sacrificed mice. One is virulent pathogenic and other is avirulent. This DNA intermediate can be incorporated into the genome of the host cell, and when the host DNA is transcribed, copies of the original retroviral RNA are produced.
Control mice that were injected with only heat-killed SI bacteria and remained unaffected are encircled in black. He also found that transformation occurred only when the inoculum size of the heat-killed S cells was much larger than that of the living R cells. Injecting mice with live S bacteria caused pneumonia and death. The final confirmation came with experiments using crude samples of the DNA-digesting enzyme deoxyribonuclease DNase , which can degrade DNA, specifically. Hershey and Chase let the labeled T2 bacteriophages infect the unlabeled bacteria and inject their genetic material into the cells Fig.
They demonstrated that the transforming principle can be destroyed by this enzyme. Expanding on these preliminary observations, he defined experimental conditions that maximized the capacity of heat-killed S cells to affect the transformation of R cells into virulent encapsulated S pneumococci. As will be discussed later, transformation of pneumococci by DNA was inefficient, especially under the less than optimal conditions that were employed by Griffith and his immediate successors. Pneumococcal TCS02 or WalRK WalRK, VicRK, YycFG and MicAB is also depicted to be involved in the regulation of competence in pneumococci Echenique and Trombe, 2001. TCS12 or ComDE competence operon is expressed as an operon along with a small peptide ComC comCDE , encoding two highly conserved proteins, namely a HK ComD and a RR ComE Havarstein et al. The principle of inheritance was discovered by Mendel. Taken together with work that had been done before, Hershey and Chase's work provided final, strong evidence to prove that DNA is the genetic material.
What happened when Griffith injected the mice with the harmless R strain bacteria alone?
The mice injected with a heat-killed smooth strain will live. As a result, this factor "transformed" the R bacteria into S bacteria. The mice died due to pneumonia and virulent smooth-type living bacteria could also be recovered from their dead bodies. The way the protective capsule in the smooth bacteria is formed is contained within the DNA of the cell. Some mice died of pneumonia and their blood contained both live Rough type bacteria and live Smooth type bacteria.
Griffith experiment: gene transformation in bacteria
As a result, the mice died and he found colonies of encapsulated bacteria in the dead mice and isolated them from the mice. Fig 1: Frederick Griffith 1879—1941; Image source: Wikipedia. From the set of observations, Griffith drew the conclusion that the S bacteria, even the dead ones, could transform the live R bacteria into S bacteria. However, it was not clear if the molecules responsible for the inheritance were proteins or the DNA, in the Frederick Griffith on Streptococcus pneumoniae in mice took the scientific community a step further towards the fact the DNA was the genetic material. Subsequently, many new phages are constructed, and the bacterial cell is lysed, releasing the progeny viruses. Because of the artificial set up of these experiments, it is difficult to evaluate the effect of CRISPR loci in the evolution of pneumococci from their results.
An antibiotic resistance marker was transferred much more efficiently from neighboring cells than from the growth medium. The nature of this ' transforming principle' was unknown. These IIIS bacte-ria were identified to the IIIS strain from which the heat killed cell prepa-ration was made. Although these experiments demonstrated that DNA is the genetic material in bacteria and viruses, it was generally accepted that DNA is a universal substance as the genetic material in eukaryotes. On the other hand, protein is found everywhere in the cell. He called this process transformation, because one type of bacteria had been changed permanently into another. He subsequently infected mice with this mixture and much to his surprise, the mice developed pneumonia and died.
Explain Griffith ‘s experiments on mice and infection of Diplococcus pneumoniae.
T-bars, inhibition; CM, cytoplasmic membrane. Current pneumococcal vaccines contain several different types of capsular polysaccharides and therefore S. Though his experiment itself did not concluded this, it led to the other scientists of carry out various experiments, finally leading to the conclusion. As such, their observations confirmed that DNA is the transforming substance. But the mechanism of how this membrane protein confers resistance is still unknown.
