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| Avery in his lab, 1948. Source: Rockefeller Archive |
Imagine a world where we build skyscrapers more than 300 meters high, where we know of nuclear fission, where the first digital computers have been designed, where the first color television has been produced, but a world where we do not even know what stuff genes are made of! [1] This is the world of the early 1940s, before Oswald Avery achieved his ‘quiet revolution’ – to borrow an expression from the biologist and author Matthew Cobb on the subject.
I knew of Avery and his experiments – as do all microbiologists – from the textbooks and microbiology class: working on pneumococci, Avery showed in 1944 that the active compound in bacterial transformation was DNA. In that he was providing an explanation to the illustrious findings of Griffith in the late 1920s, who demonstrated that, in infected mice, avirulent mutants of Streptococcus pneumoniae could be effectively transformed into the deadly kind via an unknown process involving contact with heat-killed virulent cells (again, textbook knowledge).
Alas, textbooks usually fail to convey much excitement about such groundbreaking episodes of human inquiry. To be fair, this is of course not the textbooks’ mission. This said, I can’t remember that my microbiology class fared much better in that task – often the amount of material to be covered in class does not allow for much dwelling into the historical context.
So I was thrilled to (re)discover the story of Avery in Cobb’s excellent book, Life's Greatest Secret: The Race to Crack the Genetic Code (2015), which explores in depth the work that led to the resolution of the DNA structure and later to the deciphering of the genetic code. Cobb spends a whole chapter on Avery’s work and life, and brings to light many fascinating aspects.
The career of Oswald T. Avery (1877-1955) centered in the study of pneumonia bacteria and the immunological response they elicit. We have to remember that, before antibiotics became widespread, pneumonia was among the deadliest infectious diseases in the world [sadly we should also stress that millions of people still die of pneumonia every year, although this number is decreasing]. Avery had joined the Rockefeller Institute Hospital in New York in 1913 in order to pursue this research, with the goal of developing therapies against pneumococcus. He stayed at Rockefeller thirty years, until his retirement in 1943.
Avery was interested in why certain pneumococci are more virulent than others. In the 1920s, he found that virulent strains typically embed themselves within a large polysaccharidic capsule that protects them from the human body’s defenses. In 1928, in London, Fred Griffith found that pneumonia bacteria devoid of capsule could be transformed into virulent ones producing capsules, simply by putting them into contact with dead capsule-producing bacteria. But what was the process? Griffith thought incorrectly that the avirulent bacteria were using the capsules of the dead virulent bacteria as a template to make new ones.
From the early 1930s, Avery worked to isolate what they called ‘the transforming principle’ from cultures of pneumococci, and to identify its chemical nature, with the help of the Canadian scientist Colin MacLeod. They managed this feat by using a messy, dangerous procedure with a commercial kitchen cream separator that leaked and produced tons of aerosols – I kid you not. They also found that calcium chloride precipitated nucleic acids into a white, gooey substance (if you have done DNA extraction you know how that looks like). The white precipitate, which contained nucleic acids and polysaccharides, was very potent in transforming bacteria, as demonstrated by MacLeod, who had to leave the lab in 1941.
Avery lab around 1920. Source: Rockefeller Archive
At that
time another young researcher joined the Avery lab. His name was Maclyn
McCarthy, and he made a decisive contribution by showing that the polysaccharides
could be enzymatically digested in the precipitate without it losing its
transforming ability. It became increasingly clear that most of the transforming
precipitate was made of DNA. What is more, some enzymes active on DNA were able
to block transformation.
This should have been the nail in the coffin of that story, however, resistance to grant DNA that extraordinary role was strong: DNA shows no variation whatsoever, how could it be the transforming principle? Proteins are varied and very low amounts show activity – surely they are the stuff responsible for transformation. And you can never be sure that your precipitate is totally protein-free!
So while all the evidence pointed at DNA as the agent of transformation, many were still convinced that it could not be – a powerful expectation bias that complicated the acceptance of Avery’s work. He and his team continued to work tirelessly to strengthen the case for DNA. Avery noted, in a letter to his brother:
‘We have isolated highly purified substance of which as little as 0.02 of a microgram is active in inducing transformation… this represents a dilution of 1 part in a hundred million – potent stuff that – and highly specific. This does not leave much room for impurities – but the evidence is not good enough yet.’
In 1943, Avery proposed the connection between DNA and genes. He had not solved the problem of protein impurities – after all, you cannot prove a negative – but at that time the attitude towards DNA was changing, weakening the opposition to its role. Avery started to write down all their results, a pile of evidence from different types of experiments. He wrote in that paper:
‘The inducing substance has been likened to a gene, and the capsular antigen which is produced in response to it has been regarded as a gene product.’
The paper, authored by Avery, McLoed and McCarthy, was published in February 1944 in the Journal of Experimental Medicine. Its initial reception by the scientific community was good and, in the next couple of years, Avery and McCarthy continued to publish, adding more evidence to their case. Avery received the Gold Medal of the New York Academy of Medicine. Erwin Chargaff, inspired by Avery, started to study DNA composition. In 1945, AndrĂ© Boivin at the Institut Pasteur demonstrated transformation in E. coli, supporting Avery’s findings on the importance of DNA.
After 30 years of hard work, the quiet revolution was very much underway. Avery received many awards for his work on pneumococcus, albeit never the Nobel Prize...
Reference: Cobb, Matthew. (2015) Life's Greatest Secret: The Race to Crack the Genetic Code. Basic Books, 464 pages.
[1] In 1933, Thomas Hunt Morgan noted, during his Nobel Prize acceptance lecture: ‘There is no consensus of opinion amongst geneticists as to what the genes are – whether they are real or purely fictitious.’
