came to applying genetics to the huge diversity of life on our fecund planet.
For example, did every life form, from worms to eagles, from the protozoa that crawled about in the scum of ponds to humanity itself, carry the same kinds of genes in their nuclei-bound chromosomes?
The microscopic cellular life forms, including bacteria and archaea, do not store their heredity in a nucleus. These are called the ‘prokaryotes’, which means pre-nucleates. All other life forms store their heredity in nuclei and are known collectively as ‘eukaryotes’, which means true nucleates. From the growing discoveries in fruit flies, plants and medical sciences, it was becoming rather likely – excitingly so – that some profound commonalities might be found in all nucleated life forms. But did the same genetic concepts, such as genes, apply to the prokaryotes, which reproduced asexually by budding, without the need for germ cells? At this time within the world of early bacteriology there was even a debate as to whether bacteria should be seen as life forms at all. And viruses, which were for the most part several orders of magnitude smaller than bacteria, were little understood.
Over time many researchers came to see bacteria as living organisms, classifying them according to the binomial Linnaean system; so, for example, the tuberculosis germ was labelled Mycobacterium tuberculosis and the boil-causing coccoid germ was labelled Staphylococcus aureus. Oswald Avery, with his extremely conservative nature, kept his options open, eschewing the binomial system and referring to the TB germ as the ‘tubercle bacillus’. It is instructive for our story that Dubos, who came to know Avery better than any other colleague, would observe that ‘Fess’ was similarly conservative in his approach to laboratory research. Science must adhere with a puritanical stringency to what can be logically observed and definitively proven in the laboratory.
In 1882 German physician Robert Koch discovered that Mycobacterium tuberculosis was the cause of the greatest infectious killer in human history – tuberculosis. Koch constructed a code of logic that would be applied to bugs when first determining if they caused specific diseases. Known as ‘Koch’s postulates’, this was universally adhered to, and once a causative bug had been identified it was studied further under the microscope. Thus the bug was duly classified in a number of ways. If its cells were rounded in shape it was a ‘coccus’, if a sausage shape it was a ‘bacillus’, if a spiral shape it was a ‘spirochaete’. Bacteriologists methodically studied the sort of culture media in which a bug would grow best – whether in agar alone, or agar with added ox blood, and so on. They also studied the appearance of the bacterial colonies when they were grown in culture plates – their colours, the size of the colonies, whether they were rough in outline or round and smooth, raised or flat, stellate, granular or daisy-head. So the textbooks of bacteriology extended their knowledge base on a foundation of precise factual study and observation. And as understanding grew, this newfound knowledge was applied to the war against infection.
One of the useful things they learnt about disease-causing, or ‘pathogenic’, bacteria was that the behaviour of the disease, and thus of the bug itself in relation to its infected host, could be altered by various deliberate means: for example, through repeated cultures in the laboratory, or by repeatedly passing generations of the bug through a series of experimental animals. Through such manipulations it was possible to make the disease worse or less severe by making the bug either ‘more virulent’ or ‘attenuated’. Bacteriologists looked for ways to extrapolate this to medicine. In France, for example, the eminent Louis Pasteur used this principle of attenuation to develop the first vaccine to be used successfully against the otherwise universally fatal virus infection of rabies.
One fascinating observation that came out of these studies was the fact that, once a bug had been attenuated or been driven to greater virulence, the change in behaviour could be ‘passed on’ to future generations. Could it be that some factor of the bug’s own heredity had been altered to explain the change in behaviour?
Bacteriologists talked about ‘adaptation’, using the same term that was coming into vogue with evolutionary biologists when referring to evolutionary change in living organisms as they adapted to their ecology over time. While it was too early to be sure if bacterial heredity depended on genes, these scientists linked it to the physical appearance of bugs and colonies, or to the bugs’ internal chemistry, and even to their behaviour in relation to their hosts. These were measurable properties, the bacterial equivalents of what evolutionary biologists were calling the ‘phenotype’ – the physical make-up of an organism as opposed to what was determined by the hereditary make-up, or ‘genotype’.
Bacteriologists also came to recognise that the same bacterium could exist in different subtypes, which could often be distinguished from one another using antibodies. These subtypes were called ‘serotypes’. In 1921 a British bacteriologist, J. A. Arkwright, noticed that the colonies of a virulent type of dysentery bug, called Shigella, growing on the jelly-coated surfaces of culture plates, were dome-shaped with a smooth surface, whereas colonies of an attenuated, non-virulent, type of dysentery bug were irregular, rough-looking and much flatter. He introduced the terms ‘Smooth’ and ‘Rough’ (abbreviated to S and R) to describe these colonial characteristics. Arkwright recognised that the ‘R’ forms cropped up in cultures grown under artificial conditions, but not in circumstances where bacteria were taken from infected human tissues. He concluded that what he was observing was a form of Darwinian evolution at work.
In his words: ‘The human body infected with dysentery may be considered a selective environment which keeps such pathogenic bacteria in the forms in which they are usually encountered.’
Soon researchers in different countries confirmed that loss of virulence in a number of pathogenic bacteria was accompanied by the same change in colony appearance from Smooth to Rough. In 1923, Frederick Griffith, an epidemiologist working for the Ministry of Health in London, reported that pneumococci – the bugs that caused epidemic pneumonia and meningitis which were of particular interest to Oswald Avery at the Rockefeller Laboratory – formed similar patterns of S and R forms on culture plates. Griffith was known to be a diligent scientist and Avery was naturally intrigued.
Griffith’s experiments also produced an additional finding, one that really shook and puzzled Avery.
When Griffith injected non-virulent R-type pneumococci from the strain known as type I into experimental mice, he included an additional ingredient in the injections, a so-called ‘adjuvant’, which usually pepped up the immune response to the R pneumococci. A common adjuvant for these purposes was mucus taken from the lining of the experimental animal stomach. But for some obscure reason Griffith switched adjuvant to a suspension of S pneumococci, derived from type II, that had been deliberately killed off by heat. The experimental mice died from overwhelming infection. In the blood of these dead mice Griffith expected to find large numbers of multiplying R-type I bacteria – the type that he had injected at the start of the experiment. Why then had he actually found S-type II? How on earth could adding dead bacteria to his inoculum have changed the actual serotype of the bacterium from non-virulent R-type I to highly virulent S-type II?
Researchers, including Avery himself, had previously shown that S and R types were determined by differences in the polysaccharide capsules coating the cell bodies of the bugs. Griffith’s findings suggested that the test bacteria, initially R-type pneumococci, had changed their polysaccharide coats inside the infected bodies of the mice to that of the virulent strain. But they could not have achieved this by just flinging off the old coat and putting on the new one. The coat was determined by the bacteria’s heredity – it was an inherited characteristic. Further cultures of the recovered bacteria confirmed that the S type bred true. There appeared to be only one possible explanation: adding the dead S bacteria to the living R bacteria had induced a mutation in the heredity of the living R-type bacteria, so they literally transformed into S-type II.
In the words of Dubos: ‘[At the time] Griffith took it for granted that the changes remained within the limits of the species. He
