Читать книгу Virusphere: What doesn’t kill you makes you stronger - Frank Ryan - Страница 10

In 1994 the East African nation of Rwanda erupted onto the world’s news and television screens when a simmering civil war between the major population of Hutus and minority population of Tutsis erupted into a genocidal slaughter of the minority population. But despite the deaths of half a million Tutsis, the Hutu perpetrators lost the war, causing more than two million of them to flee the country. Roughly half of these fled northwest, across the border of what was then Zaire, these days the Democratic Republic of the Congo, where they ended up around the town of Goma. Up to this point Goma had been a quiet town of some 80,000 people, nestling by Lake Kivu in the lee of a volcano. Goma now found itself overwhelmed by a desperate torrent of refugees, carrying everything from blankets to their meagre rations of yams and beans. Two hundred thousand arrived in a single day, confused, thirsty, hungry and homeless. They camped on doorsteps, in schoolyards and cemeteries, in fields so crowded that people slept standing up. Agencies from the world’s media flocked to the vicinity, reporting the chaos and the urgent need for shelter, food and water.

A reporter for Time magazine estimated that the volume of refugees needed an extra million gallons of purified water each day to prevent deaths from simple thirst, meanwhile the rescue services were managing no more than 50,000. Desperate people foraged for fresh water, scrabbling hopelessly in a hard volcanic soil that needed heavy mechanical diggers to sink a well or a latrine. Human waste from the relief camps fouled the waters of the neighbouring Lake Kivu, creating the perfect circumstances for the age-old plague of cholera to emerge. Within 24 hours of confirmation of the disease some 800 people were dead. Then it became impossible to keep count.

Viruses are not the only cause of plagues, which include a number of lethal bacteria, such as the beta-haemolytic streptococcus, tuberculosis and typhus, as well as some protists, which cause endemic illnesses such as malaria, schistosomiasis and toxoplasmosis. Cholera is a bacterial disease, caused by the comma-shaped Vibrio cholerae. The disease is thought to have originated in the Bengal Basin, with historical references to its lethal outbreaks in India from as early as 400 CE. Transmission of the germ is complex, involving two very different stages. In the aquatic reservoir the bug appears to reproduce in plankton, eggs, amoebae and debris, contaminating the surrounding water. From here it is spread to humans who drink the contaminated water, where it provokes intense gastroenteritis, which proves rapidly fatal from massive dehydration as a result of the fulminant ‘rice-water’ diarrhoea. This human phase offers a second reservoir for infection to the bug. If not prevented by strict hygiene measures, the extremely contagious and virulent gut infection causes massive effluent of rice-water stools that are uncontrollable in the individual sufferer, so that they contaminate their surroundings, and especially any local sources of drinking water, leading to a vicious spiral of very rapid spread and multiplication of the germ.

During the nineteenth century, cholera spread from its natural heartland, provoking epidemics in many countries of Asia, Europe, Africa and America. The massive diarrhoeal effluent of cholera is unlike any normal food poisoning. An affected adult can lose 30 litres of fluid and electrolytes in a single day. Within the space of hours, the victims go into a lethargic shock and die from heart failure.

The English anaesthetist, John Snow, was the first to link cholera with contaminated water, expounding his theory in an essay published in 1849. He put this theory to the test during a London-based outbreak around Broad Street, in 1854, when he predicted that the disease was disseminated by the emptying of sewers into the drinking water of the community. Snow’s thoughtful research led to the civic authorities throughout the world realising the importance of clean drinking water. Today the life of an infected person can be saved by very rapid intravenous replacement of fluid and electrolytes, but the size of the outbreak around Lake Kivu, and the relative paucity of local medical amenities, limited the clinical response. The situation was made even worse by the recognition that the cholera in the Rwandan refugee camps was now confirmed as the 01-El Tor pandemic strain of Vibrio, known to be resistant to many of the standard antibiotics. This presented immense problems for the medical staff from local health ministries and those arriving from the World Health Organization. Even though the response was one of the largest relief efforts in history – involving the Zairian armed forces, every major global relief agency and French and American army units – the spread of cholera was too rapid for their combined forces to take effect.

Three weeks after the outbreak began, cholera had infected a million people. Even with the modern knowledge and the desperate efforts of civic and medical assistance, the disease is believed to have killed some 50,000. It is hard to believe that so resistant a plague bacterium as the Vibrio cholerae might itself be prey to another microbe. But exactly such an attack, of a mystery microbe upon the cholera vibrio, had been recorded in a historic observation by another English doctor close to the very endemic heartland of the disease, a century before the outbreak at Lake Kivu.

In 1896 Ernest Hanbury Hankin was studying cholera in India when he observed something unusual in the contaminated waters of the Ganges and Yamuna Rivers. Hankin had already discovered that he could protect the local population from the lethal ravages of the disease by the simple expedient of boiling their drinking water before consumption. When, in a new experiment, he added unboiled water from the rivers to cultures of the cholera germs and observed what happened, he was astonished to discover that some agent in the unboiled waters proved lethal to the germs. It was the first inkling that some unknown entity in the river waters appeared to be preying upon the cholera bacteria.

Hankin probed the riddle further. He found that if he boiled the water before adding it to the cholera germ cultures this removed the bug-killing effect. This suggested that the agent that was killing the cholera germs was likely to be of a biological nature. He needed to know if it was another germ – sometimes germs antagonised one another – or if it was something completely different, a truly mysterious agent, that was killing the germs. Hankin decided that he would set up a new experiment using a device known as a Chamberland-Pasteur ‘germ-proof’ filter, which had been developed 12 years earlier by the French microbiologists Charles Chamberland and Louis Pasteur. The Chamberland-Pasteur filter was a flask-like apparatus made out of porcelain that allowed microbiologists to pass fluid extracts through a grid of pores varying from 0.1 to 1.0 microns in diameter – from 100-billionths to 1,000-billionths of a metre – that were designed to trap bacteria but allow anything smaller to pass through. Two years after the filter’s invention, a German microbiologist, Adolf Mayer, showed that a common disease of tobacco plants, known as tobacco mosaic disease, could be transmitted by a filtrate that had passed through the finest Chamberland-Pasteur filter. Unfortunately, he persuaded himself that the cause of the disease must somehow be a very tiny bacterium. In 1892 a Russian microbiologist, Dmitri Ivanovsky, repeated the experiment to get the same results. He refuted a bacterial cause, but still arrived at the mistaken conclusion that there must be a non-biological chemical toxin in the liquid extract. Finally, in 1896, the same year that Hankin was looking for his mystery agent in the Indian river waters, a Dutch microbiologist, Martinus Beijerinck, repeated the filter experiment with tobacco mosaic disease; but Beijerinck concluded that the causative agent was neither a bacterium nor a chemical toxin but rather ‘a contagious living fluid’. Although Beijerinck was closest of all to the truth, he was once again wrong. Today we know that the cause of tobacco mosaic disease is a virus – the tobacco mosaic virus. But thanks to Beijerinck’s mistaken finding of a ‘contagious fluid’, the current Oxford English Dictionary definition of a ‘virus’ has it as: ‘a poison, a slimy fluid, an offensive odour, or taste’.

Viruses are not poisons, or slimy fluids, or offensive odours or tastes, but rather organisms – truly remarkable organisms – that are different from bacteria, indeed utterly different from any other organisms on Earth. The great majority of viruses are very small, tiny enough to pass through Chamberland-Pasteur filters.

Of course, Hankin knew nothing of the existence of viruses when he passed the river water through the refined sieve of a Chamberland-Pasteur filter. Although he was in no position to offer a likely explanation or name for the mystery agent, he had discovered one of the most important and ubiquitous of viruses on Earth: a member of the group known today as ‘bacteriophage’ viruses, so-named from the Greek phagein, which means to devour. That is exactly what was happening to the cholera germs in Hankin’s experiments: they were being ‘devoured’ by bacteriophage viruses.

The true nature of Hankin’s discovery remained a mystery until 1915, when English bacteriologist Frederick Twort discovered a similarly minuscule agent that could pass through the Chamberland-Pasteur filters and yet remained capable of killing bacteria. By now viruses were known to exist even though biologists knew very little about them. Twort surmised that he was observing either a natural phase of the life cycle of the bacteria, the result of a fatal enzyme produced by the bacteria themselves, or a virus that grew on and destroyed the bacteria. Some two years later, a pioneering, self-taught, French-Canadian microbiologist, Félix d’Herelle, finally solved the mystery.

D’Herelle was born in the Canadian city of Montreal but considered himself a citizen of the world. Before becoming involved with viruses, he had already travelled widely, working in numerous American, Asian and African countries, to finally settle at the Pasteur Institute in Paris. At this time the discipline of microbiology was a fashionable scientific research endeavour and it was rapidly expanding its knowledge base. During his researches in Tunisia, d’Herelle had come across what was probably a virus infecting a bacterium that itself caused a lethal plague in locusts. Now working at the famous Institute, even as the First World War raged nearby, he took a particular interest in the grimy disease known as bacterial dysentery, which was killing soldiers in their muddy trenches.

Bacterial – as opposed to amoebic – dysentery is caused by a genus called Shigella, which is passed on from the infected individuals through faecal hand-to-mouth contagion. The resultant illness ranges from a mild gut upset to a severe form, with agonising griping spasms of the bowel accompanied by high fever, bloody diarrhoea and what doctors call ‘prostration’. In July and August 1915 there was an outbreak of haemorrhagic bacterial dysentery among a cavalry squadron of the French army, which was stalemated on the Franco-German front little more than 50 miles from Paris. The urgent microbiological investigation of the outbreak was assigned to d’Herelle. In the course of intensive investigation of the bugs responsible, he discovered ‘an invisible, antagonistic microbe of the dysentery bacillus’ that caused clear holes of dissolution in the otherwise opaquely uniform growth of dysentery bacteria on agar culture plates. Unlike his earlier colleagues, he had no hesitation in recognising the nature of what he had found. ‘In a flash I understood: what caused my clear spots was … a virus parasitic on the bacteria.’

D’Herelle’s hunch proved to be correct. Indeed, it would be d’Herelle who would give the virus the name we know it by today: he called it a ‘bacteriophage’. Then the French-Canadian microbiologist had an additional stroke of luck. When studying an unfortunate cavalryman suffering from severe dysentery, he performed repeated cultivations of a few drops of the patient’s bloody stools. As usual, he grew the dysentery bug on culture plates and passed a fluid extract through a Chamberland-Pasteur filter, thus obtaining a filtrate that could be tested for the presence of virus. Day after day, he tested the filtrate by adding it to fresh broth cultures of the dysentery bug in glass bottle containers. For three days the broth quickly turned turbid, confirming teeming growth of the dysentery bug. On the fourth day new broth cultures initially became turbid as usual, but when he incubated the same cultures for a second night he witnessed a dramatic change. In his words, ‘All the bacteria had vanished: they had dissolved away like sugar in water.’

D’Herelle deduced that what he was witnessing was the effects of a bacteriophage virus, which must also be present in the cavalryman’s gut – a bacteriophage virus that was capable of devouring the Shigella germ. But then he had an additional stroke of genius. What if the same thing was happening inside the infected patient? He dashed to the hospital to discover that during the night the cavalryman’s condition had greatly improved and he went on to make a full recovery. At this time bacterial infections, such as dysentery, typhoid fever, tuberculosis and the streptococcus, were a major cause of disease and death throughout the world. With no known antibiotics to treat infections, there was a desperate need for any form of therapy. His observations with the dysentery bug bacteriophage gave d’Herelle the idea that, perhaps, phage viruses might be cultivated with the express purpose of treating dangerous bacterial infections.

During the 1920s and 1930s, d’Herelle conducted extensive research into the medical applications of bacteriophages, introducing the concept of phage therapy for bacterial infections. This therapy saw widespread use in the former Soviet Republic of Georgia, and also the United States, continuing in use until the discovery of antibacterial drugs in the 1930s and 1940s. The use of drugs was much simpler to apply and proved dramatically effective, thus supplanting bacteriophage therapy. But this did not stop d’Herelle from continuing to study this marvellous if deadly entity that was so very tiny that it was completely invisible even to the most powerful light microscope, and yet appeared to be so powerful when it came into contact with its prey bacteria.

In 1926, d’Herelle published a now-historic book, The Bacteriophage, in which he described his work, and thoughtful extrapolations, concerning bacteriophage viruses. As we shall duly discover, the importance of the bacteriophage, as we recognise it today, has eclipsed all that even its pioneering researcher, Félix d’Herelle, could possibly have imagined in those early years.

In retrospect, it is remarkable that, even so many decades ago, d’Herelle clearly grasped that he was dealing with a wonder of the natural world, declaring in his book that these agents that were so dreadfully lethal to bacteria were also capable of exerting an extraordinary balancing effect in the interactions between the bacteriophage virus and its host bacterium. In his words: ‘A mixed culture results from the establishment of a state of equilibrium between the virulence of the bacteriophage corpuscles and the resistance of the bacterium. In such cultures a symbiosis obtains, in the true sense of the word: parasitism is balanced by the resistance to infection.’ This is the first use of the term ‘symbiosis’ in reference to viruses in microbiological history. In a footnote, d’Herelle took the implications further by drawing a parallel between what he was observing in the interaction of the bacteriophage virus and bacterium and the symbiosis that had recently been discovered in all land plants, where fungi in soil invade the plant roots to form a ‘mycorrhiza’, whereby the fungus feeds the plant with water and minerals and the plant feeds the fungus with the energy-giving metabolites that derive from the photosynthetic capture of sunlight. In d’Herelle’s words: ‘The respective behaviour between the bacterium and the bacteriophage is exactly that of the seed of the orchid and the fungus.’

D’Herelle is now recognised by many scientists as the father of both virology and molecular biology. But it would take many years before the world of virology, and microbiology in general, would come to rediscover d’Herelle’s original vision of the symbiotic nature of the bacteriophage.