Many diseases of humans, other animals and plants are caused by small organisms; microbes, such as bacteria, fungi and viruses which are present in the environment and can be passed on from already infected individuals.
Bacteria or fungi may enter the body and reproduce in certain organs or tissues. Viruses are not independent organisms but are packets of genetic information which cannot survive on their own but can invade healthy cells and make them produce copies of the virus, usually killing the cell in the process. The symptoms of a disease are caused by damage to the tissues, by the toxins produced by the microbes or sometimes by the immune system itself.
The body can defend itself against infections or other foreign tissue with its immune system. White blood cells and antibodies both play a role in the specific immune response. A different antibody is required to kill each different type of microbes. An individual who survives an infection by a particular microbes is then able to make those specific antibodies very quickly and is thus protected against future invasion by that organism.
Immunity can be provided artificially by exposing the individual to a form of the microbe that has been altered so that it is unable to cause disease but will still stimulate the production of antibodies. The process is known as vaccination. It is proving very difficult to develop an effective vaccine against some diseases such as the common cold, malaria and HIV. This is partly because the microbes mutate rapidly and are no longer recognised by the antibodies.
An individual can be infected by microbes in several ways which include: directly from an infected person, from a contaminated environment including air and water, or via an insect vector. The route of transmission depends on the microbes.
Antibiotics are chemicals which kill or inhibit the growth of certain bacteria or fungi. They can be used to treat infections by these organisms. They have no effect on viruses. However, over a period of time the bacteria or fungi become resistant to an antibiotic. Random gene mutations sometimes lead to individuals which are less affected by the antibiotics. These have a better chance of surviving a course of antibiotic treatment. These resistant individuals then reproduce, resulting in resistant strains of microbes.
Pasteur and Koch the microbe hunters
The superstar of medical researchers, Louis Pasteur (1822-95) was a chemistry graduate. He was an outstanding microscopist whose interest in micro-organisms was stirred by studies of fermentation in connection with wine and beer making and he devised elegant experiments to scotch the old theory of spontaneous generation. Maggots, he showed, arose from insect-laid eggs and from organisms in the atmosphere. He developed his acclaimed method for eliminating microbes from milk: ‘pasteurization’ – heating to a prescribed temperature to kill them – ensuring that milk would cease to be a source of tuberculosis and other ailments.
The problem of aetiology – what causes disease was one of medicine’s key unresolved questions and it was brought to a head by the terrible wave of of epidemics blighting Europe at the time. Many espoused the ‘miasmatic’ theory – the idea that disease originated in effluvia and other emanations from the soil and atmosphere. Others embraced ‘contagionism’ – disease was something passed from person to person.
Pasteur by no means invented the ‘germ theory’ that disease is caused by microscopic living organisms, but he was the first to show that particular microbes caused particular diseases. His researches into chicken cholera, swine erysipelas and anthrax led to new ‘vaccines’ – the term he coined to honour Edward Jenner who had championed cowpox inoculation against smallpox (vacca is Latin for cow).
The efficacy of Pasteur’s anthrax vaccine was shown in one of the many spectacular experiments which were his forte. On 28 April 1881 he injected 24 sheep with his new vaccine, repeating it after 3 weeks. A fortnight later, this group, along with a control group of unvaccinated animals was implanted with virulent anthrax bacilli. When the sheep were again inspected on 2 June, all the vaccinate animals were healthy and all the unvaccinated ones were dead or dying. Pasteur’s crowning achievement, the rabies vaccine he developed in 1885, was for a ghastly and fatal disease which, like anthrax, killed both animals and human beings.
Pasteur’s linking of streptococci and staphylococci to specific diseases put bacteriology on the scientific map. But it was his younger German contemporary, Robert Koch, later professor of public health in Berlin, whose meticulous demonstrations clinched the microbial theory of disease. In 1879 Koch published a paper which differentiated between different bacteria, connected specific micro-organisms to specific infections and sought to prove that bacteria were the cause of infections. To this end he spelt out what are known as ‘Koch’s postulates’ – four requirements to prove that a particular micro-organism produces a particular condition:
- The specific organism must be present in every instance of the infectious disease.
- The organism must be capable of cultivation in pure culture.
- Inoculating an experimental animal with the culture would reproduce the disease.
- The organism could be recovered from the inoculated animal and grown again in a pure culture.
As lately with AIDS, ‘Koch’s postulates’ are still invoked in attempts to test whether a specific micro-organism is the true – necessary and sufficient – cause of a disease. Koch’s greatest discoveries were the bacillae which produce tuberculosis (1882) and cholera (1883). His students and rivals went on to use his methods to identify the causal microbes for typhoid, diphtheria, pneumonia, gonorrhoea, meningitis, leprosy, tetanus, plague, syphilis, whooping cough and many other staphylococcal and streptococcal infections.
From Blood and Guts by Roy Porter (Penguin, 2002) chapter 4: The Laboratory
- Briefly define the following key words: Antibody, Bacterium (plural: bacteria), Fungus (plural: fungi), Immune system, Immunity, Microbe, Toxin, Vaccination, Vector, Virus.
- Research 5 of the diseases mentioned in the readings above and for each one describe the symptoms, the micro-organism which causes it, and what the current treatment is.
- Work on the sheets on John Snow and epidemiology and Koch and tuberculosis from the Nuffield website here: please attempt the questions before looking at the answers!