microorganisms

microorganisms These are microscopic forms of life which are ubiquitous in the environment and on the human body. They were first detected in 1675 by a Dutch draper, Anthony van Leeuwenhoek, who noticed tiny ‘animalcules’ in droplets of rainwater under his microscope. He went on to discover that they were present in dental plaque, faeces, and many other substances.

Most microorganisms are harmless or even beneficial to man. Only a minority cause disease in healthy humans (although many more may do so in patients with damaged immune systems). Thus it may not be so surprising that it was another 112 years after van Leeuwenhoek's discovery before it was shown that these minute creatures were also under certain circumstances the agents of disease: this was first demonstrated by Agostino Bassi in 1835 for a bacterial infection of silkworms. The German Robert Koch was the first to prove that a bacterium could cause a human disease, namely anthrax, in 1876. Naturally this discovery aroused huge scientific and public interest, although of course there were those in both the lay and the scientific communities who were opposed to the new theory of infection. This is the subject of Ibsen's play An Enemy of the People, in which the town doctor learns that the presence of bacteria in the water supply might transmit diseases such as typhoid and cholera. He campaigns to have the water cleaned, but the whole idea of tiny invisible ‘animals’ is met with ridicule by the community and his career is ruined.

The study of microorganisms is known as microbiology and not surprisingly much of the study is directed at those organisms which do cause human disease. It is now realized that not only are microorganisms responsible for what are conventionally considered ‘infectious diseases’ but they may also contribute to such diverse illnesses as peptic ulcers, angina, and cervical cancer. There is no doubt that in the future microorganisms will be found to be involved in many more ‘non-infectious’ diseases. However, they are also essential to human life. Every square inch of our body surface is colonized by many thousands of organisms which help to protect the body from invasion by other potentially harmful organisms. If this normal ‘flora’ is damaged, for instance by a course of antibiotics, it leaves the way open for the harmful organisms to get a foothold and establish themselves instead. Microorganisms are also employed in a wide variety of home and industrial processes. For example, yeasts are essential for making bread rise and for the process of alcohol fermentation; genetically engineered bacteria are used to make insulin and in the production of genetically modified foods.

Microorganisms are classified into bacteria, viruses, fungi, and parasites. Prions, which are thought to be infective protein particles rather than live organisms, are included also in the field of microbiology. These groups are totally unrelated to one another, the only common factor being that they are all microscopic.

Bacteria

These are single-celled organisms, usually either rod-shaped or roughly spherical. They are classified according to their reaction to Gram's stain: those that go blue with this stain are termed Gram positive, those staining red are Gram negative. This reflects a fundamental difference in the structure of their cell walls. They are responsible for diseases ranging from typhoid, plague, cholera, meningococcal meningitis, tuberculosis, tetanus, gonorrhoea, and syphilis, to the more mundane urinary tract infections, boils, and acne. They are killed by antiseptics and by boiling, although they may produce toxins which are not destroyed. Many were originally sensitive to antibiotics such as penicillins, but overuse of these drugs has resulted in many multi-resistant bacteria. The most notorious is multi-drug resistant tuberculosis. In terms of numbers, however, the greatest problem in the UK has been methicillin resistant Staphylococcus aureus, or MRSA. Although often it merely colonizes the skin and causes no harm, it can cause a wide range of infections and is difficult to treat. Great attempts have been made to limit its spread in hospitals, for instance by placing patients carrying the organism in isolation rooms.

Viruses

These are even tinier than bacteria and cannot be seen through a light microscope: electron microscopy is required. Their name comes from the Latin for ‘toxin’. Their existence was first suspected when it was found that some infectious agents could pass through a fine filter, unlike bacteria, which were too large. They are very simple life forms, often consisting simply of a DNA or RNA core and a protein coat. They cannot reproduce except inside another living cell, which they must hijack in order to do so. They are susceptible to heat and to some antiseptics.

The most high-profile virus of recent years has been the human immunodeficiency virus — HIV — which causes AIDS. Other viral illnesses include influenza, the common cold, Lassa fever, and ebola. There have been few anti-viral drugs available, and the main line of defence has been vaccination, which has eradicated smallpox and may do the same for polio in the near future. Unfortunately, many viruses, including those causing HIV and the common cold, have a very high rate of genetic change so that they can evade the immune system, which makes the development of vaccines extremely difficult. Recently there have been great advances in the treatment of HIV with the development of ‘triple therapy’, or ‘highly active anti-retroviral therapy’ (HAART). This consists of the use of three or sometimes more drugs together to combat the infection, and has been highly successful in slowing the progression of the disease to full-blown AIDS. The use of the combination of drugs prevents the virus from becoming drug-resistant so rapidly.

Fungi

Like bacteria, microscopic fungi are everywhere. They may be yeasts or moulds. Yeasts have been used for centuries by peoples worldwide to ferment sugar to alcohol; the drug penicillin was found in a mould. They have a very resilient cell wall, which makes them difficult to treat. The commonest fungal infections are vaginal thrush, which often occurs after a course of antibiotics has destroyed the normal vaginal bacteria, and nail and skin infections such as ‘ringworm’.

In the UK more serious fungal infections are generally only found in seriously immune-deficient patients, such as leukaemia or AIDS patients. However, in other parts of the world there are fungi which can cause severe disease in healthy individuals.

Parasites

These are organisms which live in or on the body of another known as the ‘host’. The host may provide a source of nutrients or a safe haven in which to reproduce. They range in size from single cells, like the malaria parasite, to tapeworms which may reach thirty feet in length and hence are not microscopic at all! Parasites are mainly a problem of developing countries, although some, such as thread worm, are also common in the UK.

Prions

These are the simplest infectious particles known, now widely accepted as the cause of a group of degenerative diseases of the brain that includes Creutzfeld–Jacob disease (CJD) in humans, bovine spongiform encephalopathy (BSE) in cattle and scrapie in sheep. Since the BSE epidemic in Britain in the 1980s there has been concern that a new variant of CJD may be transmitted to humans who eat the meat (specifically nervous tissue) of infected cattle. Prions are thought to consist merely of a protein with no DNA or RNA so in this sense are not ‘live’. They are nonetheless infectious because when the abnormal prion protein comes into contact with a similar but normal protein it causes a change in the way the normal protein molecule is folded up. This turns the normal protein into another abnormal protein, which alters other nearby proteins in turn, and so on until disease is evident. No treatment is available for prion diseases; also prions can withstand heat, desiccaion and conventional antiseptics, making safe disposal of infected material very difficult.

In the 1960s and 70s the rapid expansion in the number of antibiotics being developed led man to believe that he had won the battle against infection with microorganisms, but it is now evident that this is far from being the case. The emergence of new viruses and prion diseases, together with ever more antibiotic-resistant organisms, makes the study of microorganisms as urgent as ever.

Angharad Puw Davies

See also epidemics; immunization; infectious diseases; prions.