The most common image evoked by the term micro-organism (microbe) is likely to be that of "germs", i.e. disease-causing microbes. Some micro-organisms do cause human disease but to think of this entire class of biological organisms in this manner is to grossly underestimate its importance and role.
The American biologist Lynn Margulis (with Dorian Sagan) has described the universal importance of micro-organisms in her book Microcosmos (Allen and Unwin, 1987), as also has Bernard Dixon in his book Power Unseen: How Microbes Rule The World (W.H. Freeman, 1994).
Micro-organisms are of microscopic size and the term covers a wide variety of organisms such as bacteria, protozoa, viruses, yeasts etc. Micro-organisms include the oldest, most numerous, most widespread and most successful forms of life on Earth.
They were originally seen under a microscope by the famous Dutch microscopist Anton van Leeuwenhoek (1632-1723). For a great many years micro-organisms were considered a mere oddity, their impact minimised in the scientific consciousness largely because they are invisible to the naked eye.
However, they began to assume much more importance in the 19th century, not least because of the work of the German naturalist Christian Gottfried Ehrenberg (17951876) who showed, by studying the fossils of micro-organisms, that these tiny creatures were present in every corner of the Earth's crust.
The work of Louis Pasteur (1822-1895), for example, on rabies, and of the great German bacteriologist Hermann Robert Koch (1843-1910), on cholera and TB, greatly shaped the public perception of micro-organisms as agents of infectious disease.
Life on Earth began about 3.5 billion years ago as a micro-organism, specifically as a simple form of bacterium. All of the life-forms that now inhabit the Earth are descended from that original simple form.
Margulis proposes that bacteria, during the first two billion years of evolution, learned all the basic chemical pathways used subsequently by all life forms and learned everything necessary for organising all living systems.
Margulis boldly proposes that larger and more complex cells than bacteria gradually evolved when different types of bacterial cells combined with each other, living together in a symbiotic relationship in a larger whole. (Symbiotic means mutually dependent.) Animals, including humans, are composed of cells that are 25 times greater in diameter than bacteria, but, according to Margulis's vision, our cells are simply highly evolved examples of different types of bacteria living in smoothly functioning symbiosis. We are, in essence, highly organised bacteria.
This is far more than an intriguing conjecture: there is much evidence to support the hypothesis. For example, all animal and plant cells contain little bodies called mitochondria that have many properties characteristic of bacteria. The mitochondrion generates most of the chemical energy required by the cell.
Each mitochondrion is about the size of a modern bacterium, i.e. about two micro-metres (millionths of a metre) long by about one micro-metre wide. Each mitrochondrion contains its own genetic material (DNA) which bears many similarities to bacterial DNA.
The mitochondrion divides in two by a process called binary fission, i.e. it duplicates its genetic material and the mitochondrion divides in two, each half inheriting a copy of the genetic material.
Bacteria also reproduce themselves by binary fission. Mitochondria are surrounded by a double membrane: many bacteria are surrounded by a double cell wall. It appears almost certain that the mitochondrion was once a free-living bacterium that invaded another larger bacterium and the two formed a stable symbiotic relationship.
There are equally good reasons to believe that the chloroplast, the little body found in green plant leaf cells, that confers on plants the ability to harness the energy of sunlight and convert it into carbohydrate (a basic process upon which almost all food chains on Earth ultimately depend), was also once a free-living bacterium that entered into a symbiotic relationship with another cell.
On a more mundane scale, micro-organisms are also of great everyday practical importance, e.g. in the food and drinks industries. Various strains of bacteria and yeast are used in the fermentation processes that are used to produce various products such as yoghurt, cheese, bread, beer and wine.
A question: Name the substance upon which the modern industrial state pre-eminently depends? Answer: Crude oil. Question: Where does crude oil come from? Answer: From micro-organisms. Crude oil is thought to be formed from un-decomposed organic matter, principally single-celled marine organisms that settle to the bottom of marine basins and are rapidly buried within sequences of mud-rock and limestone. Oil and natural gas are formed as such rocks sink into the earth, becoming compacted and hot.
Disease-causing microbes have awesome power, but this can lead to unexpected consequences. The bubonic plague (Black Death) was caused by the bacterium Yersinia pestis, and by 1351, four years after the outbreak, had killed more than 25 million Europeans, one-third of the population.
This greatly decreased competition for food, shelter and work and left the survivors to inherit the riches of the dead. These general conditions led to the great flowering of creativity called the Renaissance.
William Reville is a senior lecturer in biochemistry in UCC and director of Microscopy