What is a Microorganism?

Organismós, "organism"; also spelled micro-organism, micro organism or microörganism) or microbe is a microscopic organism that comprises either a single cell (unicellular), cell clusters,1 or multicellular relatively complex organisms. The study of microorganisms is called microbiology, a subject that began with Anton van Leeuwenhoek's discovery of microorganisms in 1675, using a microscope of his own design. Microorganisms are very diverse; they include all of the prokaryotes, namely the bacteria and archaea; and various forms of eukaryote, comprising the protozoa, fungi, algae, microscopic plants (green algae), and animals such as rotifers and planarians.

Some microbiologists also classify viruses as microorganisms, but others consider these as nonliving. 23 Most microorganisms are unicellular (single-celled), but this is not universal, since some multicellular organisms are microscopic, while some unicellular protists and bacteria, like Thiomargarita namibiensis, are macroscopic and visible to the naked eye. Microorganisms live in all parts of the biosphere where there is liquid water, including soil, hot springs, on the ocean floor, high in the atmosphere and deep inside rocks within the Earth's crust.

Microorganisms are critical to nutrient recycling in ecosystems as they act as decomposers. As some microorganisms can fix nitrogen, they are a vital part of the nitrogen cycle, and recent studies indicate that airborne microbes may play a role in precipitation and weather. Microbes are also exploited by people in biotechnology, both in traditional food and beverage preparation, and in modern technologies based on genetic engineering.

However, pathogenic microbes are harmful, since they invade and grow within other organisms, causing diseases that kill humans, other animals and plants. Single-celled microorganisms were the first forms of life to develop on Earth, approximately 3 billion–4 billion years ago. 789 Further evolution was slow,10 and for about 3 billion years in the Precambrian eon, all organisms were microscopic.

11 So, for most of the history of life on Earth the only forms of life were microorganisms. 12 Bacteria, algae and fungi have been identified in amber that is 220 million years old, which shows that the morphology of microorganisms has changed little since the Triassic period. Microorganisms tend to have a relatively rapid evolution.

Most microorganisms can reproduce rapidly, and microbes such as bacteria can also freely exchange genes through conjugation, transformation and transduction, even between widely-divergent species. 14 This horizontal gene transfer, coupled with a high mutation rate and many other means of genetic variation, allows microorganisms to swiftly evolve (via natural selection) to survive in new environments and respond to environmental stresses. This rapid evolution is important in medicine, as it has led to the recent development of 'super-bugs' — pathogenic bacteria that are resistant to modern antibiotics.

The possibility that microorganisms exist was discussed for many centuries before their actual discovery in the 17th century. The existence of unseen microbiological life was postulated by Jainism, which is based on Mahavira's teachings as early as 6th century BCE. 16 Paul Dundas notes that Mahavira asserted existence of unseen microbiological creatures living in earth, water, air and fire.