Microorganism

A  microorganism (from the Greek: μικρός, mikrós, "small" and ὀργανισμός, organismós, "organism"; also spelt micro-organism, micro organism ormicroörganism) or microbe is a microscopicorganism that comprises either a single cell (unicellular), cell clusters or no cell at all (acellular).^[1]. The study of microorganisms is called microbiology, a subject that began with Anton van Leeuwenhoek's discovery of microorganisms in 1675, using amicroscope of his own design.

Microorganisms are very diverse; they includebacteria, fungi, archaea, and protists; microscopicplants (green algae); and animals such as planktonand the planarian. Some microbiologists also include viruses, but others consider these as non-living.^[2][3] Most microorganisms are unicellular (single-celled), but this is not universal, since somemulticellular organisms are microscopic, while some unicellular protists and bacteria, likeThiomargarita namibiensis, are macroscopic and visible to the naked eye.