A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells.
A cluster of Escherichia coli bacteria magnified 10,000 times
Mahavira postulated the existence of microscopic creatures in the 6th century BC
Antonie van Leeuwenhoek was the first to study microscopic organisms.
Lazzaro Spallanzani showed that boiling a broth stopped it from decaying.
An organism is defined in a medical dictionary as any living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have been proposed to define what is an organism. Among the commonest is that an organism has autonomous reproduction, growth, and metabolism. This would exclude viruses, despite that fact that they evolve like organisms. Other problematic cases include colonial organisms; a colony of eusocial insects is organised adaptively, and has germ-soma specialisation, with some insects reproducing, others not, like cells in an animal's body. The body of a siphonophore, a jelly-like marine animal, is composed of organism-like zooids, but the whole structure looks and functions much like an animal such as a jellyfish, the parts collaborating to provide the functions of the colonial organism.
One criterion proposes that an organism cannot be divided without losing functionality. This basil plant cutting is however developing new adventitious roots from a small bit of stem, forming a new plant.
Apolemia, a colonial siphonophore that functions as a single individual
Insect cyborg
