In microbiology, pleomorphism, also pleiomorphism, is the ability of some microorganisms to alter their morphology, biological functions or reproductive modes in response to environmental conditions. Pleomorphism has been observed in some members of the Deinococcaceae family of bacteria. The modern definition of pleomorphism in the context of bacteriology is based on variation of morphology or functional methods of the individual cell, rather than a heritable change of these characters as previously believed.

Pierre Jacques Antoine Béchamp was a French scientist now best known for breakthroughs in applied organic chemistry and for a bitter rivalry with Louis Pasteur.