microorganism - any organism of microscopic size

kingdom Monera, Monera - prokaryotic bacteria and blue-green algae and various primitive pathogens; because of lack of consensus on how to divide the organisms into phyla informal names are used for the major divisions

archaebacteria, archaebacterium, archaeobacteria, archeobacteria - considered ancient life forms that evolved separately from bacteria and blue-green algae

eubacteria, eubacterium, true bacteria - a large group of bacteria having rigid cell walls; motile types have flagella