Micrococcus luteus is a spherical bacteria of between 0.5 and 3.5 millimeters in length that grows in aerobic environments and forms yellow colonies when grown on agar plates. Micrococcus luteus is typically Gram-positive, though it can sometimes be Gram-variable depending on the strain. Gram-positive means that the bacteria stain purple when treated with Gram stain. This is due to differences in the cell wall compared to Gram-negative bacteria.
Micrococcus luteus is generally described as non-pathogenic, which means that it does not cause illnesses in people unless their immune systems are already compromised. Colonies of Micrococcus luteus typically grow on mammalian skin and are frequently found in or on the mouth and respiratory tracts of humans. These bacteria can also grow in soil and water as long as there is enough oxygen present.
Micrococcus luteus is notable because of its fairly small genome size, especially when compared to other free-living bacteria. Its genome is only 2.5 million base pairs in size. Smaller genomes tend to occur in parasites after the genes for metabolic processes that the parasite gets from its host are lost. However, because Micrococcus luteus can live as easily in soil as in larger organisms, it is not a parasite and simply has a small genome.