Micrococcus (mi’ krō kŏk’ Əs) is a genus of bacteria in the Micrococcaceae family. Micrococcus occurs in a wide range of environments, including human skin, water, dust, and soil. Micrococci have Gram-positive spherical cells ranging from about 0.5 to 3 micrometers in diameter and are typically appear in tetrads. Micrococcus has a substantial cell wall, which may comprise as much as 50% of the cell mass. The genome of Micrococcus is rich in guanine and cytosine (GC), typically exhibiting 65 to 75% GC-content. Micrococci often carry plasmids (ranging from 1 to 100MDa in size) that provide the organism with useful traits.
Some species of Micrococcus, such as M. luteus (yellow) and M. roseus (red) produce yellow or pink colonies when grown on solid media. Isolates of M. luteus have been found to overproduce riboflavin when grown on toxic organic pollutants like pyridine. Hybridization studies indicate that species within the genus Micrococcus are not closely related, showing as little as %50 sequence homology. This suggests that some Micrococcus species may, on the basis of ribosomal RNA analysis, eventually be re-classified into other microbial genera. These are neat!
Micrococci have been isolated from human skin, animal and dairy products, and beer. They are found in many other places in the environment, including water, dust, and soil. M. luteus on human skin transforms compounds in sweat into compounds with an unpleasant odor. Micrococci can grow well in environments with little water or high salt concentrations. Most are mesophiles; some, like Micrococcus antarcticus (found in Antarctica) are psychrophiles.
Though not a spore former, Micrococcus cells can survive for an extended period of time: unprotected cultures of soil micrococci have been revived after storage in a refrigerator for 10 years. Recent work by Greenblat et al. demonstrate that Micrococcus luteus has survived for at least 34,000 to 170,000 years on the basis of 16S rRNA analysis, and possibly much longer. 
Micrococcus is generally thought to be a saprophytic or commensal organism, though it can be an opportunistic pathogen, particularly in hosts with compromised immune systems, such as HIV patients. It can be difficult to identify Micrococcus as the cause of an infection, since the organism is a normally present in skin microflora, and the genus is seldom linked to disease. In rare cases, death of immunocompromised patients has occurred from pulmonary infections caused by Micrococcus. Micrococci may be involved in other infections, including recurrent bacteremia, septic shock, septic arthritis, endocarditis, meningitis, and cavitating pneumonia (immunosuppressed patients).
Micrococci, like many other representatives of the Actinobacteria, can be catabolically versatile, with the ability to utilize a wide range of unusual substrates, such as pyridine, herbicides, chlorinated biphenyls, and oil. They are likely involved in detoxification or biodegradation of many other environmental pollutants. Other Micrococcus isolates produce various useful products, such as long-chain (C21-C34) aliphatic hydrocarbons for lubricating oils.
- Sims GK, Sommers LE, Konopka A (1986). "Degradation of Pyridine by Micrococcus luteus Isolated from Soil". Appl Environ Microbiol. 51 (5): 963–968. PMID 16347070.
- Greenblat, C.L., Baum, J., Klein, B.Y., Nachshon, S., Koltunov, V., Cano, R.J., (2004). "Micrococcus luteus – Survival in Amber". Microbial Ecology. 48: 120–127. PMID 15164240.
- Smith K, Neafie R, Yeager J, Skelton H (1999). "Micrococcus folliculitis in HIV-1 disease". Br J Dermatol. 141 (3): 558–61. PMID 10583069.
- Doddamani H, Ninnekar H (2001). "Biodegradation of carbaryl by a Micrococcus species". Curr Microbiol. 43 (1): 69–73. PMID 11375667.
- Sims GK, O'loughlin EJ (1992). "Riboflavin Production during Growth of Micrococcus luteus on Pyridine". Appl Environ Microbiol. 58 (10): 3423–3425. PMID 16348793.
- Zhuang W, Tay J, Maszenan A, Krumholz L, Tay S (2003). "Importance of Gram-positive naphthalene-degrading bacteria in oil-contaminated tropical marine sediments". Lett Appl Microbiol. 36 (4): 251–7. PMID 12641721.