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Micrococcus 

Micrococcus luteus growth on nutrient agar is bright mustard-yellow colonies as shown above image. Micrococcus luteus were discovered by Sir Alexander Fleming before he discovered penicillin in 1928. They are found in soil, dust, water and air, and as part of the normal microbiota of skin. The bacterium also colonizes the mouth, mucosae, oropharynx, and upper respiratory tract.

Scientific classification

• Kingdom: Bacteria
• Phylum: Actinobacteria
• Order: Micrococcales
• Family: Micrococcaceae
• Genus: Micrococcus
• Species: M. luteus
• Micrococcus luteus: bright mustard-yellow colonies of clusters of very tiny, spherical cells
• Micrococcus roseus: rose-colored colonies

Morphology of Micrococci

Micrococcus species are strictly aerobic Gram-positive cocci arranged in tetrads or irregular clusters, and cells range from 0.5 to 3 µm in diameter. They are seldom motile and are non-sporing. They are also catalase-positive and often weakly oxidase-positive ( modified oxidase test positive). Micrococci may be distinguished from staphylococci by a modified oxidase test as shown below-

Colony characteristics

They grow on simple media and their colonies are usually pigmented.

Laboratory Diagnosis

Gram Stain: Gram-positive cocci, arranged in clusters that differ from Staphylococcus. It may appear in irregular clusters, groups of four or eight. It is often larger than Staphylococcus.

Oxidation-Fermentation Test: Oxidative

Culture Characteristics: Colonies are shades of yellow or red. The optimum growth temperature is 25-37°C. They have a respiratory metabolism, often producing little or no acid from carbohydrates, and are usually halotolerant, growing in 5% NaCl.

Oxidase (modified oxidase)  test: Positive

They contain cytochromes and are resistant to lysostaphin.

0.04U bacitracin: Sensitive

Pathogenicity of Micrococcus

They are usually normal flora but can be opportunistic pathogens in certain immunocompromised patients.

Micrococcus lylae

Micrococcus lylae are Gram-positive cocci mostly arranged in tetrads. They are catalase and oxidase positive whereas urease negative.  They grow in circular, entire, convex, and usually non-pigmented or cream white colonies with diameters of approximately 4 mm after 2-3 days on the plate at 37°C. M. lylae can be distinguished from the closely related species Micrococcus luteus by lysozyme susceptibility, genetic composition, and the type of cell-wall peptidoglycan. It has been isolated from human skin.

Micrococcus luteus

Micrococcus luteus are Gram-positive cocci mostly arranged in tetrads and larger than Staphylococcus. They are positive for catalase and oxidase ( modified). They grow in circular, entire, convex, and creamy yellow-pigmented colonies with diameters of approximately 4 mm after 2-3 days at 37°C. Several uncommon strains produce raised colonies with translucent, depressed centers. Colony pigmentation varies considerably but is usually different shades of yellow or cream-white. Growth or weak growth is observed at 45°C, at pH 10, and in the presence of 10% sodium chloride (NaCl). There are 3 biovars of this organism and they possess quite diverse chemotaxonomic features with respect to their menaquinone systems, cell-wall compositions, and Fourier transform-infrared (FT-IR) spectroscopy patterns, as well as biochemical properties. The identification of three different biovars within the species M. luteus has the advantage that the three groups can be differentiated without nomenclatural changes having to be introduced. It has been isolated from human skin.

Key Notes

• Even being normal flora, Micrococcus luteus have been obtained from blood cultures of pulmonary hypertension (PH) patients who were receiving continuous infusion therapy with epoprostenol.
• Catheter-related septic shock by Micrococcus in an autologous hematopoietic stem cell transplantation recipient.
• Organisms that are similar to Micococcus are Staphylococcus and Rothia.
• The differences between Staphylococcus and Micrococcus are on the basis of the following features-

1. Anaerobic growth
2. Arrangement of cells (predominant)
3. Mol % G+C of DNA
4. Carbohydrate utilization
5. Oxidase (modified oxidase  test)
6. 0.04U bacitracin
7. Lysostaphin
8. Teichoic acid in cell wall

Staphylococcus versus Micrococcus

References

1. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/887570/UK_SMI_ID_07i4.pdf
2. https://en.wikipedia.org/wiki/Micrococcus_luteus
3. https://europepmc.org/article/med/14576986
4. https://www.ajicjournal.org/article/S0196-6553(13)01146-2/fulltext