Micrococcus luteus

Micrococcus luteus is gram-positive, spherical (cocci), non-fermenting, and belongs to the family Micrococcaceae. Depending upon growth conditions, M. luteus can present as singular balls, in pairs (diplococci), in tetrads (groupings of four), and even as irregular clusters. Being an obligate aerobe, this species is found very commonly on animal skin microbiota and mucous membranes, in the environment (soil and water) as well as on abiotic surfaces such as dust. Because of this, it is extremely common to recover this species in clinical settings and is one of the more common contaminants identified in routine monitoring of cleanrooms.

The genus, Micrococcus, currently consists of only a few species since modern molecular techniques have re-classified many organisms that were once mis-categorized under the genus, with Micrococcus luteus being the type species. Micrococcus is typically considered non-pathogenic in most cases, though parasitic characteristics could arise in particular instances, particularly for immunocompromised patients, often influenced by environment, host risk factors, and medical intervention (i.e., the presence of an unhygienic long-term in-dwelling catheter).

M. luteus is not capable of forming a thick endospore for protection from harsh environments. However, M. luteus can form biofilms, particularly when in conjunction with other avid biofilm formers, such as Pseudomonas aeruginosa.

Infections caused by M. luteus can vary in severity and progression, often influenced by host defense factors. Symptoms typically present in localized inflammation of the colonized region with very rare instances resulting in system-level infection such as blood-stream infections.

Epidemiological investigations of M. luteus must be proactive in identifying the actual status of the organism compared to the patient host as either contaminant, colonizer, or infectious pathogen. Being a common commensal organism, M. luteus may continuously be present in clinical specimens and can be exposed to various rounds of antibiotic treatment, urging appropriate action to deter the development of antibiotic-resistant strains.

Relevance of pathogen in transmission in endoscopy

• Gastroenterology: Not relevant

• Pulmonology: Low

• Ear, nose, and throat: Not relevant

• Urology: Low

Relevance for endoscope surveillance

• Low concern organism (Heightened levels of Micrococcus luteus are often be indicative of environmental cleanliness or lapses in endoscope handling, drying, and storage.)

Transmission route

Micrococcus luteus can be recovered from soil, water, animal skin, the human nasopharyngeal complex, plant material, food, and dust ^[1][4].

Transmission methods typically occurs via direct skin-to-skin contact with colonized individuals, or through reservoirs in contaminated soil, air, water, counter surfaces, etc. ^[2][4].

Transmission risk during endoscopy is influenced by lapses in endoscope handling and/or drying and storage.

Cases of transmission are not always certain and well-researched without requiring intense laboratory testing and sometimes uncomfortable sample collection from the patient . This is due to the fact that Micrococci are generally non-pathogenic and they colonize their human hosts in a commensal and mutually beneficial relationship. Because of this relationship, it is possible for the presence of M. luteus to confound epidemiological investigations by mis-identifying M. luteus as contaminant or pathogenic agent without proper assessments.

Resistance to antibiotics

Due to Micrococcus luteus’ low virulence and reported pathogenic incidence rate, knowledge of M. luteus’ known antibiotic resistance is very limited. Guidance is also limited around this species, due to the low incidence rate, however limited case studies have reported success in identifying sensitivities utilizing Kirby-bauer disc infusions and MALDI-TOF analyses ^[3][4].