Is Micrococcus luteus always considered a contaminant?

Micrococcus luteus as a Contaminant

Micrococcus luteus is typically considered a contaminant when isolated from clinical specimens, but in rare cases, particularly in immunocompromised patients, it can cause significant infections. 1

Classification and Identification

Micrococcus luteus is commonly found as part of the normal flora of human skin and mucous membranes. When isolated from clinical specimens, particularly blood cultures, it is generally classified among organisms that are considered contaminants rather than true pathogens. According to the American Society for Microbiology (ASM) guidelines, M. luteus is specifically listed as one of the organisms typically considered a contaminant when isolated from blood cultures, alongside:

• Coagulase-negative staphylococci
• Corynebacterium species (diphtheroids)
• Bacillus species (not B. anthracis)
• Certain alpha-hemolytic streptococci
• Cutibacterium acnes 1

Environmental Presence

M. luteus is ubiquitous in the environment and has been isolated from:

• Human skin and mucous membranes
• Soil 2
• Water systems
• Deep-sea environments, including extreme locations like the Mariana Trench 3

Clinical Significance

When to Consider as a Contaminant

M. luteus should be considered a contaminant when:

• Isolated from a single blood culture bottle when multiple sets were collected
• Found in specimens likely to be contaminated during collection (e.g., sputum samples)
• No clinical signs of infection correlate with the isolation
• The patient is immunocompetent without prosthetic devices

When to Consider as a Pathogen

Although rare, M. luteus can cause true infections in certain circumstances:

1. Immunocompromised hosts: Particularly patients with:

   • Hematologic malignancies (e.g., non-Hodgkin's lymphoma) 4
   • Severe immunosuppression

2. Presence of foreign bodies:

   • Prosthetic heart valves
   • Ventriculoperitoneal shunts 5
   • Indwelling catheters

3. Multiple positive cultures: When isolated from multiple specimens or blood culture sets

Documented Infections

Despite being predominantly a contaminant, M. luteus has been documented as causing:

• Native valve endocarditis 4
• Meningitis (particularly in patients with ventriculoperitoneal shunts) 5
• Septic shock (rare but reported) 6

Best Practices to Reduce Contamination

To minimize contamination with skin flora like M. luteus:

1. Proper skin antisepsis before specimen collection:

   • Avoid benzalkonium chloride (Zephiran) as it allows growth of certain microorganisms
   • Use appropriate antiseptics like chlorhexidine or alcohol 1

2. Blood culture collection technique:

   • Use diversion devices to collect the first few milliliters of blood (which may contain skin plugs)
   • Follow evidence-based collection protocols 1

3. Specimen handling:

   • Prompt processing of specimens
   • Proper storage conditions before processing

Conclusion

While M. luteus is most commonly a contaminant in clinical specimens, clinicians should consider the possibility of true infection in immunocompromised patients or those with prosthetic devices when this organism is isolated from multiple specimens or in a clinically compatible scenario. The context of isolation (patient factors, specimen type, collection method) is crucial for proper interpretation.