Skin Flora

The skin is the body’s primary barrier, and it is colonized by a diverse population of microorganisms. Understanding skin flora and their potential roles in bloodstream infections and bone marrow infections is crucial for interpreting culture results and managing patient care

Normal Skin Flora

Composition: The skin flora is a complex and dynamic community of bacteria, fungi, and viruses. The composition varies based on factors like:
- Anatomic Site: Different areas of the skin have different conditions (moisture, sebum production, etc.) influencing which microbes thrive
- Age: Flora changes throughout life
- Environmental Exposure: Factors like hygiene, contact with other individuals, and exposure to environmental microbes
Predominant Organisms
- Gram-Positive Bacteria
  - Coagulase-Negative Staphylococci (CoNS): Staphylococcus epidermidis is the most abundant. Other CoNS include S. haemolyticus, S. hominis, S. capitis, S. lugdunensis
  - *Staphylococcus aureus:** Found on the skin of some individuals. Can colonize the anterior nares
  - Corynebacterium spp.: (diphtheroids). Includes C. xerosis, C. striatum, C. jeikeium
  - Micrococcus spp.
  - Propionibacterium acnes (Cutibacterium acnes): Commonly found in hair follicles and associated with acne
  - Bacillus spp.
  - Enterococcus spp.: Can be transient colonizers
- Gram-Negative Bacteria
  - Acinetobacter spp.: Can colonize the skin, especially in healthcare settings
  - Klebsiella spp.
  - Pseudomonas spp.
  - Moraxella spp.
- Fungi
  - Malassezia spp.: Yeast, often associated with seborrheic dermatitis
  - Candida spp.: Usually present in low numbers

Role of Skin Flora in Bloodstream and Bone Marrow Infections

Contamination vs. Infection: Differentiating between skin flora contamination and true infection is a critical step in interpreting blood culture results
Contamination
- Source: Skin flora organisms can contaminate blood culture samples during collection. This is more common when aseptic technique is not strictly followed
- Appearance: Contaminants are typically low in number and may be the only organism isolated from one blood culture set
- Common Contaminants: CoNS (especially S. epidermidis), Corynebacterium spp., Bacillus spp.
- Clinical Significance: In the absence of clinical signs of infection (e.g., fever, chills, signs of sepsis) and in the absence of isolation from multiple blood culture sets, contamination is the most likely explanation
Skin Flora as Pathogens
- Entry Points: Bacteria from the skin can enter the bloodstream through various routes
  - Catheters and Intravascular Devices: A major risk factor for bacteremia and sepsis. Organisms can colonize the catheter and then enter the bloodstream. A major route of infection is along the external surface of the catheter
  - Skin Wounds and Trauma: Breaks in the skin barrier provide opportunities for entry
  - Invasive Procedures: Surgical procedures, dental procedures, and other invasive interventions can introduce organisms into the bloodstream
  - Indwelling Medical Devices: Intravenous catheters, central lines, and other indwelling devices create an entry point for skin flora to enter the bloodstream
- Organisms of Clinical Significance
  - CoNS: The most common cause of catheter-related bloodstream infections, especially S. epidermidis. Can also cause endocarditis and other infections. Often resistant to multiple antibiotics
  - Staphylococcus aureus: Can cause a wide range of infections, including bacteremia, endocarditis, osteomyelitis, and septic arthritis. Virulent pathogen
  - Corynebacterium spp.: Can cause bloodstream infections, especially in immunocompromised patients or those with central lines
  - Cutibacterium acnes: Can cause bloodstream infections, particularly in patients with prosthetic devices
  - Bacillus spp.: Uncommon cause of bacteremia, but can occur, especially in immunocompromised patients

Clinical Significance and Interpretation

Key Factors
- Number of Positive Cultures: Isolation of the same organism from multiple blood culture sets strongly suggests a true infection, regardless of whether it is a skin flora organism
- Clinical Presentation: Presence or absence of symptoms (e.g., fever, chills, hypotension, altered mental status, and signs of local infection) is critical for assessing the significance of a positive blood culture
- Other Laboratory Results: Elevated white blood cell count, elevated inflammatory markers (e.g., CRP, procalcitonin), and abnormal organ function tests support the diagnosis of a bloodstream infection
- Antimicrobial Therapy: If the patient is already on antibiotics, the presence of an organism in the blood culture suggests that the therapy is ineffective or that there is a secondary infection
- Source of Infection: Consider the presence of intravascular devices, skin wounds, or other potential sources of infection
Interpretation Guidelines
- CoNS Isolation
  - Single positive blood culture: More likely to be contamination, unless the patient has a clear clinical picture of infection, or is immunocompromised
  - Multiple positive blood cultures: High probability of bloodstream infection, especially if the patient has an intravascular device
  - Isolation from a catheter tip: Supports the diagnosis of catheter-related bloodstream infection
- Staphylococcus aureus Isolation: Always considered a pathogen. Should be taken very seriously
- Corynebacterium spp. and Bacillus spp. Isolation: Assess clinical correlation. More likely to be pathogenic in patients with indwelling devices, central lines, or immunocompromised
- Prompt Reporting: All positive blood cultures should be promptly reported to the physician, along with Gram stain results and preliminary identification

Aseptic Technique and Specimen Collection

Meticulous Aseptic Technique: The single most important factor in reducing the risk of contamination
- Site Preparation: Prepare the venipuncture site thoroughly with antiseptic solution (e.g., 70% isopropyl alcohol followed by Chloraprep or povidone-iodine), allowing adequate dry time
- Blood Collection: Avoid touching the venipuncture site
- Equipment Sterility: Use sterile needles and syringes
Blood Culture Bottles
- Disinfect bottle tops: Before inoculating blood culture bottles, disinfect the rubber stoppers with 70% alcohol to avoid contamination
- Proper Collection Volumes: Collect the appropriate volume of blood, as indicated in your procedures
Catheter-Associated Infections
- Collect from multiple sites: Collect blood cultures from both the catheter and a peripheral site
- Catheter Removal and Tip Culture: If the catheter is being removed, send the catheter tip for culture

Key Terms

Skin Flora: The community of microorganisms that normally colonize the skin
Commensal Organism: A microorganism that lives on the host but does not cause harm
Contamination: The presence of microorganisms in a sample that were not present in the patient
Bacteremia: The presence of bacteria in the bloodstream
Sepsis: A life-threatening organ dysfunction caused by a dysregulated host response to infection
Coagulase-Negative Staphylococci (CoNS): A group of staphylococci that do not produce coagulase and are common skin flora organisms (e.g., Staphylococcus epidermidis)
Catheter-Related Bloodstream Infection (CRBSI): A bloodstream infection that originates from an intravascular catheter
Aseptic Technique: A set of procedures used to prevent contamination of a specimen with microorganisms from the environment
Venipuncture: The process of drawing blood from a vein
Antiseptic: A substance that kills or inhibits the growth of microorganisms on the skin
Culture: The growth of microorganisms in a laboratory setting for identification and susceptibility testing
Gram Stain: A staining technique used to differentiate bacteria based on cell wall characteristics

Rapid ID Methods

Colony Morphology & ID