Oral Flora

Understanding the organisms that constitute the normal oral flora is essential in the analysis of lower respiratory tract specimens. Due to the close proximity of the upper and lower respiratory tracts, specimens from the lower respiratory system are often contaminated by oral flora. Recognizing these organisms and their characteristics is crucial for interpreting culture results and differentiating true pathogens from contaminants

General Principles

Contamination Risk: Lower respiratory tract specimens, especially sputum samples, are prone to contamination by oral flora
Differentiation Challenges: Distinguishing between pathogens and contaminants requires careful consideration of colony morphology, Gram stain characteristics, and the patient’s clinical presentation
Specimen Quality Assessment: The quality of sputum specimens is assessed by the presence of squamous epithelial cells and neutrophils, and the laboratory can use a scale to evaluate the sample
Pathogen Identification: Even in contaminated samples, it is critical to identify potential pathogens

Species Comprising Oral Flora

Aerobic and Facultative Anaerobic Bacteria
- Viridans streptococci
  - Characteristics: Gram-positive cocci in chains or pairs, non-hemolytic or alpha-hemolytic on BAP
  - Colony Morphology
    - Small, translucent or gray colonies. S. pneumoniae may appear as a flattened coin
    - Alpha-hemolysis (greening) is often present
  - Gram Stain: Gram-positive cocci in chains
- Staphylococcus epidermidis and other Coagulase-Negative Staphylococci (CoNS)
  - Characteristics: Gram-positive cocci in clusters
  - Colony Morphology
    - White, cream, or yellow colonies on BAP
    - Non-hemolytic or weakly hemolytic
  - Gram Stain: Gram-positive cocci in clusters
- Staphylococcus aureus
  - Characteristics: Gram-positive cocci in clusters
  - Colony Morphology
    - Golden-yellow colonies
    - Beta-hemolytic
  - Gram Stain: Gram-positive cocci in clusters
- Corynebacterium spp. (Diphtheroids)
  - Characteristics: Gram-positive, pleomorphic rods
  - Colony Morphology
    - Small, whitish, non-hemolytic colonies
  - Gram Stain: Gram-positive rods. Can have a “Chinese letter” or “palisades” arrangement
- Neisseria spp. (non-pathogenic species)
  - Characteristics: Gram-negative diplococci
  - Colony Morphology
    - Small, gray, translucent
  - Gram Stain: Gram-negative diplococci
- Moraxella catarrhalis
  - Characteristics: Gram-negative diplococci
  - Colony Morphology
    - Gray, opaque colonies
  - Gram Stain: Gram-negative diplococci
- Haemophilus spp. (non-encapsulated)
  - Characteristics: Gram-negative coccobacilli
  - Colony Morphology
    - Small, gray colonies. Require X and V factors (for growth on BAP)
  - Gram Stain: Gram-negative coccobacilli
Anaerobic Bacteria
- Prevotella spp.
  - Characteristics: Gram-negative rods or coccobacilli
  - Colony Morphology
    - Variable, often pigmented (e.g., black-pigmented colonies)
    - B. melaninogenicus
  - Gram Stain: Gram-negative rods
- Fusobacterium spp.
  - Characteristics: Gram-negative rods, often with tapered ends
  - Colony Morphology
    - Tan or gray colonies
  - Gram Stain: Gram-negative rods with pointed ends
- Peptostreptococcus spp.
  - Characteristics: Gram-positive cocci in chains
  - Colony Morphology
    - Small, gray colonies
  - Gram Stain: Gram-positive cocci in chains
- Actinomyces spp.
  - Characteristics: Gram-positive rods, filamentous
  - Colony Morphology
    - Rough, “molar tooth” appearance
  - Gram Stain: Gram-positive rods, filamentous

Additional Considerations

Interpretation: The presence of oral flora in a lower respiratory specimen does not automatically indicate contamination
Clinical Correlation: Always interpret the culture results in light of the patient’s clinical presentation, including:
- Symptoms (cough, fever, chest pain, sputum production)
- Radiographic findings
- Underlying medical conditions
Sputum Quality: Assess the quality of sputum specimens based on the presence of squamous epithelial cells and neutrophils
- Reject samples with excessive squamous epithelial cells
Reporting: Report both the number of each type of organism that is observed, as well as whether the sample is of good quality or not

Key Terms

Oral Flora: The normal microbial community of the mouth
Contamination: The presence of organisms in a sample that were not present in the patient
Lower Respiratory Tract: Lungs, bronchi, and bronchioles
Sputum: Secretions from the lower respiratory tract
BAP (Blood Agar Plate): A general-purpose culture medium
CHOC (Chocolate Agar Plate): An enriched culture medium
Gram Stain: Staining technique to differentiate bacteria
Alpha-Hemolysis: Partial lysis of red blood cells
Beta-Hemolysis: Complete lysis of red blood cells
Squamous Epithelial Cells: Cells from the mouth and throat
Neutrophils: A type of white blood cell
Culture: The growth of microorganisms in a laboratory setting
Anaerobic: Growing without oxygen
Colony Morphology: The visual characteristics of bacterial colonies on culture media (e.g., size, shape, color, texture)
Pathogen: An organism that can cause disease
Opportunistic Pathogen: An organism that causes disease primarily in a host with a weakened immune system
Clinical Presentation: Refers to the patient’s symptoms, physical examination findings, and other relevant clinical information

Quantitative Reporting

Colony Morphology & ID