What is the characteristic appearance of Bartonella henselae on gram stain?

Bartonella Henselae Gram Stain Characteristics

Bartonella henselae is a fastidious gram-negative rod or coccobacillus that typically does not stain well with conventional Gram staining techniques, requiring specialized stains such as Warthin-Starry silver stain for proper visualization in tissue samples. 1, 2

Microscopic Appearance

• Gram stain characteristics:
  • Gram-negative rod or coccobacillus 2, 3
  • Often appears faintly stained or may not be visualized at all with standard Gram stain
  • Described as "fastidious" due to difficulty in visualization 2

Diagnostic Staining Methods

• Warthin-Starry silver stain is the preferred method for visualization in tissue samples 1

  • Demonstrates numerous bacilli in infected tissues
  • Particularly useful for identifying the organism in bacillary angiomatosis (BA) lesions
  • Shows characteristic vascular proliferation in BA lesions

• Other staining characteristics:

  • Tissue Gram staining is typically negative 1
  • Acid-fast staining is negative 1
  • Electron microscopy can be used for detailed visualization 2

Clinical Relevance of Staining Properties

• The poor staining with conventional Gram stain contributes to diagnostic challenges 2, 3
• This characteristic explains why diagnosis often relies on:
  • Serology (primary diagnostic method) 4
  • PCR for identification and speciation 1, 4
  • Histopathologic examination of biopsied tissue 1

Common Pitfalls in Identification

• B. henselae is biochemically inert due to slow growth, making conventional biochemical identification difficult 2

• Culture is challenging as the organism:

  • Does not grow on traditional media 3
  • Requires specialized media (chocolate agar or brain-heart infusion agar with 5% fresh rabbit blood) 5
  • Needs extended incubation periods (at least 4 weeks) 5
  • Requires specific conditions (35°C, 5% CO2, high humidity) 5

• Misdiagnosis is common due to:

  • Clinical presentation resembling tuberculosis or hematological malignancies 5
  • Fastidious and fragile nature of the organism requiring careful handling 2
  • Need for high index of suspicion, especially in atypical presentations 3

The difficulty in visualizing B. henselae with standard Gram stain underscores the importance of using appropriate diagnostic methods when this infection is suspected, particularly in patients with compatible clinical presentations such as regional lymphadenopathy following cat exposure or in immunocompromised individuals with vascular proliferative lesions.