What is the histological appearance of Staphylococcus (S.) aureus?

Histological Appearance of Staphylococcus aureus

Staphylococcus aureus appears histologically as Gram-positive cocci arranged in grape-like clusters, which is the characteristic morphology used for identification in clinical specimens. 1

Key Morphological Features

• S. aureus is a spherical (cocci) bacterium with a diameter of approximately 0.5-1.0 μm that forms distinctive grape-like clusters when viewed under microscopy 1
• On Gram staining, S. aureus appears purple (Gram-positive) and typically forms clusters resembling bunches of grapes, which is a defining characteristic used for identification 2
• This clustering pattern is distinctive from streptococci, which typically arrange in chains rather than clusters 2

Histological Identification in Clinical Specimens

• In respiratory samples, S. aureus can be identified by the presence of Gram-positive cocci in clusters with a reported sensitivity of 68% (95% CI 49-83%) and specificity of 95% (95% CI 86-98%) 2
• The high specificity of identifying these grape-like clusters on Gram stain makes it a valuable tool for preliminary identification of S. aureus infections 2
• When examining tissue samples, S. aureus may be surrounded by neutrophilic infiltrates, particularly in abscess formations 3

Tissue Reaction to S. aureus

• In skin and soft tissue infections, S. aureus often induces abscess formation characterized by:
  • A central collection of neutrophils (pus) 3
  • Surrounding fibrin capsule that walls off the infection 3
  • Peripheral rim of inflammatory cells 2
• In furuncles and carbuncles, histology reveals suppuration extending through the dermis into subcutaneous tissue with S. aureus visible as grape-like clusters within the purulent material 2

Special Considerations in Histological Diagnosis

• The morphological evaluation of S. aureus in Gram staining has high diagnostic value with a diagnostic odds ratio of 38 (95% CI 13-106) 2
• In scenarios where the prevalence of S. aureus is between 5% and 20%, positive and negative predictive values are 62% (95% CI 47-77%) and 95% (95% CI 82-100%), respectively 2
• Histological identification may be affected by prior antibiotic treatment, which can alter the typical appearance of bacterial colonies 2

Clinical Implications of Histological Identification

• Rapid identification of S. aureus through its characteristic grape-like clustering pattern can guide early appropriate antibiotic therapy, particularly important in severe infections 2
• The high negative predictive value of Gram stain examination (when S. aureus is not visualized) can help clinicians rule out staphylococcal infections in many clinical scenarios 2
• In skin and soft tissue infections, the visualization of Gram-positive cocci in clusters within abscess material strongly supports S. aureus as the causative pathogen 2

Understanding the histological appearance of S. aureus is crucial for rapid preliminary diagnosis and appropriate management of staphylococcal infections, particularly in settings where culture results may be delayed.