Histological Identification of Staphylococcus aureus

Key Morphological Features

The characteristic morphology of S. aureus, as identified by the Infectious Diseases Society of America, is a spherical (cocci) bacterium that forms distinctive grape-like clusters when viewed under microscopy, which is a defining characteristic used for identification 1
The clustering pattern of S. aureus is distinctive from streptococci, which typically arrange in chains rather than clusters, as noted by the American Society for Microbiology 2

In respiratory samples, the presence of Gram-positive cocci in clusters has a reported sensitivity of 68% (95% CI 49-83%) and specificity of 95% (95% CI 86-98%) for identifying S. aureus, according to the European Society of Clinical Microbiology and Infectious Diseases 3
The high specificity of identifying these grape-like clusters on Gram stain makes it a valuable tool for preliminary identification of S. aureus infections, as recommended by the Clinical and Laboratory Standards Institute 4

In skin and soft tissue infections, S. aureus often induces abscess formation characterized by a peripheral rim of inflammatory cells, as described by the Infectious Diseases Society of America 5
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, according to the American Academy of Dermatology 5

The morphological evaluation of S. aureus in Gram staining has high diagnostic value with a diagnostic odds ratio of 38 (95% CI 13-106), as reported by the European Society of Clinical Microbiology and Infectious Diseases 6
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, according to the American Society for Microbiology 2
Histological identification may be affected by prior antibiotic treatment, which can alter the typical appearance of bacterial colonies, as noted by the Clinical and Laboratory Standards Institute 7

Rapid identification of S. aureus through its characteristic grape-like clustering pattern can guide early appropriate antibiotic therapy, particularly important in severe infections, as recommended by the Infectious Diseases Society of America 7
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, according to the European Society of Clinical Microbiology and Infectious Diseases 7
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, as described by the World Society of Emergency Surgery 8