Primary Axillary Actinomycosis: A Case Report on the Integration of Culture and Molecular Diagnostics for Accurate Diagnosis of Polymicrobial Infections

Actinomycosis is a chronic suppurative granulomatous disease caused by <i>Actinomyces</i> spp. Although cutaneous actinomycosis is rare, dermatologists must consider it due to its potential coexistence with other pathogens, often as part of polymicrobial infections. We present a rare case of primary axillary cutaneous actinomycosis in a young woman, likely triggered by cosmetic axillary hair removal and home shaving practices. Histological examination revealed characteristics of actinomycosis, including sulfur granules and Gram-positive filamentous structures. Bacterial cultures failed to isolate <i>Actinomyces</i>, but identified <i>Staphylococcus epidermidis</i>, <i>S. aureus</i> (MRSA), and <i>Corynebacterium simulans</i>, suggesting a polymicrobial infection contributing to the inflammatory response. Molecular analysis of DNA extracted from formalin-fixed paraffin-embedded (FFPE) tissue yielded a 675 bp PCR product using <i>Actinomyces</i>-specific primers. BLAST analysis confirmed the species as <i>A. gerencseriae</i>, establishing the diagnosis of actinomycosis. However, a 1000 bp PCR product obtained using universal 16S rDNA primers could not be sequenced successfully, likely due to the presence of multiple bacterial species. Notably, <i>Actinomyces</i> was detected only through molecular methods, while bacterial cultures identified the aforementioned bacteria. This discrepancy between FFPE-PCR results and bacterial culture findings demonstrates a key challenge in the microbiological diagnosis of polymicrobial infections. This case highlights the importance of integrating histopathological, microbiological, and molecular techniques for accurate pathogen identification in polymicrobial infections. The failure to detect <i>Actinomyces</i> in bacterial cultures, despite its presence in FFPE-PCR, suggests that conventional culture methods alone may be insufficient for diagnosing such infections. Extended culture durations, selective anaerobic culture techniques, and molecular diagnostic methods are essential for a comprehensive evaluation. Recognizing <i>Actinomyces</i> as more than a contaminant is important for timely diagnosis and effective treatment. Increased awareness of its potential involvement in polymicrobial infection should improve clinical outcomes.