Identification of Aerococcus viridans in granulomatous mastitis utilizing nanopore targeted sequencing: case report and short literature review

<h4>Background</h4>Granulomatous lobular mastitis (GLM) is a benign, nonspecific inflammatory disease associated with autoimmune dysregulation, pathogenic infection, endocrine disorders, and other etiological factors. Although various treatments for GLM exist, there is currently no standardized approach. <i>Aerococcus viridans</i> (<i>A. viridans</i>), an opportunistic pathogen typically non-pathogenic to humans, has been reported to cause infection in immunocompromised hosts. To date, there have been no documented cases of female granulomatous mastitis associated with <i>A. viridans</i>.<h4>Case description</h4>A 33-year-old female presented with a right-breast mass. Ultrasound revealed multiple heterogeneous masses consistent with chronic GLM. Diseased glandular tissue was obtained under aseptic conditions for pathological examination and nanopore targeted sequencing (NTS). Pathology showed granulomatous inflammation, and NTS detected <i>A. viridans</i> as the dominant taxon.<h4>Conclusions</h4>Granulomatous mastitis is a heterogeneous inflammatory condition in which pathogenic infection may contribute to disease development, although the spectrum of implicated microorganisms remains incompletely defined. <i>Aerococcus viridans</i> is rarely reported in this setting, and its detection in the present case broadens the range of microorganisms associated with GLM. The dominant detection of <i>A. viridans</i> by nanopore-targeted sequencing (NTS) indicates a potential association with the lesion; however, causality cannot be inferred from a single case. NTS is a valuable tool for rapid, accurate pathogen identification, supporting timely, rational antimicrobial decision-making. In this case, NTS-guided antibiotic therapy was followed by significant clinical improvement.