A 73-year-old woman with COPD and a 40-pack-year smoking history without immunosuppression was evaluated for a slowly enlarging right upper lobe mass and recurrent hemoptysis. In 2018, a spiculated lung nodule (24mm×36mm) was noted on chest CT. Bronchoalveolar lavage was positive for galactomannan, and culture grew Aspergillus fumigatus, for which she received voriconazole. The mass persisted, and interval imaging was planned; however, the patient was lost to follow-up.

She had chronic small-volume hemoptysis for several years. In December 2024, she presented with five days of dyspnea and chest tightness. She was afebrile with normal vital signs and unremarkable lung auscultation. Her WBC count was 8740cells/mm3, procalcitonin<0.10μg/L, and a normal basic metabolic panel. CT showed interval enlargement of the mass (38mm×37mm), and PET-CT showed intense FDG uptake, concerning for primary lung carcinoma. Endobronchial ultrasound-guided transbronchial needle aspiration and cryobiopsy were negative for malignancy, though cultures grew Peptostreptococcus species. A percutaneous CT-guided core needle biopsy performed was nondiagnostic.

Over the following months, the mass further enlarged (48mm×39mm) with worsening hemoptysis. Given progressive hemoptysis and concern for vascular involvement, she was referred for thoracic surgery evaluation and underwent right upper lobectomy with additional wedge resections. Intraoperatively, purulent fluid and dense scar tissue were noted. Empirical therapy with piperacillin–tazobactam and linezolid was initiated post-operatively.

Gross pathology revealed an abscess cavity. Cultures grew Parvimonas micra alone. H&E staining showed inflammation with cavitation. Gram stain identified G+ cocci and filamentous G+ organisms. GMS staining highlighted filamentous organisms. Targeted 16S/ITS next-generation sequencing identified>40,000 reads of P. micra and ∼3800 reads of Actinomyces israelii (A. israelii), without evidence of fungal invasion or malignancy. Broad-spectrum antibiotics were discontinued, and targeted amoxicillin–clavulanate was prescribed. Dental evaluation revealed no major abnormalities. Follow-up CT showed stable postoperative changes.

P. micra (formerly known as Peptostreptococcus micros and Micromonas micros) is an anaerobic, G+ coccus belonging to normal oral and gastrointestinal flora [1]. Although classically associated with periodontal disease, P. micra has increasingly been recognized in systemic infections [2–4]. Pulmonary infections caused by P. micra are rare and often present with non-specific features that pose diagnostic challenges.

Our systematic review (PubMed, Embase, Web of Science) identified 44 reported cases of pulmonary P. micra infection (Supplementary figure). None demonstrated a similarly prolonged indolent course. Most cases occurred in men, frequently involved the upper lobes, and were associated with aspiration, polymicrobial infection, and poor oral hygiene.

Fig. 1.

Timeline of the patient's medical history. Serial chest CT scans demonstrating progressive growth of the right upper lobe mass. (A) CT June 2018: a spiculated nodule measuring 24mm×36mm. (B) CT January 2025: the nodule enlarged to 38mm×37mm. (C) CT May 2025: further enlarged to 48mm×39mm. (D) PET-CT demonstrating intense FDG uptake mimicking carcinoma. A spiculated solid mass in the central right upper lobe measuring 3.8cm demonstrated intense FDG uptake, radiographically concerning for primary lung carcinoma. The lesion extended centrally along the right upper lobe bronchovascular bundle and crossed the right oblique fissure into the superior segment of the right lower lobe. A prominent right interlobar lymph node showed mild FDG uptake, suggestive of possible nodal involvement. No evidence of distant metastatic disease was identified. (E) Gross pathology specimen of the right upper lobe. (F) H & E stain showing the abscess contents (scale bar=1mm). (G) Gram stain showing G+ cocci (yellow arrows) and filamentous G+ bacteria (green arrows) (scale bar=20μm). (H) GMS stain showing black filamentous organisms within a green-counterstained tissue background (scale bar=20μm).

Although A. israelii was detected in this case, multiple features support P. micra as the primary pathogen in this case, including monomicrobial culture growth and a substantially higher sequencing read burden. The absence of sulfur granules, granulomatous inflammation, along with the low-level sequencing detection, suggests that A. israelii most likely represented a secondary co-pathogen or colonizer.

We conclude that this case demonstrates that chronic pulmonary infection caused by P. micra can closely mimic lung cancer over many years, expands the clinical spectrum of pulmonary P. micra infection, and highlights the diagnostic value of integrating pathology with NGS to avoid misdiagnosis and delays in management.