A previously healthy 10-year-old boy presented to our department with a 5-day history of intermittent, left-sided chest pain exacerbated by deep breathing. He denied any current fever, cough, hemoptysis, nausea, vomiting, diarrhea, palpitations, night sweats, or weight loss. Additionally, he also denied previous surgery, trauma, sick contacts, dental work, aspiration episodes, and exposure to hazardous and/or infectious materials. He had no remarkable medical history and had received all scheduled vaccines, including BCG, without any serious adverse events. There was no family history of asthma, diabetes, immunodeficiency, malignancy, and tuberculosis. On day 3 of chest pain, he sought treatment at a local hospital. Chest radiography demonstrated left lobe pneumonia (Figure 1A), while blood examination revealed a white blood cell count (WBC) of 10.4x 109/L with 58.5% neutrophils and C-reactive protein levels < 0.50 mg/L. Thus, he was initially treated for community-acquired pneumonia with ceftriaxone and azithromycin for 2 days without relief. A subsequent plane computed tomography (CT) scan of the chest disclosed a round soft tissue mass and a small amount of pleural effusion in the left lung (Figures 1B,C). Then, the patient was transferred to our hospital for further evaluation and treatment at the request of his parents.
His vital signs upon arriving to us were as follows: body temperature, 37.1 C; pulse rate, 92/min; respiratory rate, 28/min; and blood pressure, 113/67 mmHg. Physical examination showed no abnormalities. Blood routine and biochemical tests were all within normal ranges. Empiric antibiotics with azithromycin and amoxicillin/clavulanate were given. The HIV test, viral hepatitis serologies, antinuclear antibody, serum parasite IgG, serum galactomannan, serum 1, 3-beta-D-glucan, serum Cryptococcus antigen test, T-spot test, oncological biomarkers, Legionella antigen, Mycoplasma pneumoniae antigen, and respiratory virus antigen were all negative. Due to concern of endocarditis and other occult sources of infection, echocardiogram and abdominal ultrasonography (US) were also ordered and were unremarkable. The patient underwent conventional flexible bronchoscopy and bronchoalveolar lavage (BAL). The utility of bronchoscopy revealed no unsuspected endobronchial lesions. BAL fluid (BALF) yielded only benign bronchial epithelial cells and macrophages, with no bacterial, viral, or fungal inclusions. The initial blood, sputum, and BALF specimens were all negative on both Gram stain and culture. These results yield no clear etiology for the formation of the nodule. However, his symptoms persisted. On hospital day 5, a chest X-ray revealed a left lung nodule measuring 2 cm in diameter (Figure 1D), which was similar to the previous. The thoracic US showed a hypoechoic lesion, 2.2 cm x 1.9 cm x 1.6 cm. This was confirmed by a contrast-enhanced CT scan revealing a rim-enhancing lesion in the left lower lobe lung (Figures 1E,F). The pulmonary abscess was highly suspected based on these findings. He was then sent for a US-guided lung biopsy on hospital day 7. Metagenomic next-generation sequencing (mNGS) showed 416 sequence reads of S. intermedius in lung biopsy tissue. Consistently, 5 days later, the culture of the specimen yielded pure growth of S. intermedius, with the pathological evaluation of the mass demonstrating fibroblast proliferation, no evidence of malignancy, lymphoma, or soft tissue tumor. The microorganism isolated from the lesion was susceptible to penicillin G, ceftriaxone, vancomycin, linezolid, levofloxacin, and chloramphenicol. Antibiotics were then narrowed to amoxicillin/clavulanate. However, following the biopsy, he had an intermittent slight fever since hospital day 7. A repeated chest X-ray showed partial resolution of the mass (Figure 2A) on hospital day 8, but an ultrasound revealed pleural effusion on hospital day 11. Considering the unsatisfactory effectiveness of amoxicillin/clavulanate, his antibiotic regimen was switched to linezolid on day 11. A thoracentesis was performed evacuating 190 ml of reddish-brown purulent pleural fluid (Figure 2B). Pleural fluid glucose was 6.07 mmol/L, protein 54.4 g/L, lactic dehydrogenase (LDH) 240 U/L, triglyceride 0.42 mmol/L, adenosine deaminase (ADA) 16.2 U/L, and white blood cell count 6,836.0 x106/L (64.0% polymorphonuclear cells and 36.0% mononuclear cells). Accordingly, pleural fluid analysis with mNGS reported 110 sequence reads of S. intermedius. The pleural fluid culture was negative. Clinically, his fever subsided on hospital day 13, and the patient was also much improved. A repeat chest scan confirmed the obvious resolution of the lung nodule and disappearance of the pleural effusion on hospital day 20 (Figures 2C,D). The patient was then discharged home with the plan to finish a 4-week course of linezolid and continued to be asymptomatic. Follow-up CT after 3 months showed nodule resolution without any residual lesion (Figures 2E,F).