Schizophyllum commune is a species of fungus in the genus Schizophyllum, family Schizophyllaceae, order Agaricales, class Agaricomycetes, and phylum Basidiomycota. It is one of most widely distributed fungal species, often found on dead and decaying organic matter [1]. Since the first report of probable pathogenic behavior of S. commune, it has been rarely reported in medical literature in association with human diseases compared to other disease-causing fungi [2]. The most frequently infected regions are the sinus, sino-orbital, bronchopulmonary, and extra-pulmonary areas, ranging from allergic respiratory conditions to severe life-threatening brain lesions in both immunocompetent and immunocompromised hosts [3]. Here, we report a rare case of bronchiectasis with mucoid impaction caused by S. commune in an immunocompetent woman in South Korea.

A 52-year-old female patient was referred to our institution from a local clinic for further examination of suspected bronchitis. The patient had symptoms of cough, whitish sputum, and chest discomfort lasting approximately one month. The patient was a non-smoker and had no medical history of pulmonary tuberculosis, asthma, rhinitis, diabetes, hypertension, allergy, or gastroesophageal reflux disease. She was not receiving any medical treatments. Her occupation was a shoe vendor, and her family history included her brother’s lung cancer. The patient’s body temperature was 36.9°C, and her respiratory examination was normal, without wheez-ing. Upon laboratory investigation, white blood cell count was 5,490/µL with 373/µL eosinophils. Serum C-reactive protein levels were elevated to 1.35 (reference range <0.6), and total IgE was 216 kU/µL (<100). The culture and smear of a series of three sputum specimens and a serum aspergillus antibody test were all negative. Posteroanterior chest radiography revealed a mass-like consolidation in the right lower lobe. Mucoid impaction manifested as high-density beaded lesions in the right middle lobe (RML) and atelectasis of RML on pre-contrast axial CT scan. A bronchoscopic examination revealed thick yellowish mucoid impaction with total obstruction of the RML bronchus (Fig. 1).

Figure 1. (A) Postero-anterior X-ray revealed a mass-like consolidation in the right lower lung field (red arrow). (B) Mucoid impaction manifested as a high density on pre-contrast CT scan (yellow arrow). (C) The bronchoscopic examination showed thick yellowish mucoid impaction with total obstruction of the right middle lobe bronchus (blue arrow).

Histopathological examination with Gomori’s methenamine silver (GMS) staining on a prepared paraffin block revealed a few scattered degenerated fungal hyphae. Bronchoscopic aspirate specimens were plated as a fungal culture on Sabouraud dextrose agar and incubated at 36.5°C for 24 hours and then at 25°C for three days, showing dense, fine, soft, hair-like white mycelial growth, defined as fungal hyphae under GMS staining. Microscopic examination revealed several thin hyaline hyphal filaments without clamp connections or spicules (Fig. 2).

Figure 2. (A) Gomori’s methenamine silver (GMS) stain of a prepared paraffin block revealed fungal hyphae (GMS stain, ×200). (B) White, woolly colonies grew on Sabouraud dextrose agar after incubation of bronchoscopic aspirate specimens for 3 days. (C) GMS stain revealed several thin hyaline hyphal filaments without clamp connections or spicules on microscopic examination (GMS stain, ×400). (D) The red arrow indicates the narrower width hyphae, and the black arrow indicates the wider hyphae (lactophenol cotton blue stain, ×400).

We performed molecular identification by DNA amplification and sequencing analysis of the Internal Transcribed Spacer (ITS) gene to identify the fungus. The primer pair used for amplifying ITS sequencing was ITS1 (5′-TCC GTA GGT GAA CT GCG G-3′) and ITS4 (5′ TCC TCC GCT TAT TCA TAT GC-3′). We used the basic local alignment search tool (BLAST) of the GenBank (http://blast.ncbi.nlm.nih.gov/) database to analyze the sequencing results. Comparative analysis of the partial ITS sequence demonstrated 100% identity with S. commune (access number MK029865.1).

The patient was diagnosed with bronchiectasis with mucoid impaction caused by S. commune fungal infection. The patient was given oral corticosteroids at 15 mg and 10 mg twice daily and the antifungal agent itraconazole 200 mg twice daily for 4 weeks. Her symptoms of cough, whitish sputum, and chest discomfort resolved after one month of treatment. The patient recovered fully without any complications.

The rate of human infections caused by S. commune has been increasing recently [3]. Several cases have been reported in immunocompromised and immunocompetent patients [4-16]. We reviewed published reports of S. commune-induced invasive infections in immunocompetent patients (Table 1). Even in immunocompetent patients, S. commune induced serious inflammation such as pneumonia or epidural abscess [4, 5]. For example, in the US, an immunocompetent 58-year-old man died of pneumonia and brain abscess caused by S. commune [4]. In another three case reports in France, S. commune caused invasive rhinosinusitis in immunocompetent patients [6]. Globally, S. commune infection in a healthy human without an underlying disease is rare [2]. Therefore, when S. commune is isolated from an immunocompetent patient with mild symptoms, it might be considered a contaminant microorganism instead of suspected as a potential pathogen. Culture positivity alone is insufficient as a diagnostic method for fungal infection, and it is necessary to determine whe-ther the sample is pathogenic or a contaminant. Furthermore, S. commune isolated from human specimens typically exhibits no distinctive microscopic morphological characteristics of its dikaryotic stage, such as spicules or clamp connections on hyphae [7]. We identified the corresponding sequences of S. commune by molecular genetic analysis of the ITS region. In our case, specimens were obtained using sterile techniques, directly GMS-stained fungal hyphae were found, and antifungal therapy was effective, so the possibility of contamination was excluded. A limitation of our study is that we could not conduct an antifungal susceptibility test for our case. In general, S. commune infections are easily manageable, and the mean minimum inhibitory concentrations observed for itraconazole remain low [8, 17].

Table 1 . Literature review of Schizophyllum commune invasive infection in immunocompetent patients

No.	Sex/Age (yr)	Diagnosis	Confirmation method	Treatment	Outcome	Country, Year [Reference]
1	M/58	Several lung and brain abscesses	Histologic finding	Amphotericin B deoxycholate and itraconazole	Died	The USA, 1996 [4]
2	M/53	Epidural abscess	Sequence analysis	Vancomycin, meropenem, and liposomal amphotericin B	Cured	Japan, 2018 [5]
3, 4, 5	F/45, F/37, F/30	Rhinosinusitis with mucosal and/or bone invasion	MALDI-TOF MASS spectrometry and sequence analysis	Surgery and voriconazole	Cured	France, 2015 [6]
6	M/56	Bronchogenic cyst	Sequence analysis	Surgery	Cured	Serbia, 2006 [7]
7	F/30	Sino-orbital infection	Sequence analysis	Amphotericin B deoxycholate and voriconazole	Cured	Korea, 2012 [11]
8	F/54	Mucoid impaction of the bronchi	Histologic finding	Itraconazole	Cured	Japan, 2007 [14]
9	F/50	Sino-orbital infection	Cycloheximide susceptibility (400 µg/mL) and tolerance to benomyl (10 µg/mL)	Surgery and amphotericin B deoxycholate	Cured	India, 2020 [15]
10	F/59	Pulmonary mycosis	Sequence analysis	Voriconazole and pulmicort inhalation	Cured	China, 2016 [16]

In Korea, several cases of S. commune infections in immunocompromised patients have been reported [8, 9]. However, there have only been two case reports in immunocompetent patients, including allergic fungal sinusitis and sino-orbital infection caused by S. commune [10, 11]. Here, we report a rare case of bronchiectasis with mucoid impaction caused by S. commune in an immunocompetent woman in South Korea. This report shows that S. commune can cause serious infections even in immunocompetent patients.

None declared.