Diagnosis of Pulmonary Actinomycosis Caused by Actinomyces graevenitzii: a Case Series of Three Patients.

Background: Actinomyces graevenitzii is a relatively uncommon Actinomyces species, which is an oral species and predominantly recovered from respiratory locations [1,2]. It is a gram-positive anaerobic bacteria or microaerobic filamentation bacteria, which can induce pyogenic and granulomatous inflammation characterized by swelling and concomitant pus, sinus formation, and the formation of yellow sulfur granules. All tissues and organs can be infected; the most common type involves the neck and face (55%), followed by the abdominal and pelvic cavities (20%). Chest involvement is the third most common type (15%), affecting the lung parenchyma, central airway, pleura, mediastinum, and chest wall [3-5]. Diagnosis of actinomycosis mainly relies on sputum, pus or biopsy specimens to find actinomycetes. Metagenomics next-generation sequencing (mNGS) in recent years has been increasingly valued and recognized for its application in infectious diseases. It provides a more efficient and accurate means for the pathological diagnosis of respiratory infections, updating the diagnostic strategy for lower respiratory tract infections [6].

Methods: This study systematically summarized the clinical characteristics of Actinomyces graevenitzii infection by analyzing three cases of this pathogen. Bronchoalveolar lavage fluid (BAL) samples were collected for bacterial culture and mNGS. The isolated strains were routinely identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS).

Results: Microbiological analysis demonstrated Actinomyces graevenitzii growth in all three bronchoalveolar la-vage (BAL) cultures. Metagenomic next-generation sequencing (mNGS) results showed concordant detection of A. graevenitzii in two cases, while unexpectedly identifying Tropheryma whipplei as the predominant pathogen in the remaining case.

Conclusions: Actinomycosis infections often present with subtle and nonspecific clinical manifestations, making them difficult to distinguish from pulmonary tuberculosis, fungal infections, and lung malignancies through imaging studies alone, frequently resulting in misdiagnosis or delayed diagnosis. Early and accurate diagnosis can be achieved through timely analysis of BAL fluid using advanced diagnostic techniques, including MALDI-TOF/MS for bacterial identification and mNGS, facilitating prompt and appropriate treatment.