Bacillus megaterium infection presenting as pulmonary alveolar proteinosis, a case report | BMC Infectious Diseases

To our knowledge, this is the first PAP in adults after B. megaterium Infection has been described in the medical literature. Previous literature reported few cases of B. megaterium Infection at specific sites, including the cornea, soft tissue of the skin, brain and pleura [1]Most cases occurred in immunocompromised patients, triggered by surgery or trauma. All patients were completely cured with appropriate antimicrobial treatment.

PAP is a diffuse lung disease characterized by the accumulation of lipoproteinaceous material in the alveoli and dysfunction of alveolar macrophages. It occurs in approximately 7 out of a million people in the general population. PAP can be classified into several types depending on the pathogenetic mechanism. Primary PAP is caused by the disruption of GM-CSF signaling and can be further divided into an autoimmune and a hereditary form. Secondary PAP results from various underlying diseases, including malignancies and infections. Congenital PAP is caused by mutations in genes involved in surfactant production.

Common symptoms of PAP include shortness of breath, cough, and chest pain, while up to one-third of patients remain asymptomatic. Rals may occur but are not always present. Antibodies to GM-CSF are detected in both serum and BALF in almost all autoimmune PAP. Other possible serological markers, including LDH, CEA, and Cyfra21-1, have been studied as surrogates for disease severity and progression, but clinical availability, sensitivity, and specificity are unknown. PFT typically reveals restrictive impairment and a reduction in diffusion capacity. High-resolution CT usually shows a “crazy paving” pattern characterized by diffuse, ground-glass opacities with septal thickening and subpleural recession. BALF shows an opaque, milky appearance. Histological evaluation shows granular eosinophilic material filling the alveolar spaces and reacting positively to PAS staining. The diagnosis of PAP is made by specific CT scans, BALF and histological patterns. For the treatment of PAP, in addition to oxygen therapy and smoking cessation, some patients with mild symptoms may experience spontaneous remission. Inhaled GM-CSF therapy has been particularly targeted at autoimmune PAP. In patients with secondary causes, treatment of the underlying diseases should be considered. Pulmonary lavage (WLL) is the cornerstone of PAP therapy when patients suffer from severe hypoxemia or an intrapulmonary shunt and remission has not been achieved by the above treatments. [2, 3].

In the setting of chronic infection, treating the infection alone may improve the patient's condition. Patients with PAP also have an increased risk of secondary infection with common and opportunistic pathogens. Due to the slightly elevated GM-CSF antibody, this could be a case of autoimmune PAP with secondary infection. A previous study showed that the commonly reported opportunistic infections in patients with PAP included nocardiosis, mycobacteria, and fungi [4]. B. megaterium Infections have been reported rarely. Antimicrobial or antifungal therapy should be considered along with WLL. In both cases, once infection is confirmed, antimicrobial therapy is recommended and improvement is expected.

In summary, PAP should be considered in apparently immunocompetent patients who have an opportunistic infection, such as B. megaterium. PAP may be a focal histological finding that cannot always be detected by BAL or transbronchial biopsy. Apart from mNGS, traditional methods such as percutaneous needle biopsy or surgical biopsies should not be neglected.