Arthritis caused by Legionella micdadei and Staphylococcus aureus: metagenomic next-generation sequencing provides a rapid and accurate access to diagnosis and surveillance

Legionella spp. is an important pulmonary pathogen but rarely causes extra-pulmonary infections. We report a case of joint infection caused by Legionella micdadei and Staphylococcus aureus in a 54-year-old male with medication history of oral steroid for systemic lupus erythematosus (SLE). He developed arthritis in his right metacarpophalangeal (MCP) joints without precursor pneumonia. In the joint aspirate, S. aureus was detected through culture. The existence of L. micdadei and S. aureus were indicated by metagenomic next-generation sequencing (mNGS) and confirmed by 16S rRNA sequence analysis. After oral levofloxacin treatment for 54 days, the patient's symptoms ameliorated and blood test results improved, which were consistent with the dynamic trend of reads numbers in mNGS data. Our case included, arthritis caused by Legionella spp. have been reported in 11 patients. However, our case is the first to report septic arthritis caused by L. micdadei in native joints and monitored by mNGS. This case demonstrated an application of mNGS for etiological diagnosis and semi-quantification in joint aspirate. mNGS may serve as a promising tool for rapid and accurate etiological diagnosis and surveillance, contributing to appropriate antimicrobial drug applications and timely medication adjustments when necessary.