A 45-year-old man presented at the emergency department of our tertiary teaching hospital with a septic arthritis of the left hip. Four weeks before presentation the hip became painful. Since then the pain worsened and the patient became febrile. He did not suffer from night sweats.
The patient had no significant medical history. He originated from former Yugoslavia, where he spent the holidays once or twice a year. His occupation was supervisor in a metal recycling company in the harbour of Rotterdam.
On physical examination the hip was mildly swollen and painful at palpation. Laboratory investigation showed an increased C reactive protein (CRP) of 234 mg l-1 (normal <10) with a normal leukocyte count. Magnetic resonance imaging (MRI) of the hip was performed, which showed a high-intensity signal and a lot of intra-articular fluid at the swollen side of the hip. With an ultrasound guided biopsy synovial fluid was aspirated and sent to the microbiology department.
The working hypothesis was an ordinary septic arthritis. However, a gram stain of the synovial fluid did not show any micro-organisms. Antibiotic treatment was started immediately with intravenous cefuroxime and one dose of gentamicin, according to local antibiotic guidelines. After starting antibiotic treatment the fever and increased CRP persisted. Therefore, a chest radiograph was taken, which showed bilateral, perihilar infiltrates consisting of several small nodules. A chest computed tomography (CT) showed perihilar diffuse nodular infiltrates (Fig. 1).
With a differential diagnosis of pulmonary and extrapulmonary tuberculosis, the hip synovial fluid was stained with auramine-rhodamine and then acid-fast bacilli could be visualized. Next, GeneXpert MTB/Rif (Cepheid, Sunnyvale, CA, USA), a molecular test based on nucleic acid amplification, confirmed  M. tuberculosis  complex in the sample from the synovial fluid. The normal aerobic and anaerobic cultures were negative. The mycobacterial culture, using our BD BACTEC MGIT 960 mycobacteria culture system (BD Diagnostics, Sparks, MD, USA) was negative after 6 weeks of culture. Given the positive GeneXpert and no history of pre-treatment, it was decided to extend the culture period for another 6 weeks. After a total of 75 days of culture, the mycobacterial culture became positive (Fig. 2). In order to differentiate within the  M. tuberculosis  complex we used the GenoType MTBC molecular genetic assay (HAIN Lifescience, Nehren, Germany) and identified the strain as an  M. microti . Additionally, the GenoType MTBDRplus (HAIN Lifescience) was performed in order to identify molecular genetic resistance towards isoniazid (INH) and rifampicin (RMP). This hybridization assay showed no mutations in the rpoB gene (associated with RMP susceptibility) and no mutations in the katG or inhA genes (associated with INH susceptibility).
The sputum of the patient was also tested for mycobacteria, but turned out to be negative by auramine-rhodamine staining, GeneXpert and (mycobacterial) culture. Eventually, broncho-alveolar lavage was performed, but with the previous three tests  M. microti  could not be detected.
The  M. microti  infection was treated with INH, RMP, pyrazinamide and ethambutol according to the World Health Organization recommended dosage. Initially the patient recovered and the fever and pain in the hip disappeared. Therefore, the pyrazinamide and ethambutol were discontinued after 2 months. However, shortly thereafter the former complaints reoccurred and the pyrazinamide and ethambutol were restarted. A CT scan of the hip revealed bone destruction at the anterior side and multiple abscesses in the adjacent muscles (Fig. 3). A radiological drainage of these abscesses was performed. Therapeutic drug monitoring of RMP and INH showed that drug levels of both drugs were subtherapeutic. Hereafter, the dose of INH was increased to 900 mg per day and RMP to 1800 mg per day. Shortly after this treatment modification the patient got complaints of polyneuropathy. Therefore, the INH was discontinued. The other drugs were continued and eventually, the patient improved clinically.
Chest computed tomography, showing perihilar consolidations at both sides with underlying bronchiectasis. In the periphery of the lungs smaller nodular infiltrates are visible. These are indicated by the arrows.