Septic arthritis is an inflammatory joint disease produced by invasion and multiplication of pathogenic microorganisms. Most cases of septic arthritis are caused by microorganisms of the genus Staphylococcus and Streptococcus. Non-fermenting Gram-negative bacilli (Pseudomonas, Stenotrophomonas, Acinetobacter, and Burkholderia) are a frequent causes of nosocomial infection associated with immunosuppression situations, and Sphingomonas paucimobilis (S. paucimobilis) (formerly Pseudomonas paucimobilis) is also a Gram-negative aerobic fermentor that now emerges as an opportunistic pathogen.

We present a case of septic arthritis by S. paucimobilis in a 41-year-old man with a history of frequent episodes of hyperuricemia and gout in his left knee. In the past year, he presented several episodes of acute monoarthritis treated with nonsteroidal anti-inflammatory drugs, colchicine, and intraarticular infiltration of triamcinolone. The patient presented with symptoms of pain and swelling of left knee which had lasted for 3 months without fever. Physical examination showed: temperature 36.5°C and left knee arthritis with preserved but painful active and passive mobility. Arthrocentesis was performed, resulting in inflammatory synovial fluid, without evidence microcrystals under the polarized light microscope. The fluid was sent to the microbiology department in a sterile syringe and blood culture bottles for aerobic and anaerobic culture.

Gram stain showed abundant polymorphonuclear leukocytes and intracellular Gram-negative bacilli, and the culture-negative bacilli isolated were identified as S. paucimobilis by ID32GN Api (bioMerieux, Marcy L’etoile 3. France), sensitive to beta-lactams, aminoglycosides, quinolones, and cotrimoxazole. Laboratory analysis upon the patient's admission showed 8070 leukocytes/L (70% neutrophils and 30% lymphocytes), erythrocyte sedimentation rate of 42mm the first hour and CRP 8.04mg/dl. CBC, coagulation, and biochemistry were normal. Echocardiogram was normal and X-rays showed a slight increase in soft tissue suprarrotulian density, indicative of effusion; the CT with intravenous contrast observed loosening of articular recesses, with slight enhancement of the synovium.

Daily articular drainage was performed and the patient was treated with ceftazidime (6g/24h) plus gentamicin (240mg/24h), with clinical improvement, so gentamicin was discontinued after 8 days of treatment, continuing only with ceftazidime. After 15 days and with several negative cultures we reisolated S. paucimobilis, so treatment was changed to meropenem and ceftazidime (4g/24h/7 days) to complete 21 days of intravenous antibiotic therapy in total. The patient underwent surgical treatment consisting of joint lavage and synovectomy. After 22 days of treatment, it was substituted to oral therapy with ciprofloxacin (1.5g/24h) plus trimethoprim-sulfamethoxazole (800/160mg/12h) for 3 months.

S. paucimobilis is isolated in nature from soil and water,1 and in hospital settings from distilled water equipment, dialysis fluids, nebulizers, and other instruments used for respiratory therapy.2–4 It can cause a variety of nosocomial infections such as community acquired pneumonia, bacteremia, catheter-related infection, osteomyelitis, septic arthritis, meningitis, peritonitis, postoperative endophthalmitis, pleural empyema, and infections of the urinary tract, and bile ducts. These infections are manifested mainly in patients with some form of immunosuppression, patients undergoing an invasive procedure or patients with peritoneal5–7 dialysis catheters. The microorganism has not demonstrated a high degree of virulence and no cases of death from infection due to S. paucimobilis are reported. Its low pathogenicity is due to the lack of lipopolysaccharide in the outer membrane of the Gram-negative cell wall, which is associated with endotoxic activity, and this would explain the good condition of the patient despite successive positive cultures and the prognosis of most cases of infection described by S. paucimobilis.8 There was no knowing what was the origin of the infection by S. paucimobilis was. It is possible that the patient was colonized and transient bacteremia allowed the organism to reach the joint, or that the patient became infected due to an improper procedure (direct inoculation) in one of the infiltrations to which he was subjected.

Particularly striking is the extreme difficulty in eradicating S. paucimobilis, despite antibiotic therapy and adequate daily articular drainage. Although there are only two cases of arthritis described by S. paucimobilis, both in immunocompromized patients,9,10 it seems necessary to monitor infections caused by opportunistic pathogens in immunocompetent patients, if only because of the high hospital cost involved.