Medicine – Bacteria Associated with Septic Arthritis

Septic arthritis is an infection of a synovial joint caused by invasion of the joint space by a pathogenic microorganism, most commonly bacteria. It is a medical emergency because rapid inflammation can cause irreversible destruction of articular cartilage and subchondral bone. Patients typically present with an acutely painful, swollen, warm joint with markedly restricted movement, sometimes accompanied by fever and systemic illness. The causative organism varies according to age, immune status, underlying disease, and route of infection.

1. Staphylococcus aureus

Staphylococcus aureus is the most common cause of native-joint septic arthritis in adults. Infection usually reaches the joint through haematogenous spread, although direct inoculation following trauma, surgery, or joint procedures can also occur. Risk is increased in patients with diabetes, immunosuppression, intravenous drug use, prosthetic joints, or pre-existing joint disease such as rheumatoid arthritis.

Methicillin-resistant S. aureus (MRSA) should also be considered when appropriate risk factors are present. Because S. aureus can rapidly destroy cartilage, early joint drainage and appropriate antimicrobial therapy are essential.

2. Streptococcus pyogenes

Streptococcus pyogenes, or group A streptococcus, is another recognised cause of septic arthritis. Joint infection may develop following bacteraemia originating from skin, soft-tissue, or other streptococcal infections. The disease can progress rapidly and may be associated with significant systemic toxicity.

3. Streptococcus pneumoniae

Streptococcus pneumoniae is a less common but important cause of septic arthritis. It may occur following pneumococcal bacteraemia associated with infections such as pneumonia or meningitis. Patients with impaired immunity, advanced age, or asplenia may be particularly vulnerable to invasive pneumococcal disease.

4. Mycobacterium tuberculosis

Mycobacterium tuberculosis produces a more chronic form of infectious arthritis rather than the typical rapidly progressive presentation of acute bacterial septic arthritis. Tuberculous arthritis usually develops through haematogenous dissemination from another site of infection and commonly affects a single large weight-bearing joint, particularly the hip or knee.

Symptoms tend to develop gradually, with persistent pain, swelling, stiffness, and progressive loss of joint function. Constitutional manifestations such as fever, night sweats, or weight loss may be absent. Untreated disease can eventually cause extensive joint destruction.

5. Neisseria gonorrhoeae

Neisseria gonorrhoeae is an important cause of infectious arthritis, particularly in sexually active adolescents and younger adults. Disseminated gonococcal infection may initially produce a characteristic combination of migratory polyarthralgia, tenosynovitis, and pustular skin lesions.

Some patients subsequently develop a more localised purulent monoarthritis, frequently involving the knee, wrist, ankle, or elbow. A sexual history and appropriate testing from genital and other relevant mucosal sites are important because cultures obtained directly from the affected joint may sometimes be negative.

6. Salmonella Species

Salmonella species are particularly associated with septic arthritis and osteomyelitis in patients with sickle cell disease or other haemoglobinopathies, although infection can occur in other immunocompromised individuals. Bacteraemia can allow the organism to seed a joint and produce an acute inflammatory arthritis.

Diagnosis and Clinical Importance

When septic arthritis is suspected, urgent arthrocentesis and synovial fluid examination are central to diagnosis. Synovial fluid should generally undergo cell count, Gram staining, culture, and other appropriate microbiological investigations. Blood cultures are also important, particularly when bacteraemia is suspected.

Treatment usually requires prompt antimicrobial therapy together with drainage of the infected joint. Whenever clinically feasible, synovial fluid and blood cultures should be obtained before antibiotics are started, but treatment should not be unnecessarily delayed in a severely ill patient.

Key Clinical Pattern

The major bacterial causes to remember are Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumoniae, Neisseria gonorrhoeae, and Salmonella species, while Mycobacterium tuberculosis is an important cause of chronic infectious arthritis. Among these, S. aureus is the most important and common cause of acute native-joint septic arthritis in adults.