Bacterial infection is the most common cause of septic arthritis

Septic arthritis — an infection inside a joint — is caused by bacteria in the vast majority of cases. The most frequent culprit is Staphylococcus aureus, a bacterium that lives on skin and can enter the joint through an injury, surgery, or bloodstream infection. Other common bacteria include Streptococcus species and, in some older adults, gram-negative organisms. Fungal and viral infections of joints are rare by comparison.

The reason bacteria cause most joint infections is straightforward: joints are enclosed spaces with fluid inside, and once bacteria reach that fluid, they multiply quickly in an environment with few defenses. A single bacterium entering through a cut, puncture wound, or surgical procedure can trigger a serious infection within hours or days.

Older adults face higher risk because immune function declines with age, skin becomes more fragile, and chronic conditions like diabetes or rheumatoid arthritis make infection more likely. If you have had a recent joint injection, surgery, or puncture wound and develop sudden joint pain, swelling, warmth, or fever, medical attention is urgent — the longer bacteria sit in a joint, the more damage they cause.

Key Takeaways

  • Staphylococcus aureus causes the majority of septic arthritis cases, followed by other common skin and respiratory bacteria.
  • Bacteria enter joints through puncture wounds, surgical procedures, injections, or travel through the bloodstream from infection elsewhere in the body.
  • Older adults are at higher risk because of weaker immune response, thinner skin, and conditions that lower infection resistance.
  • Sudden joint pain, swelling, warmth, and fever — especially after an injury or procedure — are warning signs that require same-day medical evaluation.
  • Early treatment with antibiotics and sometimes drainage of the infected fluid can prevent permanent joint damage.

How bacteria reach the joint

Bacteria can enter a joint in three main ways. The most direct route is through the skin — a puncture wound from a needle, thorn, or bite, or a cut that reaches deep enough to breach the joint capsule. This is why stepping on a nail or being pricked by a contaminated needle carries real risk.

The second route is surgery or medical procedures. Any time a surgeon or physician enters the joint space — whether for arthroscopy, injection, or repair — there is a window for bacteria to slip in. This is why sterile technique matters so much, and why infection rates after joint surgery, though low, are not zero.

The third route is the bloodstream. If you have a bacterial infection elsewhere — a urinary tract infection, pneumonia, skin abscess, or infected wound — bacteria can travel through your blood and lodge in a joint. This is more common in people with weakened immune systems, those on dialysis, or those with prosthetic joints.

Why Staphylococcus aureus is the top culprit

Staphylococcus aureus accounts for roughly half of all bacterial joint infections. It lives naturally on human skin and in the nose, so it is everywhere — on your hands, under your fingernails, on surfaces you touch. When you have a break in the skin, this bacterium is often the first to colonize the wound.

Staph is also aggressive. It produces toxins and enzymes that help it invade tissue and resist immune attack. Some strains, including methicillin-resistant Staphylococcus aureus (MRSA), are harder to treat with standard antibiotics, which is why doctors often start with broad-spectrum coverage while waiting for culture results.

The second most common cause is Streptococcus pyogenes (Group A Strep), which usually enters through a skin wound or follows a respiratory infection. In older adults, gram-negative bacteria like E. coli and Pseudomonas are seen more often, particularly in people with urinary catheters or recent urinary procedures.

Risk factors that make septic arthritis more likely

Age itself is a risk factor. Immune function declines gradually after age 65, so older adults clear infections less efficiently. Skin becomes thinner and more prone to tears, and reflexes slow — increasing the chance of falls and puncture wounds.

Certain medical conditions raise risk sharply. Rheumatoid arthritis and other autoimmune diseases damage joints and suppress immunity. Diabetes makes wounds heal slowly and reduces immune response. Chronic kidney disease, liver disease, and HIV all weaken infection-fighting ability. If you take medications that suppress immunity — including some arthritis drugs — your risk is higher.

Recent procedures or injuries are major risk factors. Joint injections (for steroid treatment or hyaluronic acid), arthroscopic surgery, joint replacement, and even dental work can introduce bacteria. Puncture wounds, animal bites, and cuts that penetrate deep into tissue are obvious risks. Intravenous drug use carries extremely high risk because needles bypass skin defenses entirely.

Prosthetic joints — artificial hips, knees, or shoulders — are vulnerable to infection for months or even years after implant. Bacteria can form a biofilm on the prosthetic surface that antibiotics struggle to penetrate, sometimes requiring removal and replacement of the joint.

Symptoms that warrant when ready medical attention

Septic arthritis usually announces itself suddenly. You may wake with a joint that is swollen, hot to the touch, and extremely painful — often so painful that you cannot bear weight or move the joint. The knee is the most common site, followed by the hip, shoulder, and ankle, though any joint can be infected.

Fever often accompanies joint pain, though not always — some people have infection without fever, especially if they are older or immunocompromised. You may feel generally unwell, with chills or fatigue. Some people notice redness or warmth over the joint, or fluid draining from a wound near it.

The speed matters. Septic arthritis can cause permanent cartilage damage within days if untreated. If you have sudden joint pain and swelling — especially after a recent injury, procedure, or injection — or if you have fever plus joint symptoms, seek medical evaluation the same day. Do not wait to see if it improves on its own.

How doctors identify the bacteria

Diagnosis starts with imaging and blood tests, but the definitive step is joint aspiration — a needle inserted into the joint to withdraw fluid. This fluid is sent to the lab for culture, which identifies the exact bacterium and tests which antibiotics will kill it. This takes 24 to 48 hours, but it is the only way to know for certain what you are treating.

While waiting for culture results, doctors typically start antibiotics based on the most likely organisms — usually covering Staphylococcus aureus and Streptococcus. Once the culture comes back, antibiotics can be narrowed to target the specific bacterium, which reduces side effects and improves outcomes.

In some cases, especially if the joint is very swollen or if antibiotics alone are not working, the infected fluid must be drained surgically. This relieves pressure, removes bacteria and toxins, and allows antibiotics to work more effectively. The decision to drain depends on which joint is infected, how sick you are, and how quickly you improve on antibiotics.

Treatment and recovery

Treatment combines antibiotics and sometimes drainage. Antibiotics are given intravenously at first — usually for two to four weeks — then switched to oral medication to complete a course that typically lasts four to six weeks total. The exact duration depends on the bacterium, the joint involved, and how quickly the infection clears.

Rest and immobilization of the joint are important early on to reduce pain and prevent spread of infection. Physical therapy begins once the acute infection is controlled, to restore movement and strength. Recovery is usually gradual — some people regain full function, while others are left with stiffness or chronic pain if cartilage was damaged before treatment started.

The outcome depends heavily on how quickly treatment began. Joints treated within days usually recover well. Joints left infected for weeks or months may have permanent damage even after the infection clears. This is why recognizing symptoms and seeking prompt medical care makes such a difference.

Frequently Asked Questions

Can you get septic arthritis from a regular cut or scrape?

A shallow cut or scrape on the skin above a joint is unlikely to cause infection unless it is very deep or heavily contaminated. Risk is higher with puncture wounds (stepping on a nail, being pricked by a needle) because bacteria are driven deep into tissue. Any wound that reaches the joint space itself — rare but possible with severe injuries — carries real risk.

Is septic arthritis contagious?

No. The infection is inside the joint fluid, not in your blood or respiratory secretions. You cannot catch septic arthritis from another person. However, if someone has an open wound with bacteria like MRSA, and you have a cut or puncture wound, you could theoretically acquire that bacterium — but it would have to enter your joint to cause septic arthritis, which is uncommon.

What happens if septic arthritis is not treated?

Untreated bacterial infection in a joint destroys cartilage, bone, and surrounding tissue. Within weeks, permanent damage can occur, leaving you with a stiff, painful joint that may require surgical fusion or replacement. Infection can also spread to the bloodstream, causing sepsis — a life-threatening condition. Early treatment prevents these outcomes.

Can you get septic arthritis from a joint injection?

Joint injections carry a small risk of infection — typically less than 1 in 1,000 when performed under sterile conditions. Risk is higher if you have a weakened immune system, diabetes, or a prosthetic joint. If you develop pain, swelling, or fever within days of an injection, contact your doctor when ready.

Are older adults more likely to have atypical symptoms?

Yes. Older adults may have less fever, less obvious swelling, or milder pain than younger people with the same infection. Some have only fatigue, confusion, or loss of appetite. This is why any new joint pain in an older adult — even mild — warrants evaluation, especially if there was a recent injury or procedure.