Staphylococcus Aureus Bacteremia

Staphylococcus aureus is a gram-positive bacterium, existing as a commensal in the human nares, skin, throat, and gastrointestinal tract in about 30% of people. S aureus can become virulent if it breaches the skin or mucosal barriers and accesses normally sterile sites such as the bloodstream. Staph aureus is the leading cause of death from bacteremia worldwide, with an estimated 300,000 deaths per year. The case fatality rate is 15 to 30%. 

After entering the bloodstream, Staph aureus can attach to the surface of tissues or implanted devices. Staph aureus possesses surface proteins called MSCRAMMS (microbial surface components recognizing adhesive matrix molecules) that facilitate binding to many human proteins, including fibronectin, fibrinogen, collagen, von Willebrand factor, and platelets. After attaching to a surface, aggregates of Staph aureus bacteria can produce a biofilm matrix of polysaccharides, proteins, and extracellular DNA that shields the bacteria from detection by the human immune system. Staph aureus then enters a low metabolic state, resulting in reduced susceptibility to antibiotics that are active against replicating bacteria.

In more than one-third of cases, Staphylococcus aureus bacteremia may lead to lead to metastatic infection, including endocarditis, septic arthritis, vertebral osteomyelitis, spinal epidural abscess, psoas abscess, splenic abscess, septic pulmonary emboli, emboli to implantable medical devices. Staph aureus may cause urinary tract infection after catheterization or instrumentation. 

Risk factors for Staph aureus bacteremia include implantable cardiac devices, dialysis vascular catheters, recent surgical procedures, injection drug use, diabetes, and previous Staph aureus infection. 

Patients with Staph aureus bacteremia commonly present with fever or symptoms from metastatic infection, such as pain in the back, joints, abdomen or extremities, and/or change in mental status. 

Staphylococcus aureus bacteremia is detected with blood cultures. Gram stain of positive cultures shows gram-positive cocci in clusters. Rapid molecular tests can identify Staphylococcus species within several hours. They can also detect the mecA gene that confers methicillin resistance.

Staphylococcus aureus is categorized as methicillin-susceptible (MSSA) or methicillin-resistant (MRSA) based on susceptibility to β-lactam antibiotics. Initial treatment for S aureus bacteremia typically includes antibiotics active against MRSA such as vancomycin or daptomycin. Once antibiotic susceptibility results are available, antibiotics should be adjusted. Cefazolin or anti-staphylococcal penicillins should be used for MSSA. Vancomycin, daptomycin, or ceftobiprole should be administered for MRSA.

Reference

Tong SYC et al. Management of Staphylococcus aureus Bacteremia: A Review, JAMA, published online April 7, 2025;AMA.2025;334(9):798-808.doi:10.1001/jama.2025.4288