Acute infection and emergencies

Staphylococcus aureus bacteraemia

Overview and approach

Staphylococcus aureus bacteraemia is clinically significant and can seed endocarditis, bone, joints and implanted devices.

S. aureus (SAB) is a leading cause of community- and hospital-acquired bloodstream infection. Patients can develop a broad array of complications that may be hard to recognise initially and that increase morbidity and mortality. In one NSW health district, about 12% of community-acquired SAB was MRSA, with 30-day age-adjusted mortality of 6.9% (1).
Key principles: every SAB needs infectious diseases (ID) or microbiology input, removal of any removable focus, repeat cultures, an echocardiogram and a planned duration of IV therapy (1)(2).
Organism: most virulent staphylococcus; Gram-positive cocci in clusters; catalase positive (staph not strep); coagulase positive (not S. epidermidis or S. saprophyticus); beta-haemolytic with yellow/golden colonies.
Diseases: systemic: hospital-acquired pneumonia, ventilator-associated pneumonia (staphylococcus > Pseudomonas aeruginosa), osteomyelitis (unless sickle cell = Salmonella), acute endocarditis (subacute = viridans streptococci; catheter-associated = S. epidermidis). Local: skin (cellulitis, impetigo).
Virulence: binds the Fc fragment of IgG (enhanced inflammation); superantigens driving TNF-alpha release: toxic shock syndrome (fever, hypotension, rash; tampon use a risk factor), staphylococcal scalded skin syndrome; food poisoning from pre-formed toxins (onset 1-6 hours, watery diarrhoea; meat, custard, potato salad).

Assessment and clinical features

Search for skin, line and deep sources; ask about back pain, joint pain, embolic symptoms and implanted material.

Initial exam: look for metastatic foci, prosthetic material and endocarditis; check for previous MRSA (1).
Daily review: peripheral stigmata of endocarditis, new murmur, vertebral percussion tenderness, abdominal palpation (splenic/visceral abscess), large joints and long bones (1).
Likely sources: skin and soft tissue, intravascular lines, bone and joint, endovascular; removable foci include lines and drained skin abscesses (1)(2).

Differentials and investigations

Obtain follow-up blood cultures to document clearance and echocardiography as indicated; image focal symptoms and involve infectious diseases.

Repeat blood cultures 48-72 hours after starting appropriate antibiotics (and until negative). Persistent bacteraemia means ID review (1)(2).
Echocardiography: TTE in all adults (best timed around days 5-7); add TOE for a high suspicion of endocarditis with negative TTE, complicated SAB with negative TTE, a prosthetic valve, or poor TTE images (1)(2).
Monitoring bloods: FBC, CRP, electrolytes and LFTs every 3 days for 2 weeks, then weekly during IV therapy (2).
Complicated SAB = any of: bacteraemia persisting at 48-72 h; fever >72 h; abnormal valve/endocarditis on echo; no identifiable focus or a focus that has not been removed (removable = lines, drained skin abscesses); metastatic foci (e.g. endocarditis, vertebral osteomyelitis, visceral abscess). Uncomplicated = none of these (1).
Imaging (MRI spine, CT, PET-CT) for focal symptoms is individualised.

Management and follow-up

Use susceptibility-directed IV therapy and source control. Duration depends on clearance, endocarditis, metastatic sites and source; arrange follow-up.

first line = methicillin-class (in Australia this means flucloxacillin); MRSA = vancomycin; VRSA = quinupristin/dalfopristin (streptogramins) or linezolid.
Empirical therapy (Gram-positive cocci in clusters in blood): an anti-staphylococcal beta-lactam (flucloxacillin) PLUS vancomycin to cover MRSA until susceptibilities or PCR are back, then narrow (1)(2). The combination for over 48 hours is potentially nephrotoxic, so narrow promptly (1). Non-immediate beta-lactam allergy (e.g. rash): cefazolin 2 g IV 8-hourly is used instead of flucloxacillin (1).
MSSA definitive therapy: the SNAP platform trial found cefazolin non-inferior to flucloxacillin/cloxacillin for 90-day mortality, with less acute kidney injury (3).
MRSA: vancomycin IV, dosed by drug exposure (AUC or level) (1). In haemodialysis: cefazolin 2 g IV post dialysis for MSSA (1).
Source control: remove or replace infected lines, drain abscesses; PICC for prolonged IV therapy once cultures are negative (except in dialysis patients) (2).
Duration: uncomplicated adult SAB 14 days IV; MRSA minimum 14 days IV; complicated SAB at least 4 weeks IV, extended to 6 weeks if slow response or specific complication. Count from first negative culture / source removal (1)(4). Agree the plan with ID and document it (1).
early oral switch is a research question (SNAP sub-study); do not switch without ID advice.

Red flags and clinical pearls

Do not dismiss S. aureus in blood as contamination. Persistent positivity, prosthetic devices or new focal pain should trigger deeper investigation.

Never treat S. aureus in blood as a contaminant; it needs a full work-up and ID involvement (1)(2).
Persistent bacteraemia at 48-72 h or fever beyond 72 h, prosthetic valve/device, or new focal pain (back, joint) points to complicated disease and deep foci (1).
Patients on dialysis, with MRSA or with prosthetic material need ID-led plans (1).
Viva: list the criteria for complicated SAB; why add vancomycin initially and why stop it early; which patients need TOE.

References

Last updated from Sen’s notes on 9 October 2026.