Staphylococcus aureus bacteraemia

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Staphylococcus aureus bacteraemia is bloodstream invasion by a virulent organism able to adhere to endothelium and foreign material, form biofilm, destroy tissue and seed distant sites. Even a single credible positive episode has substantial mortality and recurrence risk.

Common portals are vascular catheters, skin and soft-tissue infection, surgical wounds, bone and joint infection, injection exposure and infected prostheses. Sometimes the initial source is no longer obvious because bacteraemia has already established endocarditis or another deep focus.

Clinical improvement after the first antibiotic dose does not establish uncomplicated disease. Repeated negative cultures, echocardiographic assessment, source control and a symptom-led search for dissemination are required.

Definitive beta-lactam therapy is preferred for methicillin-susceptible isolates. Dose, duration and outpatient suitability must reflect clearance, endocardial risk, implanted material, renal and liver function and whether all infected tissue has been controlled.

Key points

Treat every credible S aureus blood-culture episode as true bacteraemia until a complete assessment proves otherwise.
Repeat peripheral blood cultures every 24 to 48 hours until documented clearance; the clearance date helps define duration.
Examine skin, wounds, vascular access, heart, spine, joints, neurological system and every implanted prosthesis for source and spread.
Remove infected or unnecessary intravascular catheters promptly and drain abscesses or wash out infected joints.
Perform echocardiography; use TOE when clinical probability, persistent bacteraemia, prosthetic material or inadequate TTE makes occult endocarditis plausible.
Use intravenous flucloxacillin for MSSA when safe and a monitored MRSA-active regimen when resistance or allergy requires it.
Uncomplicated disease requires all criteria: rapid clearance, defervescence, controlled removable source, no implanted prosthesis, no endocarditis and no metastatic focus.
Treat truly uncomplicated bacteraemia for at least 14 days of effective therapy from clearance; complicated infection generally needs four to six weeks or longer according to focus.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Vascular catheter entry

Skin or hub organisms colonise intravenous devices and gain direct access to blood, often with adherent thrombus.

Skin and soft tissue

Abscess, cellulitis, ulcers, wounds and injection sites breach the epithelial barrier and release organisms systemically. in susceptible exposed adults.

Bone and joint infection

Osteomyelitis, septic arthritis and infected prosthetic joints may be either the primary source or a seeded complication.

Infected prosthetic material

Valves, vascular grafts and cardiac devices support biofilm that intermittently sheds S aureus into blood. in susceptible exposed adults.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Adhesion and invasion
Surface proteins bind fibrinogen, fibronectin and endothelial tissue, permitting rapid colonisation of valves, thrombus and prostheses.
2
Immune evasion
Capsule, protein A and intracellular persistence interfere with opsonisation and phagocyte-mediated clearance. during active invasive disease.
3
Tissue destruction
Toxins, enzymes and intense neutrophilic inflammation create abscesses, valve destruction and local necrosis. during active invasive disease.
4
Metastatic seeding
Sustained bloodstream organisms lodge in high-flow endothelium, vertebrae, joints, viscera and previously implanted material. during active invasive disease.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Acute bloodstream sepsisRed flag

Fever, rigors, hypotension, confusion or organ dysfunction may accompany any source and requires immediate active intravenous treatment.

Vascular-access sourceRed flag

Tenderness, purulence, thrombosis or rigors during line use suggest catheter infection; an apparently clean site does not exclude intraluminal biofilm.

Endocardial diseaseRed flag

Murmur, embolus, heart failure, conduction change or persistent bacteraemia raises endocarditis probability, but none is required for diagnosis.

Spinal disseminationRed flag

New focal back pain, radicular pain, weakness or sphincter disturbance may reflect vertebral infection or epidural abscess.

Joint and bone seedingRed flag

A hot swollen joint, prosthetic-joint pain or focal bone tenderness during bacteraemia needs urgent aspiration or imaging and source control.

Skin and injection portal

Cellulitis, abscess, ulcers, recent cannulation or injection sites may provide entry and reveal further undrained infection.

Red flags requiring action

Staphylococcus aureus in a correctly collected blood culture is not dismissed as contamination.
Positive cultures beyond 48 to 72 hours of active treatment predict an uncontrolled source, endocarditis or metastatic infection.
New murmur, heart failure, conduction delay or embolic deficit requires urgent endocarditis evaluation.
Back pain, focal bone tenderness, hot joint or weakness may indicate vertebral osteomyelitis, epidural abscess or septic arthritis.
Any retained prosthetic valve, vascular graft, joint, cardiac device or intravascular catheter increases complicated-infection risk.
Hypotension, confusion, oliguria, mottling or rising lactate requires immediate sepsis escalation.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Repeat peripheral blood culturesFirst step
Why
Document sterilisation, detect persistent bacteraemia and establish the clearance date.
Interpretation and limitations
Obtain new sets every 24 to 48 hours until negative; persistent positivity is a complication marker, not a reason simply to prolong unchanged therapy.
02
Susceptibility and rapid species confirmation
Why
Distinguish MSSA from MRSA and narrow promptly to the most effective agent.
Interpretation and limitations
Review every bottle and prior isolate with microbiology; susceptible S aureus generally receives an anti-staphylococcal beta-lactam rather than continued vancomycin.
03
TTE and selected TOE
Why
Detect vegetation, valve destruction and haemodynamic consequences of endocarditis.
Interpretation and limitations
TTE provides the initial study; TOE is needed with high clinical probability, persistent growth, prosthetic material, inadequate images or no convincing removable source.
04
Full blood count, CRP, renal and liver profile
Why
Establish organ injury, treatment safety and response trajectory.
Interpretation and limitations
Inflammatory-marker improvement supports response but cannot replace culture clearance or exclusion of deep infection.
05
Source-directed MRI, CT, ultrasound or aspiration
Why
Investigate focal back, joint, abdominal, neurological or limb symptoms and obtain deep cultures.
Interpretation and limitations
MRI is preferred for suspected spinal infection; aspirate a hot joint before antibiotics when safe but never delay therapy in sepsis.
06
Prosthesis and line assessment
Why
Identify biofilm-bearing material requiring removal, washout or specialist retention strategy.
Interpretation and limitations
Review operative records and examine all devices; normal superficial appearance does not exclude deep graft, valve or joint infection.
07
ECG and neurological assessment
Why
Detect peri-annular conduction injury and embolic or epidural complications.
Interpretation and limitations
New block, focal deficit or sphincter dysfunction triggers urgent targeted imaging and procedural review.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Blood-culture contamination

Common skin flora often contaminate sampling, but S aureus rarely does and requires a complete evaluation before that conclusion.

Coagulase-negative staphylococci

These organisms more often contaminate cultures yet cause genuine infection in intravascular and prosthetic-material settings. on focused clinical assessment.

Streptococcal bacteraemia

Streptococci share fever and endocardial risk but suggest different oral, gastrointestinal or soft-tissue portals and directed regimens.

Sterile inflammatory illness

Vasculitis, malignancy and drug fever raise inflammatory markers but do not produce repeated matching blood-culture isolates.

Alternative septic source

Gram-negative urinary, biliary or pulmonary sepsis can cause similar organ dysfunction and must be distinguished microbiologically.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01FIRST HOURSConfirm significance and stabiliseFirst stepA laboratory reports S aureus from blood or Gram-positive cocci strongly suggest it in an unwell patient.

1Contact the treating team urgently, use ABCDE and ensure an active intravenous regimen is prescribed with allergy and renal status documented.
2Obtain repeat peripheral cultures before the next dose when feasible without delaying urgent therapy.
3Remove unnecessary or infected vascular catheters and drain readily accessible pus; send deep material for culture.
4Ask specifically about valves, all prostheses, back or joint pain, injection exposure, recent surgery and neurological symptoms.

02CLEARProve bloodstream clearanceActive directed intravenous therapy has already started.

1Repeat peripheral blood-culture sets every 24 to 48 hours until negative and record the first sustained clearance date.
2EscalationEscalate persistent growth to infection and source-control specialists and reassess line removal, abscess drainage, prosthetic infection and drug exposure.
3Perform TTE and determine promptly whether TOE is required rather than waiting until the end of treatment.
4Confirm fever resolution and review new focal symptoms daily because metastatic foci may declare themselves after initial stabilisation.

03CLASSIFYSeparate uncomplicated from complicatedCultures have cleared and initial source management is complete.

1Verify that endocarditis is excluded, there is no implanted prosthesis, cultures cleared within 48 to 72 hours and fever resolved promptly.
2Confirm the primary source is removed or drained and careful examination and symptom-directed imaging show no metastatic infection.
3If every criterion is met, complete at least 14 days of effective treatment from clearance using the specialist plan.
4If any criterion is absent or uncertain, manage as complicated disease with focus-specific imaging, source control and usually four to six weeks or longer.

04DIRECTOptimise definitive treatmentDefinitiveMethicillin susceptibility, allergy assessment and deep-focus status are known.

1For MSSA use high-dose intravenous flucloxacillin when safe and stop broader empirical agents.
2For MRSA use a monitored protocol such as vancomycin, ensuring weight-based loading, renal adjustment and target exposure.
3Seek specialist alternatives for immediate beta-lactam allergy, toxicity, treatment failure or reduced susceptibility rather than under-dosing.
4Before outpatient therapy confirm reliable access, adherence, laboratory monitoring, source stability and an immediate route back for deterioration.

Key medicines and prescribing safety4 treatments · regimens, roles and cautions

Provides preferred bactericidal definitive treatment with superior activity for susceptible isolates.

Flucloxacillin for MSSA bacteraemia

Give 2 g intravenously every four hours in adults with methicillin-susceptible Staphylococcus aureus bacteraemia when beta-lactam therapy is safe.

Clarify immediate penicillin allergy, monitor liver, renal, blood count and sodium exposure, and adjust the total course to clearance and infection focus.

Treats methicillin-resistant S aureus bacteraemia when susceptibility supports glycopeptide therapy.

Vancomycin for MRSA

Use the local intravenous loading dose based on actual body weight followed by renal-adjusted maintenance and measured serum exposure.

Monitor creatinine and concentrations, minimise nephrotoxins and investigate persistent cultures for source failure or need for a specialist alternative.

Avoids inferior glycopeptide exposure for MSSA when the reported penicillin allergy does not represent a severe immediate or delayed reaction.

Alternative beta-lactam in non-immediate allergy

Use the infection-service-approved intravenous cephalosporin or other beta-lactam regimen only after detailed allergy phenotype and susceptibility review.

Do not challenge severe cutaneous or organ-involving reactions; involve allergy or infection specialists and document the future antibiotic label clearly.

Treats fever and pain while focal symptoms are investigated and source control proceeds.

Analgesia without diagnostic masking

Give paracetamol 500 mg to 1 g orally or intravenously up to four times daily, maximum 4 g in 24 hours for a suitable adult.

Reduce the maximum dose in low body weight, frailty, liver disease or heavy alcohol use; analgesia must not defer spinal, joint or vascular imaging.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Infective endocarditis

Valve colonisation produces vegetation, regurgitation, peri-annular abscess, emboli and persistent bloodstream infection. without timely definitive management.

Vertebral infection

Haematogenous seeding causes discitis, osteomyelitis and epidural abscess with pain, weakness or sphincter compromise. without timely definitive management.

Septic arthritis

Organisms enter native or prosthetic joints, producing destructive inflammation that usually requires drainage or washout. without timely definitive management.

Septic thrombophlebitis

Infected venous clot sustains bacteraemia and can embolise despite catheter removal and active antimicrobial therapy. without timely definitive management.

Recurrent bacteraemia

Unrecognised biofilm, endocarditis or metastatic focus causes relapse after an apparently complete short course. without timely definitive management.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Repeat blood cultures every 24 to 48 hours until sustained negativity and re-culture any recurrent fever.
Review temperature, blood pressure, urine output, mental state and oxygen requirement during acute treatment.
Perform daily directed examination of heart, access sites, skin, spine, joints, neurological system and prostheses.
Monitor renal, hepatic and haematological toxicity and antimicrobial serum concentrations where relevant.
Track CRP as supportive trajectory information while giving priority to culture clearance and controlled anatomy.
Reassess the need for TOE or focal MRI when growth persists or new symptoms emerge.
At discharge document source, clearance date, complexity classification, treatment end date, access plan and relapse warning symptoms.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

One bottle can matter

S aureus is sufficiently virulent that even limited credible growth warrants urgent clinical assessment and repeat cultures.

Persistence is prognostic

Continued positivity despite active treatment is strongly associated with endocarditis, retained biofilm and metastatic foci.

All uncomplicated criteria apply

Rapid improvement alone is insufficient; absence of prosthesis, endocarditis and dissemination must also be demonstrated.

Back pain is microbiology

Focal spinal pain during S aureus bacteraemia should be treated as possible vertebral or epidural infection until assessed.

MSSA deserves a beta-lactam

Vancomycin is not continued merely for convenience when a safe high-dose anti-staphylococcal beta-lactam is available.

Source control is treatment

Removing an infected catheter or draining a joint changes clearance and outcome more than repeatedly broadening active antibiotics.

11Common pitfallsFrequent interpretation and management errors.

01
Do not label S aureus in blood as a contaminant without senior microbiology review.
02
Do not stop repeat cultures after the first positive set.
03
Do not declare disease uncomplicated because fever settled quickly.
04
Do not retain an unnecessary infected catheter or leave a drainable focus untreated.
05
Do not rely on a negative low-quality TTE when endocarditis probability remains high.
06
Do not convert to short oral treatment before clearance, source control and complication assessment are secure.