Is Staphylococcus aureus (Staph aureus) usually a contaminant?

Staphylococcus aureus is NOT Usually a Contaminant

Staphylococcus aureus should always be considered a true pathogen rather than a contaminant when isolated from clinical specimens, particularly from blood cultures, due to its significant association with morbidity and mortality. 1

Clinical Significance of S. aureus

S. aureus is a virulent organism with significant pathogenic potential:

• It is the leading cause of death from bacteremia worldwide, with a case fatality rate of 15-30% 2
• S. aureus bacteremia causes metastatic infection in more than one-third of cases, including endocarditis (≈12%), septic arthritis (7%), and vertebral osteomyelitis (≈4%) 2
• When isolated from blood cultures, S. aureus represents true infection in the vast majority of cases, unlike coagulase-negative staphylococci which are frequent contaminants 1

Distinguishing S. aureus from Common Contaminants

True Contaminants vs. S. aureus

• Coagulase-negative staphylococci are the most frequent contaminants in blood cultures and other specimens 1
• S. aureus is distinguished from coagulase-negative staphylococci by its coagulase-positive status and should be treated as a true pathogen 1

Diagnostic Considerations

• For coagulase-negative staphylococci, diagnosis of true infection should be based on more than one set of positive blood cultures 1
• For S. aureus, even a single positive blood culture warrants treatment as it rarely represents contamination 1

Clinical Management Algorithm

1. When S. aureus is isolated from blood cultures:

   • Remove intravascular catheters if present 1
   • Initiate appropriate antibiotic therapy immediately 1, 2
   • Perform transthoracic echocardiography for all patients 2
   • Consider transesophageal echocardiography for high-risk patients (persistent bacteremia, fever, metastatic infection) 2

2. When S. aureus is isolated from other specimens:

   • Evaluate clinical context to confirm infection
   • Implement source control measures (drainage of abscesses, debridement) 2
   • Initiate appropriate antibiotic therapy based on susceptibility 1, 3

Risk Factors for S. aureus Infection

• Intravascular catheters and implantable devices 3, 2
• Recent surgical procedures 2
• Injection drug use 3, 2
• Diabetes mellitus 3
• Previous S. aureus infection or colonization 3
• Nasal carriage (present in 20-40% of general population) 3

Prevention Strategies

• Appropriate hand and skin hygiene 3
• Cleaning and disinfecting shared equipment 3
• Prompt treatment of skin wounds 3
• Patient decolonization protocols for high-risk individuals 1

Common Pitfalls to Avoid

1. Misinterpreting S. aureus as a contaminant: Unlike coagulase-negative staphylococci, S. aureus is rarely a contaminant and should prompt immediate clinical action 1

2. Inadequate source control: Failure to remove infected devices or drain abscesses can lead to persistent bacteremia and increased mortality 1, 2

3. Insufficient treatment duration: S. aureus infections typically require longer courses of antibiotics (minimum 14 days for uncomplicated bacteremia) 1, 2

4. Missing metastatic foci: S. aureus has a high propensity for hematogenous spread, requiring thorough evaluation for metastatic infection 3, 2

In conclusion, S. aureus should be considered a true pathogen rather than a contaminant in virtually all clinical scenarios, and its isolation should prompt appropriate diagnostic workup and treatment to prevent significant morbidity and mortality.