Is Staphylococcus (Staph) hemolyticus always considered a contaminant?

Is Staphylococcus haemolyticus a Contaminant?

Staphylococcus haemolyticus is frequently a blood culture contaminant, but it can also be a true pathogen in certain clinical scenarios, particularly in immunocompromised patients or those with indwelling medical devices.

Understanding S. haemolyticus in Clinical Specimens

S. haemolyticus is a coagulase-negative staphylococcus (CoNS) that is part of the normal human skin microbiota. When isolated from blood cultures, it presents a common diagnostic challenge for clinicians:

When S. haemolyticus is likely a contaminant:

• Single positive blood culture bottle when multiple sets are drawn 1
• Absence of clinical signs of infection
• No indwelling medical devices
• Immunocompetent host
• Recovery from other sites where it represents normal flora

When S. haemolyticus should be considered a true pathogen:

• Multiple positive blood cultures with the same organism 2
• Presence of pyuria (in urinary isolates) 2
• Immunocompromised patient 3
• Presence of prosthetic devices or catheters 3
• Clinical signs consistent with infection

Clinical Significance and Pathogenicity

S. haemolyticus has emerged as the second most frequently isolated CoNS from clinical specimens after S. epidermidis 4. Despite often being dismissed as a contaminant, it has been associated with:

• Bloodstream infections including sepsis
• Meningitis
• Endocarditis
• Prosthetic joint infections
• Peritonitis
• Catheter-associated infections 3

The pathogenicity of S. haemolyticus is attributed to:

• Biofilm formation capability
• Production of enterotoxins and hemolysins
• Fibronectin-binding proteins that facilitate adherence and invasion 5
• Unusual genome plasticity that contributes to antibiotic resistance 4

Diagnostic Approach

When S. haemolyticus is isolated from blood cultures, consider these factors to determine significance:

1. Number of positive cultures: Multiple positive cultures with the same organism strongly suggest true infection rather than contamination 6
2. Clinical context: Evaluate for signs of infection, presence of indwelling devices, and immune status
3. Time to positivity: Early growth in blood culture bottles may indicate higher bacterial load and true infection
4. Laboratory parameters: Elevated inflammatory markers support true infection

Antibiotic Resistance Concerns

S. haemolyticus isolates frequently demonstrate multi-drug resistance:

• High rates of methicillin resistance
• Increasing resistance to glycopeptides
• May serve as a reservoir of resistance genes for other staphylococci, including S. aureus 4

Management Considerations

For confirmed S. haemolyticus infections:

• Remove indwelling devices when possible 2
• Select antibiotics based on susceptibility testing
• Consider antibiotic lock therapy for catheter-related infections when catheter removal is not feasible 2
• Standard duration of therapy for complicated infections is 10-14 days 2

Common Pitfalls to Avoid

1. Dismissing all S. haemolyticus isolates as contaminants: This can lead to missed diagnosis of true infections, especially in vulnerable populations
2. Overtreatment of likely contaminants: The IDSA guidelines discourage vancomycin use in response to a single positive culture for CoNS when other cultures are negative 2
3. Failure to obtain adequate blood culture samples: Multiple sets increase the ability to differentiate contaminants from true pathogens
4. Ignoring host factors: Immunocompromised patients and those with prosthetic devices are at higher risk for true infection with organisms typically considered contaminants

In conclusion, while S. haemolyticus is often a contaminant in blood cultures, clinical judgment considering multiple factors is essential to determine its significance in each case. The increasing antibiotic resistance and pathogenic potential of this organism warrant careful consideration in vulnerable patient populations.