α-Hemolysin activity of methicillin-susceptible Staphylococcus aureus predicts ventilator-associated pneumonia

Area of Science:

• Microbiology and Infectious Diseases
• Critical Care Medicine
• Molecular Epidemiology

Background:

• Staphylococcus aureus colonization of the lower airways is a known risk factor for ventilator-associated tracheobronchitis (VAT) and ventilator-associated pneumonia (VAP).
• Limited understanding exists regarding the specific virulence factors of methicillin-sensitive (MSSA) and methicillin-resistant (MRSA) S. aureus that contribute to colonization and disease progression in ventilated patients.

Purpose of the Study:

• To investigate the genotypic characteristics and alpha-hemolysin activity of MRSA and MSSA isolates from endotracheal aspirates (ETA).
• To correlate these factors with the development of VAT and VAP in mechanically ventilated patients.

Main Methods:

• Analysis of serial S. aureus ETA isolates from 231 ventilated patients.
• Determination of methicillin resistance, molecular typing (MLST, spa-typing), and alpha-hemolysin activity (blood agar hemolysis, epithelial cell cytolysis).
• Virulence assessment of selected strains in a mouse pneumonia model.

Main Results:

• Staphylococcus aureus was detected in ETA samples from 25% of patients; one-third developed VAP.
• Ventilator-associated pneumonia (VAP) cases were predominantly infected by MSSA (87%), while colonization without progression was mainly associated with MRSA (68%).
• MSSA isolates from VAP patients showed significantly higher alpha-hemolysin activity compared to colonized or VAT patients; this correlation was not observed for MRSA. Alpha-hemolysin activity predicted virulence in mice.

Conclusions:

• MSSA strains exhibiting strong blood agar hemolysis and high alpha-hemolysin activity are significant markers for VAP, but not VAT.
• These findings suggest that alpha-hemolysin activity of MSSA can aid in differential diagnosis and inform the initiation of therapy for VAP in ventilated patients.