Vancomycin-heteroresistant phenotype in invasive methicillin-resistant Staphylococcus aureus isolates belonging to...

Area of Science:

• Microbiology
• Infectious Diseases
• Molecular Epidemiology

Background:

• Methicillin-resistant Staphylococcus aureus (MRSA) poses a significant global health threat.
• Understanding the genetic diversity and resistance mechanisms of S. aureus is crucial for effective treatment.
• The emergence of vancomycin resistance, including heteroresistance (h-VISA), is a growing concern.

Purpose of the Study:

• To characterize invasive methicillin-susceptible S. aureus (MSSA) and MRSA strains from Italy.
• To investigate the prevalence of heteroresistant vancomycin-intermediate S. aureus (h-VISA) among these strains.
• To identify specific molecular types associated with h-VISA.

Main Methods:

• In vitro susceptibility testing for MSSA and MRSA isolates.
• Macro Etest (MET) and vancomycin population analysis profiles (PAP) for h-VISA detection.
• Genotyping including agr locus, SCCmec typing, spa typing, and multilocus sequence typing (MLST).

Main Results:

• Two MRSA strains were classified as vancomycin-intermediate S. aureus (VISA).
• Twelve MRSA strains tested positive by MET, with nine confirmed as h-VISA by PAP (14% of MRSA).
• The spa type t041 (ST228/273-MRSA-I, CC5) was most common, comprising 24 isolates, including one VISA and eight h-VISA cases, indicating a strong association.

Conclusions:

• A significant association was identified between h-VISA and the spa type t041.
• Molecular typing, particularly spa typing, is vital for identifying clinically relevant S. aureus clones.
• These findings underscore the importance of monitoring for h-VISA and its associated clones in Italy and Europe.