Isolates with low-level vancomycin resistance associated with persistent methicillin-resistant Staphylococcus aureus...

Area of Science:

• Microbiology
• Infectious Diseases
• Genomics

Background:

• Low-level vancomycin resistance in Staphylococcus aureus (vancomycin-intermediate S. aureus [VISA] and heterogenous VISA [hVISA]) is a growing clinical concern, often leading to treatment failure.
• The precise genetic mechanisms driving the development of hVISA and VISA phenotypes remain incompletely understood.

Purpose of the Study:

• To investigate the phenotypic, antibiotic resistance, and genomic changes associated with the emergence of hVISA/VISA from vancomycin-susceptible S. aureus (VSSA) in patients with persistent bacteremia.
• To compare isogenic pairs of VSSA and hVISA/VISA isolates to identify genetic and phenotypic alterations contributing to vancomycin resistance.

Main Methods:

• Genomic DNA microarray comparison and sequencing of selected loci were used to analyze five pairs of clinical hVISA/VISA strains and their antecedent VSSA isolates.
• Phenotypic characterization included assessment of cell wall thickness, autolytic activity, and biofilm formation.

Main Results:

• The hVISA/VISA phenotype was associated with increased cell wall thickness, reduced autolytic activity (in 4/5 pairs), and significantly decreased biofilm formation (in all pairs) compared to VSSA parent strains.
• Genomic analysis suggested that major genetic changes are not required for the development of resistance; no sequence differences were found in the agr locus or tcaRA genes.
• A marked reduction in RNAIII expression was observed in four of the five hVISA/VISA pairs.

Conclusions:

• hVISA/VISA can arise from fully susceptible VSSA strains during persistent infections that are refractory to glycopeptide therapy.
• The development of low-level vancomycin resistance is associated with significant phenotypic alterations, notably a substantial decrease in biofilm-forming capacity.
• These clinically derived isolate pairs provide a valuable resource for elucidating the genetic underpinnings of hVISA/VISA resistance mechanisms.