vancomycin-intermediate *Staphylococcus aureus*

vancomycin-intermediate *Staphylococcus aureus*
Name	vancomycin-intermediate Staphylococcus aureus
Synonyms	VISA, GISA (glycopeptide-intermediate S. aureus)
Field	Infectious disease (medical specialty), Clinical microbiology
Causes	Staphylococcus aureus

vancomycin-intermediate *Staphylococcus aureus* is a significant antimicrobial-resistant pathogen characterized by reduced susceptibility to the glycopeptide antibiotic vancomycin. First identified in Japan in 1996, these strains represent a critical evolutionary step from vancomycin-susceptible Staphylococcus aureus toward full vancomycin resistance. The emergence of VISA poses a major challenge in clinical settings, particularly for treating infections caused by methicillin-resistant Staphylococcus aureus where vancomycin is a cornerstone therapy.

Definition and Classification

VISA is formally defined by specific minimum inhibitory concentration breakpoints established by organizations like the Clinical and Laboratory Standards Institute and the European Committee on Antimicrobial Susceptibility Testing. These strains exhibit MICs to vancomycin of 4-8 µg/mL, placing them in the intermediate susceptibility category, distinct from both susceptible isolates and the fully resistant vancomycin-resistant Staphylococcus aureus. The first clinical isolate, known as Mu50, was reported by Keiichi Hiramatsu and colleagues at Juntendo University. This classification is crucial for guiding antimicrobial stewardship programs and informing treatment protocols in hospitals like the National Institutes of Health Clinical Center.

Mechanisms of Resistance

The primary mechanism involves a thickened and aberrantly structured peptidoglycan cell wall, a phenomenon often linked to mutations in regulatory genes such as walkR (also known as yycFG). This thickened wall acts as a sponge, trapping vancomycin molecules before they can reach their target on the bacterial cell membrane. Additional genetic alterations frequently involve the vraSR and graSR two-component systems, which upregulate cell wall biosynthesis pathways. Research from institutions like the Rockefeller University has shown these adaptations are distinct from the acquired vanA gene cluster found in Enterococcus species that confers high-level resistance.

Epidemiology and Risk Factors

Since the initial report from Japan, VISA strains have been identified globally, with cases documented in the United States, France, and South Africa. The epidemiology is often healthcare-associated, with outbreaks reported in facilities such as the Cleveland Clinic. Major risk factors for infection or colonization include prolonged exposure to vancomycin, prior MRSA infection, and the presence of indwelling devices like central venous catheters. The Centers for Disease Control and Prevention and the World Health Organization monitor its spread as part of global surveillance for antimicrobial resistance.

Clinical Significance and Diagnosis

VISA infections are clinically significant as they are associated with vancomycin treatment failure, leading to persistent bacteremia, increased mortality, and longer hospital stays. These infections often complicate conditions like endocarditis, osteomyelitis, and ventilator-associated pneumonia. Accurate diagnosis requires broth microdilution methods in a clinical microbiology laboratory; automated systems like VITEK 2 may misclassify these strains. Key diagnostic clues include the presence of heteroresistant vancomycin-intermediate Staphylococcus aureus subpopulations within an infection.

Treatment and Management

Managing VISA infections typically requires a combination of antimicrobial agents and source control. Alternative antibiotics may include daptomycin, linezolid, ceftaroline, or telavancin, often used in combination based on antimicrobial susceptibility testing. Consultation with infectious disease specialists at institutions like the Mayo Clinic is standard. Therapeutic drug monitoring of vancomycin trough levels is critical, and surgical intervention, such as debridement or device removal, is often necessary. The Infectious Diseases Society of America provides guidelines for these complex cases.

Prevention and Control

Prevention hinges on strict infection control practices to limit transmission in healthcare environments. Core strategies include contact precautions, meticulous hand hygiene compliance with agents like chlorhexidine, and robust environmental cleaning protocols. Antimicrobial stewardship programs, endorsed by the Society for Healthcare Epidemiology of America, are essential to reduce unnecessary vancomycin pressure. Surveillance cultures in high-risk units and decolonization regimens may be employed during outbreaks, as implemented in hospitals like Johns Hopkins Hospital.

Category:Antimicrobial resistance Category:Staphylococcus Category:Bacterial diseases