Vancomycin susceptibility of meticillin-resistant Staphylococcus aureus (MRSA) bacteraemia isolates from two UK hospitals over a decade

Importance of early detection of vancomycin-resistant subpopulations in apparently susceptible meticillin-resistant Staphylococcus aureus clinical isolates

This study aimed to investigate the presence of vancomycin-non-susceptible subpopulations in apparently susceptible meticillin-resistant Staphylococcus aureus (MRSA) and the ability of these isolates to develop into homogeneously resistant strains. Vancomycin MICs of 200 clinical MRSA isolates were determined using agar dilution (AD) and spiral gradient endpoint (SGE) technique. Isolates with an MIC≤2mg/L but displaying subpopulations with an MIC>2mg/L by SGE were re-tested by Etest and PAP-AUC and were incubated with 2mg/L vancomycin for 2 weeks. MIC testing was repeated weekly by AD, Etest and SGE to observe progression to non-susceptibility. A total of 17.5% and 16.0% of isolates were non-susceptible to vancomycin (MIC>2mg/L) by SGE and AD, respectively. Eight isolates (4%) displayed a resistant subpopulation; five met the definition of hVISA by PAP-AUC. The initial Etest MIC for these isolates was 2mg/L, but resistant subpopulations were observed in only three isolates on prolonged incubation. MICs of all eight isolates increased rapidly in the presence of vancomycin, reaching ≥3.0mg/L by Day 7 and ≥4mg/L after 14 days by all three methods. The prevalence of vancomycin-non-susceptible MRSA was high, and non-susceptibility developed rapidly in seemingly susceptible isolates with covert subpopulations. These were effectively detected by SGE. With increasing reports of vancomycin clinical failure, early detection of potentially non-susceptible isolates before or early in vancomycin therapy is essential to avoid further resistance development and poor clinical outcomes. SGE offers a novel and cost-effective technique for detection of potentially non-susceptible strains.

Reduced vancomycin susceptibility among clinical Staphylococcus aureus isolates ('the MIC Creep'): implications for therapy

Methicillin-resistant S. aureus (MRSA) has emerged as the most common hospital-acquired pathogen and is associated with increased morbidity and mortality compared with other strains. Vancomycin has been the cornerstone of treatment of patients with serious MRSA infections for some decades and while more than 99% of clinical S. aureus isolates remain susceptible to vancomycin, we are beginning to see strains of MRSA with reduced susceptibility. This review discusses this phenomenon, the predictors of infection with such forms of MRSA, and current and future management options.

An association between bacterial genotype combined with a high-vancomycin minimum inhibitory concentration and risk of endocarditis in methicillin-resistant Staphylococcus aureus bloodstream infection

INTRODUCTION

Antimicrobial resistance and bacterial virulence factors may increase the risk of hematogenous complications during methicillin-resistant Staphylococcus aureus (MRSA) bloodstream infection (BSI). This study reports on the impact of increasing vancomycin minimum inhibitory concentrations (V-MICs) and MRSA clone type on risk of hematogenous complications from MRSA BSI during implementation of an effective MRSA control program.

METHODS

In sum, spa typing, staphylococcal cassette chromosome mec allotyping, and vancomycin and teicoplanin MICs were performed on 821 consecutive MRSA bloodstream isolates from 1999 to 2009. Prospectively collected data, including focus of infection, were available for 695 clinically significant cases. Logistic and multinomial logistic regression was used to determine the association between clone type, vancomycin MIC (V-MIC), and focus of infection.

RESULTS

MRSA BSIs decreased by ∼90% during the 11 years. Typing placed isolates into 3 clonal complex (CC) groups that had different population median V-MICs (CC30, 0.5 μg/mL [n = 349]; CC22, 0.75 μg/mL [n = 272]; non-CC22/30, 1.5 μg/mL [n = 199]). There was a progressive increase in the proportion of isolates with a V-MIC above baseline median in each clonal group and a disproportionate fall in the clone group with lowest median V-MIC (CC30). In contrast, there were no increases in teicoplanin MICs. High V-MIC CC22 isolates (1.5-2 μg/mL) were strongly associated with endocarditis (odds ratio, 12; 95% confidence interval, 3.72-38.9) and with a septic metastasis after catheter-related BSI (odds ratio, 106; 95% confidence interval, 12.6-883) compared with other clone type/V-MIC combinations.

CONCLUSIONS

An interaction between clone type and V-MIC can influence the risk of endocarditis associated with MRSA BSI, implying involvement of both therapeutic and host-pathogen factors.