title: Importance of early detection of vancomycin-resistant subpopulations in apparently susceptible meticillin-resistant Staphylococcus aureus clinical isolates.
abstract: 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.
authors.affiliation: School of Nursing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. Electronic address: margaret.o.donoghue@polyu.edu.hk.
authors.forename: M M
authors.initials: MM
authors.lastname: O'Donoghue
authors.affiliation: Department of Health Technology and Informatics, The Hong Kong Polytechnic University, Kowloon, Hong Kong.
authors.forename: V C
authors.initials: VC
authors.lastname: Doddangoudar
authors.affiliation: Department of Pathology, Queen Elizabeth Hospital, Kowloon, Hong Kong.
authors.forename: D N C
authors.initials: DN
authors.lastname: Tsang
authors.affiliation: Department of Health Technology and Informatics, The Hong Kong Polytechnic University, Kowloon, Hong Kong.
authors.forename: M V
authors.initials: MV
authors.lastname: Boost
keywords: MRSA; Spiral gradient endpoint technique; Vancomycin non-susceptibility; hVISA