Clinical isolate of vancomycin-heterointermediate Staphylococcus aureus susceptible to methicillin and in vitro selection of a vancomycin-resistant derivative

Abstract

A Staphylococcus aureus strain with low-level heteroresistance to vancomycin (designated MER) but susceptible to methicillin was isolated from an outpatient with conjunctivitis who did not receive any glycopeptide antibiotics. Incubation of the parent strain, MER, with increasing concentrations of vancomycin led to rapid selection of a stable progeny homogeneously resistant to vancomycin. Electron micrographs of strain MER showed enhanced cell wall thickness and abnormal septations typically seen with methicillin-resistant S. aureus having intermediate susceptibility to vancomycin.

FIG. 1

Population analysis of parental hVISA strain MER, S. aureus ATCC 29213, and their derivatives. Bacteria grown overnight were serially diluted and plated on various concentrations of vancomycin-containing agar medium. MER-S6 was a MER-derived progeny selected at 6 μg of vancomycin per ml, MER-S12 was derived from MER-S6 selected at 12 μg of vancomycin per ml, and MER-S20 was derived from MER-S12 selected at 20 μg of vancomycin per ml. ATCC 29213-S3 was selected at 3 μg of vancomycin per ml, ATCC 29213-S4 was selected at 4 μg of vancomycin per ml, and ATCC 29213-S4′ was obtained by selecting ATCC 29213-S4 with 4 μg of vancomycin per ml. Horizontal arrows indicate the order in which the consecutive population analyses were performed.

FIG. 2

Composite picture showing the morphology of S. aureus ATCC 29213 (A) susceptible to vancomycin and the morphological abnormalities of hVISA MER grown in the absence (B) or presence (C) of 2 μg of vancomycin per ml and of strain MER-S20 (D). All cultures were grown to an optical density at 600 nm of 0.6 in brain heart infusion broth prior to processing for transmission electron microscopic examination. Magnification, ×40,000 (original magnification, ×60,000).