Disruption of d-alanyl esterification of Staphylococcus aureus cell wall teichoic acid by the β-lactam resistance modifier (−)-epicatechin gallate

OBJECTIVES: The naturally occurring polyphenol (−)-epicatechin gallate (ECg) increases oxacillin susceptibility in mecA-containing strains of Staphylococcus aureus. Decreased susceptibility to lysostaphin suggests alterations to the wall teichoic acid (WTA) content of ECg-grown bacteria. Changes in WTA structure in response to ECg were determined. METHODS: Nuclear magnetic resonance spectroscopy of purified monomers from S. aureus was used to elucidate WTA structures. Molecular modelling of WTA chains was employed to determine their spatial configuration. RESULTS: ECg-grown methicillin-resistant S. aureus (MRSA) strains BB568 and EMRSA-16 displayed markedly reduced resistance to oxacillin, had thickened cell walls and separated poorly. Growth in ECg-supplemented medium reduced the substitution of the WTA backbone by d-alanine (d-Ala); ratios of N-acetyl glucosamine to d-Ala were reduced from 0.6 and 0.49 (for BB568 and EMRSA-16) to 0.3 and 0.28, respectively. Molecular simulations indicated a decrease in the positive charge of the bacterial wall, confirmed by increased binding of cationized ferritin, and an increase in WTA chain flexibility to a random coil conformation. CONCLUSIONS: Structural elucidation and molecular modelling of WTA indicated that conformational changes associated with reduced d-Ala substitution may contribute to the increased susceptibility of MRSA to β-lactam antibiotics and account for other elements of the ECg-induced phenotype.