Anti-Staphylococcus aureus activity and oxacillin resistance modulating capacity of 3-O-acyl-catechins

Abstract

Epicatechin gallate (ECg), a component of green tea with weak activity against Staphylococcus aureus, reduces oxacillin resistance in methicillin-resistant S. aureus at concentrations below the MIC. Because catechins bind to artificial lipid bilayers, we investigated whether the anti-staphylococcal activity of catechins could be improved by increasing their capacity to interact with the cytoplasmic membrane. Substitution of the gallate group of ECg with 3-O-acyl chains of varying lengths (C₄–C₁₈) led to enhanced anti-staphylococcal activity with chain lengths of C₈ (octanoyl) and C₁₀ (decanoyl). 3-O-octanoyl catechin was bactericidal against MRSA as the result of membrane damage. 3-O-acyl catechins tested at a 1/4 MIC did not reduce the oxacillin MIC greater than two-fold. 3-O-acyl catechins warrant further investigation as anti-staphylococcal agents.

Introduction

The polyphenolic compounds (+)-catechin and (−)-epicatechin gallate (ECg), constituents of Japanese green tea (Camellia sinensis), possess weak anti-staphylococcal activity. ECg has, in addition, the capacity to reduce oxacillin resistance in Staphylococcus aureus [1], [2], [3], [4]. At ECg concentrations as low as one-tenth of the minimum inhibitory concentration an oxacillin-resistant isolate, such as EMRSA-16, can become oxacillin-susceptible [4]. In contrast, (+)-catechin does not modulate oxacillin resistance. The mechanism by which ECg modulates oxacillin resistance remains to be determined, although oxacillin-resistant S. aureus grown in the presence of sub-inhibitory concentrations of ECg have thickened cell walls and form pseudomulticellular aggregates [5]. Recent studies suggest that the capacity of a related compound, (−)-epigallocatechin gallate, to modulate oxacillin-resistance is governed by binding of the compound to peptidoglycan [6]. However, this does not appear to be the mechanism of action of ECg (Stapleton PD, Shah S, Hamilton-Miller JMT, Taylor PW. In: Proceedings of the 42nd Interscience Conference on Antimicrobial Agents Chemotherapy; 2002 [abstract 1061]).

Several studies have shown that catechins, including ECg, are capable of binding to artificial lipid bilayers [7], [8], [9], [10]. Binding affinity appears to correlate with activity; ECg has a greater propensity to intercalate into the phospholipid palisade than catechin [7]. It is therefore possible that the modulating capacity of ECg could be attributable, whole or in part, to intercalation of the compound into the staphylococcal cytoplasmic membrane.

The incorporation of fatty acid hydrocarbon chains into compounds can increase their propensity to interact with lipid bilayers. This strategy has been successfully used to increase the bactericidal activity of the synthetic antimicrobial peptide of human cathepsin G [11]. In this study, we have synthesised 3-O-acyl derivatives of catechin with various chain lengths to evaluate whether the anti-staphylococcal activity of the catechin could be improved. We have found that fatty acid chain lengths of C₈ and C₁₀ significantly enhanced the direct anti-staphylococcal activity of catechin but the derivatives had no capacity to modulate oxacillin resistance.