Abstract

Staphylococcal biofilms are problematic and play a critical role in the persistence of chronic infections because of their abilities to tolerate antimicrobial agents. Thus, the inhibitions of biofilm formation and/or toxin production are viewed as alternative means of controlling Staphylococcus aureus infections. Here, the antibiofilm activities of 560 purified phytochemicals were examined. Alizarin at 10 μg/ml was found to efficiently inhibit biofilm formation by three S. aureus strains and a Staphylococcus epidermidis strain. In addition, two other anthraquinones purpurin and quinalizarin were found to have antibiofilm activity. Binding of Ca2+ by alizarin decreased S. aureus biofilm formation and a calcium-specific chelating agent suppressed the effect of calcium. These three anthraquinones also markedly inhibited the hemolytic activity of S. aureus, and in-line with their antibiofilm activities, increased cell aggregation. A chemical structure-activity relationship study revealed that two hydroxyl units at the C-1 and C-2 positions of anthraquinone play important roles in antibiofilm and anti-hemolytic activities. Transcriptional analyses showed that alizarin repressed the α-hemolysin hla gene, biofilm-related genes (psmα, rbf, and spa), and modulated the expressions of cid/lrg genes (the holin/antiholin system). These findings suggest anthraquinones, especially alizarin, are potentially useful for controlling biofilm formation and the virulence of S. aureus.

Highlights

  • Staphylococcal biofilms are problematic and play a critical role in the persistence of chronic infections because of their abilities to tolerate antimicrobial agents

  • Further experiments showed that the addition of alizarin (0, 1, 2, 5, 10, 50, or 100 μ g/ml) at the beginning of bacterial culture dose-dependently inhibited biofilm formation by all three S. aureus strains (MSSA 6538, methicillin-sensitive S. aureus strains (MSSA) 25923, and methicillin-resistant S. aureus (MRSA) MW2) and a S. epidermidis strain (ATCC 14990) (Fig. 1a–d)

  • Alizarin decreased biofilm formation by all three S. aureus strains by ≥ 90%, whereas in the case of S. epidermidis, 50 μ g/ml was required to inhibit biofilm formation by ≥ 70%

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Summary

Introduction

Staphylococcal biofilms are problematic and play a critical role in the persistence of chronic infections because of their abilities to tolerate antimicrobial agents. Binding of Ca2+ by alizarin decreased S. aureus biofilm formation and a calciumspecific chelating agent suppressed the effect of calcium These three anthraquinones markedly inhibited the hemolytic activity of S. aureus, and in-line with their antibiofilm activities, increased cell aggregation. S. aureus, examples include; magnolol[10], ellagic acid[11], tannic acid[12], quercetin[13], ginkgolic acids[14], eugenol[15], and flavonoids[16] It has been reported staphylococcal biofilm formation is inhibited by several plant essential oils[17,18,19,20,21,22]. The identification of active compounds in plant extracts and essential oils often requires extensive investigation to identify active components, and only a limited number of organic biofilm inhibitors have been identified

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