Glycyrrhetinic acid protects mice from Staphylococcus aureus pneumonia

Abstract

In the present study, the antimicrobial activity of glycyrrhetinic acid (GA) against Staphylococcus aureus, and its influence on the production of S. aureus alpha-haemolysin (Hla) were investigated, along with the in vivo activity of GA against S. aureus-induced pneumonia. GA could not inhibit the growth of S. aureus, but the secretion of Hla by S. aureus was significantly inhibited by low concentrations of GA in a dose-dependent manner. Furthermore, in vivo data show that GA provides protection against staphylococcal pneumonia in a murine model system.

Introduction

Staphylococcus aureus is a major nosocomial pathogen. It causes numerous diseases that have high morbidity and mortality, such as toxic shock syndrome (TSS) and necrotising pneumonia [1], [2]. In the past few decades, antibiotics have been the chief method used against S. aureus infections. However, the appearance of methicillin-resistant S. aureus (MRSA), which was caused by the superfluous and unreasonable use of antibiotics, has reduced the effectiveness of traditional antibiotics in the treatment of staphylococcal infections. To address this crisis, the discovery of a new class of antibiotics with new targets seems necessary.

S. aureus produces a wide variety of exoproteins that contribute to its ability to colonise and cause disease in mammalian hosts. Alpha-haemolysin (Hla) (encoded by the hla gene) is a 33.2-kDa pore-forming cytotoxin which is produced by the majority of S. aureus strains. It is secreted as a water-soluble monomer, which binds to the membrane of the target cell [3]. After binding to the cell membrane, seven monomers interact to form a barrel-shaped pore that penetrates the membrane [4], [5]. The Hla pore complex can penetrate a broad range of host cell types, including erythrocytes and epithelial cells [6], [7]. Previous studies have indicated that Hla plays a central role in the pathogenesis of many S. aureus infections, such as pulmonary, intraperitoneal, intramammary, and corneal infection, as hla mutants are less virulent in animal models [8].

Liquorice is a popular Chinese herbal medicine derived from the dried roots and rhizomes of Glycyrrhiza uralensis, G. glabra, and G. inflata. The main bioactive components of liquorice are triterpene saponins and various types of flavonoids [9]. The triterpene saponins of liquorice include glycyrrhizin (GL). Glycyrrhetinic acid (GA) (Fig. 1) is the hydrolysis product of GL, and many of the properties of liquorice can be attributed to GA [10]. Recent research has indicated that GA and its related compounds have anti-inflammatory activity and also to inhibit liver carcinogenesis and tumour growth. GA and related compounds inhibited cell proliferation of HepG2, the human hepatoma cell line [11]. The pharmacokinetics of GA in humans and experimental animals have been well studied by Krähenbühl et al. [12], who defined the process as a biphasic elimination from the central compartment with a dose-dependent second elimination phase. Depending on the dose, GA has a half-life of 3.5 h in the second elimination phase in humans. Salari and Adkhoda reported that GA has potent antimicrobial activity against periodontopathogenic and capnophilic bacteria isolated from adult periodontitis [13]. However, no published work has addressed the effects of GA on S. aureus. In the present study, we investigated the anti-S. aureus activity of GA, and further assessed its influence on S. aureus Hla production. The in vivo effect of GA on S. aureus-induced pneumonia was also evaluated in a murine model.