Targeting ESKAPE pathogens with anti-infective medicinal plants from the Greater Mpigi region in Uganda

Abstract

Antibiotic resistance poses one of the greatest threats to global health today; conventional drug therapies are becoming increasingly inefficacious and limited. We identified 16 medicinal plant species used by traditional healers for the treatment of infectious and inflammatory diseases in the Greater Mpigi region of Uganda. Extracts were evaluated for their ability to inhibit growth of clinical isolates of multidrug-resistant ESKAPE pathogens. Extracts were also screened for quorum quenching activity against S. aureus, including direct protein output assessment (δ-toxin), and cytotoxicity against human keratinocytes (HaCaT). Putative matches of compounds were elucidated via LC–FTMS for the best-performing extracts. These were extracts of Zanthoxylum chalybeum (Staphylococcus aureus: MIC: 16 μg/mL; Enterococcus faecium: MIC: 32 μg/mL) and Harungana madagascariensis (S. aureus: MIC: 32 μg/mL; E. faecium: MIC: 32 μg/mL) stem bark. Extracts of Solanum aculeastrum root bark and Sesamum calycinum subsp. angustifolium leaves exhibited strong quorum sensing inhibition activity against all S. aureus accessory gene regulator (agr) alleles in absence of growth inhibition (IC₅₀ values: 1–64 μg/mL). The study provided scientific evidence for the potential therapeutic efficacy of these medicinal plants in the Greater Mpigi region used for infections and wounds, with 13 out of 16 species tested being validated with in vitro studies.

Introduction

The rise of antimicrobial resistance (AMR) requires mobilization of political, financial and research investment due to its emergence as a global health hazard that threatens the ability to treat infectious diseases¹. According to the World Health Organization, AMR poses “one of the biggest threats to global health, food security, and development today” and can affect anyone in any country and of any age². Today, AMR already accounts for 700,000 deaths annually. By 2050, this figure is estimated to reach more than 10 million deaths per year, which is more people than currently die from cancer³. Because effective antibiotics are critical for treatment of bacterial infections and for procedures where there is a high risk of infection, e.g. surgery, new anti-infectives are needed to overcome this global threat⁴. The issue of resistance is not uniformly spread across all bacteria⁵. Six species have been identified by the Infectious Disease Society of America (IDSA) as being especially dangerous due to their potential multidrug resistance mechanisms and virulence. They are referred to as ‘ESKAPE’ pathogens, which is an acronym for Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter species. This group of pathogenic bacteria encompasses both Gram-negative and Gram-positive species that are capable of ‘escaping’ bactericidal action of conventional antibiotics^6,7. ESKAPE pathogens are common causes of deadly or life-threatening infections, especially among children, immunocompromised, and critically-ill people⁸.

Antibiotics are not the only anti-infectives that could provide an effective weapon against these pathogens. Another therapeutic, yet non-antibiotic, strategy is targeting bacterial virulence controlled by quorum sensing processes. The quorum-sensing mechanism mediated by signal molecules regulates the expression of virulence genes in the majority of pathogenic bacteria, meaning that quorum-sensing inhibitors are expected to be one of the best alternatives to antibiotics^9,10. Autoinducers, self-secreted signal molecules, are regulated by a density-dependent synchronized gene expression system during quorum sensing¹¹. Biofilm formation, toxin production and other virulence factors are controlled by quorum sensing and the production of virulence factors can weaken the balance of host defense mechanisms⁹. Initiation of toxin production occurs when extracellular signaling and communication indicates that a threshold population of bacteria has been achieved¹². Inhibition of quorum sensing induced by secondary plant metabolites can significantly attenuate bacterial virulence and substantially enhance vulnerability to conventional antibiotics and to the immune system