Antibacterial and antibiotic resistance modulatory activities of leaves and bark extracts of Recinodindron heudelotii (Euphorbiaceae) against multidrug-resistant Gram-negative bacteria

Infectious diseases caused by multidrug-resistant (MDR) bacteria constitute nowadays a real public health concern all over the world. These bacteria drastically reduced the efficacy of antibiotic arsenal, consequently, increasing the frequency of therapeutic failure and mortality [1, 2]. In the European Union, it was estimated that 25,000 patients die annually due to infections with MDR bacteria [3]. Among these MDR bacteria, Gram-negative MDR bacteria drastically impair the efficacy of antibiotic families and consequently limit their clinical uses [4, 5]. Due to these facts, scientists are in the quest for new antimicrobial substances. Nature is a source of medicinal agents since times immemorial. The screening of plant extracts and natural products for their antimicrobial activity has shown that higher plants represent a potential source of novel antibiotic prototypes [6‐8]. In the last decades, a number of studies conducted, demonstrated that plants as well as their derived products could possess direct antimicrobial activity and resistance modifying effects [9‐14]. Thus, with increased incidence of resistance to antibiotics, natural products from plants could be an interesting alternative.

Recinodindron heudelotii (Baill.) Pierre ex Pax. known as “Djansang or Essessang” in different area in Cameroon, is a tree belonging to Euphorbiaceae family. With 20–30 m of height, that plant grows throughout the humid lowland rainforest of Cameroon [15, 16]. This plant is traditionally used in Cameroon and certain countries in Africa to treat cough, intestinal disease, dysentery and as antidote [15, 17]. Furthermore, R. heudelotii is also used to ease delivery, treat diseases such as malaria, anaemia, stomach pain, yellow fever and as aphrodisiac. Its seeds are also used as food ingredient [16]. R. heudelotii is well documented for some pharmacological properties among which antimicrobial [18, 19] and antioxidant activities [20]. In the continuous search for antibacterial agents from that plant, we have designed this study to investigate the in vitro antibacterial and antibiotic resistance modulating activities of the methanol extracts from leaves and stem bark of R. heudelotii against MDR Gram-negative bacteria.

The objective of this study was to evaluate the methanol extracts from the leaves (RHL) and bark (RHB) of R. heudelotii for their antibacterial and antibiotic resistance modulating activities against a panel of Gram-negative bacteria including multidrug resistant (MDR) phenotypes. For phytochemical agents, MICs ranging from 100 to 1000 μg/mL obtained after susceptibility tests indicate their antimicrobial activities [26]. Thus, leaves extract of R. heudelotii (RHL) displayed antibacterial activity against 75.86% of the tested bacteria whereas its bark extract (RHB) was not active. According to the cut-off value of MICs for extracts as proposed by Kuete [8], RHL presents moderate antibacterial activity (100 ≤ MIC ≤625 μg/mL) against the tested bacteria. A keen look the MICs and MBCs of RHL (Table 1) indicated that it possesses bactericidal or killing effects (MBC/MIC ≤4) [27]. R. heudelotii bark extract was previously documented for its low antimicrobial activity against some pathogenic bacteria including Gram-negative and Gram-negative bacteria [18, 19]. This confirms low antibacterial activity observed with its bark methanol extract (RHB) in the present study. By considering the MDR features of the tested bacteria, RHL could be a source for the development of new antibacterial agents.

The antibiotic resistance modulating effects of the extracts or natural compounds from medicinal plants against resistant bacteria have been already reported [11, 13, 28‐30]. In this work, the combination of extracts with antibiotics has shown that extract of the leaves of R. heudelotii (RHL) modulated 2 to 16 folds the activity of TET, KAN and CHL against selected MDR bacteria. Based on the previous work, one of the mechanisms of action of plant extracts associated with antibiotics may be the disruption of the membrane structure and the bacterium cell by the extract, increasing influx of antibiotics inside the bacteria [30]. These actions are generally attributed to some terpenoids [29] and lipophilic flavonoids [31], which can cause a disruption of the plasma membrane of the microorganisms. The extracts or natural compounds can also exert their modulating effects by inhibiting bacterial efflux pumps, allowing an increase of the intracellular concentrations of the antibiotics [24, 32]. RHL has potentiated the activity of TET and KAN on more than 70% of the tested MDR bacteria. This suggests that some compounds of that extract may act as efflux pump inhibitors [33]. View that it is the first time here to report the potential of the R. heudelotii leaves extract (RHL) to reverse antibiotic resistance in MDR bacteria, that plant extract could be used for the screening of antibiotic modulators, especially efflux pumps inhibitors.

Various classes of phytochemical compounds were previously found in the tested plant extracts [34]. The most active extract (RHL) contained terpenoids and saponins absent in the bark extract (RHB). This suggests that the pronounced antibacterial activity as well as antibiotic-modulating effects of the leaves extract of R. heudelotii could be due to the presence of these metabolites.