Antivirulence is the concept of blocking virulence factors produced by pathogenic organism. In regards to bacteria, the idea is to design agents that block virulence rather than kill bacteria population that generate more selective pressure leading to antibiotic resistance.
African plants, through their huge biodiversity, present a considerable reservoir of secondary metabolites with a very broad spectrum of biological activities, a potential source of natural products targeting such non-microbicidal mechanisms.
Antimicrobial resistance, increasingly observed within a wide range of pathogenic bacteria, has become a worldwide threat to public health [1,2]. Over time, bacteria adapt to the drugs that are designed to kill them, evolving or selecting resistance mechanisms to ensure survival. The resistance of bacteria to antibiotics is a naturally occurring phenomenon, supposedly progressive over contact with antibiotics. However, the misuse and abuse of antibiotics led to a strong and rapid selective pressure, leading to an uncontrolled widespread development of antibiotic-resistant bacteria [3,4]. Beyond resistance to antibiotics, the ability of bacteria to develop effective biofilms represents one of the major obstacles in the fight against bacterial infections. Indeed, while planktonic lifestyle allows bacteria to easily diffuse in diverse environments, their biofilm lifestyle allows efficient colonization of biotic and abiotic surfaces and protection from environmental aggression .
Undoubtedly, whenever new antimicrobial compounds would be discovered, their use will result in selective pressures, probably leading targeted bacteria to develop a resistance to these agents. This likely outcome stirs researchers to consider other strategies, notably based on the search for original compounds that impair virulence expression mechanisms and/or biofilm formation without affecting bacterial viability [6,7]. Striking such targets will likely impact invasion capabilities and aggressiveness of bacteria, as well as their ability to build protective barriers against host immune defenses or antibiotics; all the while, selective pressure would be minimized , most probably preventing or slowing down the apparition and spread of resistances.
The expression of bacterial virulence factors is generally coordinated by quorum sensing (QS) mechanisms, a cell-to-cell communication system that allows bacteria to detect their population density by producing and perceiving diffusible signal molecules that synchronize common behaviors . Depending on bacteria species, QS regulates the production of virulence factors, motilities, and/or biofilm formation. Thus, the disruption of QS signaling, also termed quorum quenching (QQ), appears as interesting adjuvant strategies in the fight against bacterial infections .
Over millions of years of co-evolution, plants accumulated highly diverse secondary metabolites (so-called ‘natural products’), developing means of surviving in hostile environments that combine herbivorous insects and pathogenic bacteria, fungi, and viruses . Given the huge diversity of flora and ecosystems in the world, the plants likely represent significant sources of innovative compounds with antivirulence properties. Indeed, several studies have already reported natural compounds, mainly isolated from plants, and synthetic compounds interfering with bacterial virulence [5,11]. For instance, ajoene, an allyl sulfide isolated from garlic (Allium sativum L., Liliaceae) and baicalin, a flavone glycoside isolated from Huangqin (Scutellaria baicalensis Georgi, Lamiaceae) have been reported to inhibit both virulence factors production and biofilm formation in P. aeruginosa through QQ pathways [12,13].
From an estimated African biodiversity of ~45,000 plant species, only 5000 have documented medicinal use . The list of drugs provided by the African flora appears quite short (less than 100 active compounds)  compared with those from other traditional medical systems such as Traditional Chinese Medicine (more than 2000 active compounds from Chinese herbal) , suggesting an unrivalled opportunity for the discovery of new drugs.
|Burkinabe Medicinal Plant||Plant Part||Tested Extract and Concentration||Production of Violacein in C. violaceum CV026||Production of Pyocyanin in P. aeruginosa PAO 1||Production of Elastase in P. aeruginosa PAO1||Production of Biofilm in P. aeruginosa PAO 1||References|
|Acacia dudgeoni Craib. ex Holl.||Stem bark||Methanol
50–400 µg/mL 2,3
|−25% to −69%||−33% to −66%||NC||−25% to −59%|||
|Balanites aegyptiaca (L.) Delille.||Leafy galls 1||Methanol
100 µg/mL 2
100 µg/mL 2
|Crossopteryx febrifuga (Afzel ex G. Don) Benth||Leave and stem||Methanol
100 µg/mL 2
|Terminalia leiocarpa (DC.) Baill. [synonym of Anogeissus leiocarpus (DC) Guill. et Perr.]||Stem bark||Methanol
100 µg/mL 2
|Terminalia macroptera Guill. and Perr.||Stem bark||Methanol
100 µg/mL 2
|Vachellia seyal (Delile) P.J.H.Hurter [synonym of Acacia seyal Delile]||Bark||Methanol
50–800 µg/mL 2,3
|−25% to −97%||−22% to −86%||−8% to −56%||At 800 µg/mL: −69%|||
|Zanthoxylum zanthoxyloides (Lam) Zepern. and Timler||Stem bark||Methanol
100 µg/mL 2
Several active fractions containing flavonoid-like compounds, obtained from the bark of Combretum albiflorum (Tul.) Jongkind, (Combretaceae), a Madagascar endemic plant , were found to inhibit the production of QS-regulated extracellular virulence factors (violacein in C. violaceum CV026 and pyocyanin in P. aeruginosa PAO1) . Among these flavonoids, the flavan-3-ol catechin, at 4 mM in P. aeruginosa PAO1, had a negative impact on the production of violacein (75 % inhibition), pyocyanin (50% inhibition), elastase (30 % of inhibition), on biofilm formation (30 % inhibition) and on the transcription of several QS-related genes (i.e., lasI, lasR, rhlI, rhlR, lasB, and rhlA). Synthetic epicatechin reduces the P. aeruginosa production of pyocyanin (50 % inhibition) and elastase (30 % inhibition) without effect on biofilm formation. By contrast, epicatechin isolated from Ficus sansibarica Warb. (Moraceae), collected in KwaZulu-Natal, South Africa, reduces the adhesion to polystyrene surfaces of Gram negative E. coli ATCC 25922 up to 15 % at 3.4 mM .
The methyl gallate (MG) isolated from the galls (produced on leaves following insect attack) methanol extract of Guiera senegalensis J. F. Gmel (Combretaceae), a traditional burkinabe treatment of cough, dysentery and malaria, has been shown to exert antivirulence activities , Methyl gallate presents MIC values of 512 and 64 µg/mL against P. aeruginosa PAO1 and C. violaceum CV026, respectively, but, at 12.5 µg/mL (67.9 µM), already inhibits the production of pyocyanin (by 65 %) and violacein (by 10 %). These antivirulence activities are in correlation with the data of Hossain et al.  who showed that the production of pyocyanin was inhibited (37 - 64 %) by MG in a concentration-dependent manner (16 - 256 μg/mL). Moreover, in P. aeruginosa PAO1, MG reduces the expression of the AHL synthetases genes (lasI and rhlI) and the QS regulator genes (lasR and rhlR) and biofilm formation.A triterpenoid coumarate ester has been isolated from Dalbergia trichocarpa Baker (Malagasy endemic species) bark extract as a major bioactive compound. Indeed, oleanolic aldehyde coumarate (OALC), at 200 µM, inhibits the formation of P. aeruginosa PAO1 biofilm (by 44 %) and its maintenance as well as the expression of the las and rhl QS systems . As a consequence, the production of QS-controlled virulence factors, including rhamnolipids, pyocyanin, elastase and extracellular polysaccharides, as well as twitching and swarming motilities are significantly reduced (75 %, 64 %, 19 %, 44 %, 40 % and 52 % inhibition, respectively). Additionally, OALC disorganizes established biofilm structure and significantly increases the bactericidal activity of tobramycin against biofilm-encapsulated PAO1 cells. Consistently, in vivo experiments indicated that OALC treatment reduces P. aeruginosa pathogenicity in Caenorhabditis elegans, a nematode.
The monocyclic diterpenoid cassipourol and the phytosterol β-sitosterol, isolated from Platostoma rotundifolium (Briq.) A. J. Paton (Lamiaceae), a Burundian anti-infectious plant , inhibit QS-regulated and QS-regulatory genes expression in las and rhl systems and disrupt the formation of biofilms by P. aeruginosa at concentrations down to 12.5 and 50 µM, respectively . Authors also isolated α-amyrin, a biosynthesis precursor of ursolic acid , that exerts antibiofilm properties at 50 µM without any effect on QS-regulatory genes expression; this suggests that other ursane and oleane-type triterpenes may exert antibiofilm properties with similar mechanisms of action. The three isolated compounds improve swimming but not twitching motilities which consequently promotes planktonic lifestyle in P. aeruginosa PAO1 and dispersal on preformed biofilms. Interestingly, the addition of cassipourol, α-amyrin and β-sitosterol (100 µM) considerably improved the effectiveness of tobramycin (50 µg/mL = 107 µM) against P. aeruginosa PA01 with a drastic reduction in cell viability of biofilm-encapsulated bacteria (89 %, 70 % and 76 % of bacterial death, respectively, versus 40 % in DMSO control treatment).
African plants screened so far provide a clear indication that we have a fairly large source of non-microbicidal natural products active on bacteria (Table 1 and 3). According to literature, the search for antivirulence activities have been shyly initiated since the last decade; by contrast, there has been a wide research on conventional antimicrobial activities (i.e, bactericidal activity) of African plants over the past thirty years [83-85]. Although African plants investigated for antivirulence activities are diverse (largely from Southern and Eastern African regions), very few studies have resulted in the characterization of the active compound(s), suggesting that this investigation is only beginning, which highlights a huge potential for new substances still to be discovered.
So far however, it should be acknowledged that the discovery of QS modulators has not yet led to major therapeutic breakthroughs; also, QS systems do not control the totality of virulence factors expression and the development of anti-QS bacterial resistance cannot be excluded . But this should not prevent further research in this promising field. Indeed, according to in vitro experiments, the combination of antibiotics and antivirulence (e.g. cassipourol) agents has already demonstrated its potential and about 33 compounds or agents, mainly from synthetic origin, that target virulence factors are under way for preclinical investigations, most of them focusing on P. aeruginosa, Enterobacteriaceae spp. and S. aureus [91-93].