Rhamnolipids (RLs) are glycolipid biosurfactants produced by various bacterial species including some Pseudomonas sp. and Burkholderia sp. [1]. The structure of RLs is highly diverse and those produced by Pseudomonas aeruginosa have been extensively studied. These RLs are amphiphilic molecules typically composed of 3-hydroxyfatty acids linked through a beta-glycosidic bond to mono- or di-rhamnoses (Figure 1) [2]. RLs have several potential functions in bacteria. They are involved in the uptake and biodegradation of poorly soluble substrates and are essential for surface motility and biofilm development [1]. From a biotechnological point of view, RLs are powerful biosurfactants with applications related to environmental concerns, such as bioremediation of hydrocarbon, organic pollutants and heavy-metal-contaminated sites. These topics have been extensively reviewed including some very recent articles [3–6]. RLs have also been used in the production of fine chemicals, surface coatings, as well as additives for food and cosmetics [7]. Finally, a new role for RLs as potential players in the combat of plants and animals against microbes has recently emerged. For years RLs have been extensively studied regarding their direct toxicity to microorganisms but recently they have also been reported to be involved in the stimulation of plant and animal defense responses. The present review provides an update of the current knowledge on the antimicrobial properties of RLs and also highlights the recent discoveries of the involvement of these molecules in the stimulation of immunity in plants and animals. The potential use of these molecules to fight against pathogenic microorganisms in medical and agricultural field will be discussed.

RLs have been shown to display antibacterial activities against plant and human pathogenic bacteria. RLs are known to be active against the Gram-negative bacteria P. aeruginosa, Enterobacter aerogenes, Serratia marcescens and Klebsiella pneumonia, as well as against Gram-positive Micrococcus sp., Streptococcus sp., Staphylococcus sp. and Bacillus sp [8–13] (Table 1). RLs have direct impact on bacterial cell surface structures. Al-Tahhan et al. [14] observed a loss of lipopolysaccharides (LPS) in P. aeruginosa strains treated with RLs at low concentrations and this resulted in increased cell surface hydrophobicity. Recently, Sotirova et al. [15] showed that RLs from Pseudomonas sp. PS-17 interact with P. aeruginosa causing a reduction in LPS content and changes in the outer membrane proteins of the bacteria. These changes had a direct impact on bacterial cell surface morphology. Sotirova et al. [15] concluded that RLs from Pseudomonas sp. PS-17 have a potential application in the field of biomedicine against pathogenic bacteria. Several studies described antifungal activity of RLs mainly against phytopathogens including Botrytis sp., Rhizoctonia sp., Pythium sp., Phytophtora sp. and Plasmopara sp. (Table 1) [16–22]. Additionally, RLs were also shown to be active against Mucor miehei and Neurospora crassa [12]. The main mode of action of RLs against zoospore-producing plant pathogens is the direct lysis of zoospores via the intercalation of RLs within plasma membranes of the zoospore which are not protected by a cell wall [16,21,23]. Recent studies also demonstrated an effect of RLs in the reduction of mycelia growth of Pythium myriotylum [18] and Botrytis cinerea [23]. These data suggest that RLs may also have an adverse effect on cell structures that are protected by a cell wall. Properties of RLs against the algae Heterosigma akashiwo, viruses, amoeba like Dictyostelium discoideum and mycoplasma have also been reported [24–29]. However, RLs’ applications have no significant effects on yeasts [10,12,17,28]. In addition to their in vitro antimicrobial activity, RLs have proven to be also efficient in in vivo plant systems. Treatments with RLs have been shown to protect pepper plants from Phytophthora blight disease and also prevent the development of Colletotrichum orbiculare infection on leaves of cucumber plants [17]. Yoo et al. [22] investigated RLs as alternative antifungal agents against typical plant pathogenic oomycetes, including Phytophthora sp. and Pythium sp. They showed that RLs significantly decrease the incidence of water-borne damping-off disease. Sharma et al. [19] obtained similar results in field trials on chili pepper and tomato. Using bacterial mutants, Perneel et al. [18] clearly showed that phenazine and RLs interact in the biological control of soil-borne diseases caused by Pythium spp. Recent studies also demonstrated that a combination mixture of SRE (Syringomycin E) and RLs is efficient against pathogenic and opportunistic fungi recovered from diseased grape [30,31].

During the last decade, pattern recognition emerged as a fundamental process in the immune response of plants and animals. Perception by pattern recognition receptors (PRRs) of molecular signatures that identify whole classes of microbes but are absent from the host allows this nonself recognition [32,33]. Once recognized, these molecular signatures, conventionally named microbe-associated molecular patterns (MAMPs) [34], trigger complex signaling pathways leading to transcriptional activation of defense-related genes and accumulation of antimicrobial metabolites in plant cells [32]. In mammals, MAMP perception leads to the inflammatory response with the production of cytokines including interleukins and the tumor necrosis factor α (TNFα). Years ago, lipopeptides were shown to stimulate human innate immune responses through the PRR Toll-like receptor TLR2 perception, by activating the transcriptional activator of multiple host defense genes NFkB, the production of interleukin (IL)-12 and the respiratory burst [35–39]. Lipopeptides are also involved in the stimulation of innate immunity in plants [40]. It is quite recent that RLs have been shown to be involved in triggering plant and animal defense responses and can be described as a new class of MAMPs.

Major breakthroughs allowing production, separation and purification of RLs in industrial quantities and laboratory purities have allowed the application of these molecules in different fields from cosmetic to industrial and more recently from agriculture to medicine. As previously stated, the major advantage of using RL biosurfactants, which have diverse roles in plant and animal systems, is that they are natural and organic biodegradable compounds, originating from a large number of bacteria [1]. RLs have also been proposed to be used in food industry applications [12]. RLs have a direct biocide action on bacteria and fungi. They also increase the susceptibility of certain Gram-positive bacteria to specific antibiotics. RLs have been demonstrated to control zoosporic pathogens through lysis of their zoospores [21]. Clinical trials using RLs for the treatment of psoriasis, lichen planus, neurodermatitis and human burn wound healing have confirmed excellent ameliorative effects of RLs when compared to conventional therapy using corticosteroids [54,55]. RLs also display differential effects on human keratinocyte and fibroblast cultures [55]. The advantages of these biosurfactants are low irritancy and even anti-irritating effects, as well as compatibility with human skin [55]. Moreover, RLs have permeabilizing effects on Gram-positive and Gram-negative human bacterial strains, reinforcing their potential in biomedicine [20]. An important issue to be taken into account is the study of side effects of biosurfactants on plants and animals. Attention should be paid while using surfactants on plants as the latter could be affected in many different ways. Parameters like negative impact on crop yield or other important agronomical traits should not be neglected and should be studied in parallel to avoid any impact on plant growth or metabolism, while boosting plant immunity. For instance, it is known that high concentrations of RLs cause necrosis in plants [23]. Dose/response experiments in the field are a necessity in order to ensure use of non-toxic concentrations of RLs. In addition, in animal systems, RLs are known as virulence factors especially for immunocompromised patients and individuals suffering from cystic fibrosis (CF) [1]. At some concentrations, RLs also have hemolytic activity [56,57]. Thus, care should be taken in the use of RLs, albeit some applications such as fungicide and bactericide are already considered especially for skin treatments [54,55].

RLs are new actors in animal and plant defense and their low toxicity and biodegradability make them promising molecules to be used against pathogens. In this respect, there are some clues now available for the success of RL applications in greenhouses to fight phytopathogens. A better understanding of RL mode of action, especially their perception and the signaling pathways activated, will be very important to potentiate their beneficial effects in plants. RLs have a dual mode of action: they are antimicrobial and also stimulate plant defense responses. This dual property is probably very important for the efficiency of new biopesticides. In animals, the use of RLs is also at an advanced stage. RLs are successfully used as antimicrobial agents, especially for skin disease treatment. Deep insight into the physiochemical effects of RLs and their biological importance would reveal new dimensions in the fields of research like agriculture and medicine, precisely in plant defense, disease control and pathogenesis. An understanding of bacterial genera producing RLs that are not yet well studied would provide light on these fascinating aspects.