Introduction

Lichens are obligate symbiotic systems consisting of a filamentous fungus and a photosynthetic partner (eukaryotic algae and/or cyanobacterium), and in some cases non-photosynthetic bacteria (Hodkinson and Lutzoni 2009; Selbmann et al. 2010). Lichens are ecologically diverse and are distributed from the tropics to the polar regions (Brodo et al. 2001). The worldwide lichen flora is estimated to include approximately 18,500 species (Boustie and Grube 2005; Feuerer and Hawksworth 2007) and cover about 8 % of the earth’s land surface (Ahmadjian 1995). Lichens are one of the slowest growing symbiotic associations and according to Coley (1988) slow growing organisms occupying low-resource habitats produce higher levels of defense chemicals in order to defend themselves from various consumers. This is certainly true in the case of lichens since they are known to produce more than a 1,000 different secondary metabolites. The main natural roles of these compounds include: protection against a large spectrum of microbes, animal predators and plant competitors; defense against environmental stress like UV radiation and desiccation; physiological regulation of metabolism, such as the ability to increase algal cell membrane permeability in order to increase the flow of nutrients to the fungal component (Huneck and Yoshimura 1996). As lichens are very capable of protecting themselves from various microbes including bacteria, non-lichenized fungi, and nematodes, the potential value of these metabolites for medicinal purposes is generating increasing research interest.

Lichens produce two different types of metabolites; primary and secondary. Primary lichen substances have structural functions and roles in cellular metabolism, similar to those in other fungi. Primary metabolites are intracellular in origin and are synthesized independently by both symbionts. Primary compounds consists mainly of chitin (in hyphal walls), lichenin, isolichenin, hemicellulose, pectins, disaccharides, polyalcohols, amino acids, enzymes, pigments like algal chromophores: chlorophyll and, β-carotenes, xanthophylls, etc. (Podterob 2008). In contrast, secondary metabolites are produced exclusively by the fungal partner and are exported outside the fungal hyphae and deposited as crystals in different parts of the thallus, often in the upper cortex or in specialized structures such as fruiting bodies (Fahselt 1994). The exclusive fungal origin of secondary metabolites has been confirmed based on the work of Culberson and Armaleo (1992); Hamada et al. (1996); Kon et al. (1997); Stocker-Wörgötter and Elix (2002). Although lichen secondary metabolites are exclusively of fungal origin, the metabolic interaction between the mycobiont and photobiont is essential to the production of these secondary chemicals. This has been documented by studies where the mycobiont grown without the photobiont does not produce the same metabolites as the intact lichen or produces a completely different suite of chemical products (Molina et al. 2003; Fazio et al. 2009). There are some situations where the photobiont, especially cyanobacteria, also produce some secondary metabolites (Cox et al. 2005; Yang et al. 1993).

Over 1,050 secondary metabolites have been reported for lichens and aposymbiotically cultured mycobionts (Molnar and Farkas 2010). Among them a relatively small number of these secondary products (50–60) occur in non-lichenized fungi or higher plants (Elix and Stocker-Wörgötter 2008). One example is the anthraquinone parietin which is present in other fungi like Aspergillus and Penicillium, as well as in the vascular plant genera Rheum, Rumex and Ventilago (Romagni & Dayan 2002). This metabolic diversity is due largely to the symbiotic relationship between the lichen partners (Lawrey 1986). Lichen secondary products can comprise up to 20 % of the thallus dry weight, but in most lichens the amount varies from 5 to 10 %.

The aim of this review paper is to provide insights regarding the antibacterial and anticancer properties of lichen chemicals (either as crude extracts or purified compounds) and also to provide information regarding the mode of action of lichen compounds against bacterial and cancer cells. The results of this review concerning the use of lichen compounds as antibacterial and anticancer therapies are based on a thorough examination of the published literature.

Role of lichens in medicine

Lichens have been used as ingredients in folk medicines for centuries and many cultures have used lichens to treat a variety of ailments as part of their traditional medicines (Dayan and Romagni 2001). The medicinal properties of some lichens are mentioned in the Ayurvedic and Unani systems where they are used to treat a broad array of common ailments, including blood and heart diseases, bronchitis, scabies, leprosy, asthma inflammations, stomach disorders, etc. (Shukla et al. 2010). Recent advances in the medical field have resulted in exploration of the biological activity of a limited number of lichen products with some studies suggesting that some lichen chemicals could possible provide a promising source of future drug therapies. Demonstrated medicinal properties based on lichen chemistry include :antibiotics (Balaji et al. 2006; Burkholder et al. 1944; Paudel et al. 2010; Turk et al. 2003), anti-proliferative (Bucar et al. 2004; Burlando et al. 2009; Kumar and Müller 1999), antioxidants (Bhattarai et al. 2008; Gulluce et al. 2006; Hidalgo et al. 1994), anti-HIV (Nakanishi et al. 1998; Neamati et al. 1997), anti-cancer (Bézivin et al. 2003; Bezivin et al. 2004; Mayer et al. 2005; O’Neill et al. 2010; Ren et al. 2009), and anti-protozoans (De Carvalho et al. 2005; Schmeda-Hirschmann et al. 2008). It should be noted that there have been reports of liver-related toxicity with the use of usnic acid in dietary supplements (Foti et al. 2008; Sanchez et al. 2006). However, other studies (Sahu et al. 2012) indicate that low non-toxic concentrations of usnic acid do not cause damage to the liver. This review paper focuses specifically on two particularly promising biological responses of lichen acids against pathogenic bacteria and cancer cell lines. We will particularly concentrate on which lichens (either as crude extracts or isolated compounds) have been tested against various bacteria and cancer cell lines while also considering the mechanism of cell death caused by the lichen metabolites.

Lichens as a source of antibiotics

The antibiotic properties of lichen metabolites represent one of the better studied biological roles for lichens. Initial testing of the antibiotic activity of lichens was done by Burkholder et al. (1944). They tested aqueous extracts of 42 lichen species against Staphylococcus aureus, Bacillus subtilis and Escherichia coli. Results from this early study showed that 27 lichens were active against S. aureus and/or B. subtilis but none of the species showed any activity against E. coli. Since Burkholder’s preliminary study various lichens (either as crude extracts or isolated compounds) have been screened against a variety of gram positive, gram negative, and mycobacteria with several lichen metabolites showing promise as potential antibiotics drug therapies (Table 1).

Table 1 A summary of the literature dealing with the antibiotic activity of lichen secondary chemistry
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The above table (Table 1) shows that lichen substances have been found to be effective against a variety of pathogenic bacteria, especially gram-positive bacteria. Table 1 also shows that among the various lichen compounds, usnic acid, pulvinic acid derivatives—vulpinic acid, lichesterinic acid (an aliphatic acid), and orcinol-type depsides and depsidones demonstrate the most promising antibiotic properties. Lichen compounds have not only shown their potency against sensitive strains of bacteria, but also against various multi-drug resistant bacterial strains (Martins et al. 2010; Kokobun et al. 2007). In addition, steps have also been taken to incorporate lichen metabolites into medical devices to enhance their activity against the formation of bacterial biofilms (Francolini et al. 2004) as well as in combination with other antibiotics for synergistic effect (Safak et al. 2009). Studies have also demonstrated that lichen compounds inhibit bacterial growth at much lower concentrations when compared to other sources of antibiotic therapies (Weckesser et al. 2007; Gordien et al. 2010). Although over 1,050 secondary metabolites have been identified from lichens, relatively small number (~50 species) have been screened for antibiotic activity. According to Vartia (1973) more than 50 % of the lichens tested show at least some antibiotic activity. These results demonstrate the need for further efforts in screening lichen extracts in general and specific lichen metabolites in particular. North America is home to a diverse assemblage of lichen species. In spite of the increased interest in the ecological and evolutionary role of North American lichens, the biological significance of their secondary metabolites remains largely unexplored. Our lab is presently screening 36 different lichen species collected from various parts of North American against four different bacterial strains—S. aureus, methacillin-resistant S. aureus COL, E. coli, and P. aeruginosa. Our preliminary results indicate that except for Lobaria pulmonaria all other lichen extracts showed at least some inhibition against all four bacterial strains at test concentrations of 500–7.8 μg/ml. Although there have been several studies showing effective inhibition of different bacterial strains by lichen metabolites, we did not find any studies that have specifically addressed the mechanism of action of these lichen compounds. We are currently exploring possible inhibition mechanisms related to lichen metabolites which cause bacterial cell death (S. aureus COL). We are specifically focusing on the effects of several promising lichen acids (e.g., Usnic acid and Vulpinic acid).

Lichens as a potential source of anti-cancer drug therapies

Worldwide, cancer is one of the most common causes of death. Focused efforts on identifying and developing new anticancer drug therapies from various natural sources, including vascular plants, fungi, prokaryotes, marine organisms, etc. are essential. Although representatives of these groups have already been screened and have been the source of pharmaceutically important anticancer drugs, there still remains a vast potential reservoir of untapped possibilities. Among the more promising possibilities are lichenized fungi with their more than 1,000 identified secondary chemicals. The use of lichen secondary products as anti-cancer drugs dates back to the late 1960s when the activity of lichen polysaccharides against tumor cells was initially explored (Fukuoka et al. 1968; Shibata et al. 1968). Similarly, Kupchan and Kopperman (1975) first reported the tumor inhibitor activity of Usnic acid extracted from Cladonia sp. against Lewis lung carcinoma. They reported a 35–52 % increase in the life span of treated mice versus the control group using a dose range of 20–200 mg/kg of usnic acid. Since these early studies many other lichens compounds, either in crude extract or purified form, have been screened against various malignant cell lines with several showing cytotoxic effects on various cancer cell lines (Bézivin et al. 2003; Bezivin et al. 2004; Kumar and Müller 1999; Zeytinoglu et al. 2008). Table 2 summarizes the results of the various studies examining the anti-cancer role of lichen metabolites.

Table 2 A summary of the literature dealing with the anti-cancer activity of lichen secondary chemistry
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Based on the information in the above table we can see that lichens are effective against various cancer cell lines both in crude form (Bézivin et al. 2003; Ren et al. 2009) and purified form (Bačkorová et al. 2011; Burlando et al. 2009; Russo et al. 2006). The literature also shows that lichen metabolites are strongly cytotoxic and have the capability of terminating cell proliferation at micro-molar concentrations (Einarsdóttir et al. 2010). Structural modification of lichen compounds has also been shown to enhance the cytotoxic capacity of many lichen compounds (Bazin et al. 2008; Tokiwano et al. 2009). In addition, the position of different functional groups in lichen compounds also affects levels of cytotoxicity (Correche et al. 2002). Regulation of the cell cycle is critical in controlling the growth and development of cancer cells. Various lichen acids have been found to stop cancer cell growth at the sub-G1 (Ren et al. 2009) or S-phase (Bačkorová et al. 2011; Liu et al. 2010) of the cell cycle. The mechanism of cell death in various cancer cell lines caused by lichen metabolites include apoptosis (Bačkorová et al. 2011; Bezivin et al. 2004; Russo et al. 2008), necrosis (Einarsdóttir et al. 2010; Russo et al. 2006), and angiogenesis inhibition (Koparal et al. 2010). Both caspase dependent (Correche et al. 2004; Liu et al. 2010; Russo et al. 2008) and caspase independent (Liu et al. 2010) pathways were found to initiate apoptosis. Caspase activation takes place along two different pathways—the cell membrane mediated death receptor pathway and mitochondria mediated pathways. There are no studies that document caspase activation though the death receptor pathway; however, there are evidences of mitochondria mediated caspase activation (Liu et al. 2010; Ren et al. 2009). Both studies showed that there is increase in the level of the Bax protein with an associated decline in the Bcl-2 protein. It is well documented that an increase in the Bax/Bcl-2 ratio can stimulate the release of cytochrome c from mitochondria into the cytosol, resulting in activation of caspase-3 which serves as an initiator of apoptosis. In addition to lichen secondary compounds, polysaccharides derived from lichens, especially β-glucan and galactomannan, have been shown to be active against several cancer cell lines (Nishikawa and Ohno 1981; Nishikawa et al. 1974; Watanabe et al. 1986). Recently, there has been additional research examining the use of lichen polysaccharides as immunostimulatory compounds and their potential role in fighting cancer (Olafsdottir and Ingolfsdottir 2001; Cordeiro et al. 2008; Karunaratne et al. 2012).

Future directions

In spite of the fact that lichens are one of the more promising reservoirs of low-molecular weight secondary compounds demonstrating some level of biological activity; a very limited number of compounds have been studied (Boustie and Grube 2005). Hence, there is an urgent need for: (1) continued screening of lichen metabolites across their diversity, (2) more in-depth studies of those compounds that have already shown promising activity against pathogenic bacteria and/or various cancer cell lines, (3) clinical trials for those compounds that have shown significant activity, and finally (4) commercial production and implementation of effective drug lines. One of the key issues in the development of therapeutic drugs is to understand how a drug interacts with our innate and adaptive immune system to combat various pathogens and cancer cells. Recently there has been growing interest in the immunostimulant role of lichen compounds especially lichen polysaccharides (Omarsdottir et al. 2007; Choi et al. 2009; Kim et al. 2010; Karunaratne et al. 2012).

One of the main issues related to the limited use of lichens compound in modern medicine is related to their slow growth rate and challenges with in vitro propagation. However, with recent advancements in technology, culturing lichens in the laboratory is achieving greater success (Behera et al. 2006; Stocker-Wörgötter 2001, 2008; Stocker-Wörgötter and Elix 2002; Yamamoto et al. 1985, 1987, 1993). Similarly, Miao et al. (2001) reviewed the possibilities of using molecular genetic techniques as an alternative approach for exploring the diversity of polyketide biosynthetic pathways in lichens. This approach can be extended to examine other pathways which can then be integrated with conventional culture methods. Also according to Miao et al. (2001) lichen genes can be introduced into a surrogate host with good fermentation characteristics and a well characterized endogenous chemical profile like Aspergillus nudulans, Neurospora crassa, Saccharomyces cerevisiae, E. coli, Streptomyces spp. etc. to produce promising lichen metabolites in larger quantities. Furthermore, researchers have now been able to synthesis usnic acid in the laboratory from commercially available starting materials. The synthesis involves the methylation of phloracetophenone followed by oxidation with horseradish peroxidase (Hawranik et al. 2009). This work also provided the impetus for synthesizing other lichen metabolites. With these recent advancements in technology, the development of cost effective options for growing and harvesting lichen metabolites commercially as a source of effective drugs against pathogenic bacteria and various forms of cancer show real promise.