Effects of lichen secondary metabolites on bacterial functions and biofilm formation

Long-living and slow growing lichens accumulate a diverse array of secondary metabolites in their symbiotic structures. These chemical products may participate in multiple functions of lichen biology, including defence against parasites. In fact, many previous studies showed various degrees of in vitro antibacterial effects of extracted secondary compounds pioneered by Burkholder et al. [1] and incorporation of lichen compounds as usnic acid into medical devices has been shown to inhibit bacterial biofilm formation on polymer surfaces [2]. However, further details are still unclear today and we have investigated the effect of some lichen compounds on biofilm formation looking for the interference with quorum sensing (QS) and influences on other bacterial functions. For this purpose we used highly purified lichen compounds and bacterial screening systems (Pseudomonas aeruginosa, Burkholderia cenocepacia, Vibrio fischeri and Chromobacterium violaceum). Initial studies focused on lactonic lichen compounds and revealed that protolichesterinic acid interferes with QS-systems of P. aeruginosa and B. cenocepacia. Vulpinic acid specifically inhibits production of the pigment violacein in Chromobacterium violaceum (also in Janthinobacterium), without effect on the QS-system. The potential utility of lichen compounds in bacterial biofilm regulation is highlighted.