Abstract Resistant bacterial infections are a major public health problem worldwide, which entails the need to search for new therapeutic agents. In this context, lichens stand out, provided that they are producers of structurally diverse compounds that have attractive biological properties, including antimicrobial activity. Thus, extracts of 12 lichen species were prepared and their potential to inhibit the growth of 5 bacterial strains was evaluated in this work. The chemical compositions of these extracts were examined using TLC and microcrystallization, being the identity of the active compounds in each extract attributed based on the bioautography technique. The most active extracts (and their identified active compounds) were from Cladonia borealis (usnic, barbatic and 4-O-demethylbarbatic acids), Cladina confusa (usnic and perlatolic acids), Stereocaulom ramulosum (atranorin, perlatolic and anziaic acids) and Canoparmelia cryptochlorophaea (cryptochlorophaeic and caperatic acids), with MICs ranging from 7.8 to 31.25 μg/mL, including for resistant clinical strains. MIC values were also obtained for substances isolated from lichens for comparison purposes. A group of four extracts containing usnic acid was analyzed by 1H NMR in order to correlate relative proportion of major metabolites and extracts activity. The less active extracts in this group, in fact, presented low proportion of usnic acid.

Extracts of six lichen species collected from Brazil and Antarctica were investigated for their potential toxicity and radical-scavenging properties. The composition of the extracts was investigated using TLC and NMR, leading to identification of atranorin (1), along with salazinic (2), barbatic (3), α-alectoronic (4), α-collatolic (5), cryptochlorophaeic (6), caperatic (7), lobaric (8), and protolichesterinic (9) acids. All acetone extracts were evaluated for their 2,2'-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging ability and subjected to Artemia salina bioassay. The free-radical-scavenging activities of each extract (100 µg) ranged from 8.9 ± 0.1% to 38.7 ± 2.5% and the EC50 values ranged from 0.24 ± 2.10 to 3.54 ± 0.28 mg mL-1, while the toxicity of the extracts against A. salina were low (151.0 to >600 µg mL-1).