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Background   Ixodes scapularis  is the most common tick species in North America and a vector of important pathogens that cause diseases in humans and animals including Lyme disease, anaplasmosis and babesiosis. Tick defensins have been identified as a new source of antimicrobial agents with putative medical applications due to their wide-ranging antimicrobial activities. Two multigene families of defensins were previously reported in  I. scapularis . The objective of the present study was to characterise the potential antimicrobial activity of two defensins from  I. scapularis  with emphasis on human pathogenic bacterial strains and important phytopathogenic fungi.    Methods  Scapularisin-3 and Scapularisin-6 mature peptides were chemically synthesised.  In vitro  antimicrobial assays were performed to test the activity of these two defensins against species of different bacterial genera including Gram-positive bacteria  Staphylococcus aureus ,  Staphylococcus epidermidis , and  Listeria spp.  as well as Gram-negative bacteria  Escherichia coli ,  Pseudomonas aeruginosa  along with two plant-pathogenic fungi from the genus  Fusarium . In addition, the tissue-specific expression patterns of Scapularisin-3 and Scapularisin-6 in  I. scapularis  midgut, salivary glands and embryo-derived cell lines were determined using PCR. Finally, tertiary structures of the two defensins were predicted and structural analyses were conducted.    Results  Scapularisin-6 efficiently killed  L. grayi , and both Scapularisin-3 and Scapularisin-6 caused strong inhibition (IC 50  value: ~1 μM) of the germination of plant-pathogenic fungi  Fusarium culmorum  and  Fusarium graminearum . Scapularisin-6 gene expression was observed in  I. scapularis  salivary glands and midgut. However, Scapularisin-3 gene expression was only detected in the salivary glands. Transcripts from the two defensins were not found in the  I. scapularis  tick cell lines ISE6 and ISE18.    Conclusion  Our results have two main implications. Firstly, the anti- Listeria  and antifungal activities of Scapularisin-3 and Scapularisin-6 suggest that these peptides may be useful for (i) treatment of antibiotic-resistant  L. grayi  in humans and (ii) plant protection. Secondly, the antimicrobial properties of the two defensins described in this study may pave the way for further studies regarding pathogen invasion and innate immunity in  I. scapularis .

Defensins from the tick Ixodes scapularis are effective against phytopathogenic fungi and the human bacterial pathogen Listeria grayi