Ilić B., Unković N., Ćoćić D., Vukojević J., Ljaljević Grbić M., Makarov S. 2020. Assessment of p-cresol and phenol antifungal interactions in an arthropod defensive secretion: the case of an endemic Balkan millipede, Apfelbeckia insculpta (L. Koch, 1867) (Diplopoda: Callipodida) // Arthropoda Selecta. Vol.29. No.4: 413–418 [in English].
University of Belgrade, Faculty of Biology, Studentski Trg 16, 11000 Belgrade, Serbia. Email: email@example.com
Abstract. Millipedes (Diplopoda) are a group of arthropods that produce and deploy chemically diverse exudates from defensive glands in the event of predator attack. These exocrine secretions are also potent antimicrobials. In view of the fact that the defensive secretion of the endemic Balkan millipede, Apfelbeckia insculpta (L. Koch, 1867) consists of only two compounds with known antifungal properties (p-cresol and phenol), it represents an ideal model for studying the contribution of individual compounds to the overall antifungal activity of this natural product, thereby enabling us to define the nature and type of interactions between the main compound (p-cresol) and the trace compound (phenol). Twenty-five combinations of concentrations, ranging from 0.1 to 1.0 mg mL–1, were tested on 14 filamentous fungi belonging to the genera Aspergillus, Cladosporium, Fusarium, Penicillium and Trichoderma; two yeasts (Sporobolomyces roseus and Meyerozyma guilliermondii); and one yeast-like filamentous fungus (Aureobasidium pullulans var. melanogenum) using the checkerboard method. Among the 29 interactions observed, the tested combinations of concentrations showed mainly additive (16 instances) and, to a lesser extent, indifferent (12 instances) properties, with notably 2–8 times lower concentrations of compounds needed to suppress fungal growth than those recorded for the individual compounds. A synergistic effect was observed only for Aspergillus niger when 0.1 mg mL–1 of p-cresol was supplemented with 0.2 mg mL–1 of phenol. Furthermore, A. niger was the only fungus where all three types of documented antifungal interactions, i.e., synergism, additivism and indifference, were observed. No antagonism between compounds was documented in any of the tested combinations. Meyerozyma guilliermondii was the only tested fungus where no interactions could be determined (MIC >1.0 mg mL–1).
KEY WORDS: allomones, antimycotics, checkerboard method, millipede, phenolics.