Inhibitory effects of the Iranian propolis ethanolic extract on different life stages of two Saprolegnia parasitica isolates recovered from rainbow trout (Oncorhynchus mykiss) eggs

The present study investigated the potential anti-Omycetes activity of the ethanolic extract of Iranian propolis on two Saprolegnia parasitica isolates (KMG2 and KMG3) obtained from infected rainbow trout eggs in Iran. The initial minimum inhibitory concentration (MIC) of propolis extract was determined by the hemp (Cannabis sativa L.) seed MicroPlate (HeMP) method. Then, the effect of propolis extract on hyphal radial growth and cyst germination was assessed by an agar dilution method and 48-well tissue culture plates, respectively. Also, the impact of propolis on the sporulation activity of Saprolegnia sporangia was evaluated by using Saprolegnia-colonized hemp seed in sterile distilled water (SDW). The HeMP method showed that propolis had some anti-oomycete activity on S. parasitica with a MIC value of 1000 ppm. According to the agar dilution method, complete inhibition of hyphal growth was achieved at 250 ppm for KMG2 and 500 ppm for KMG3. Cyst germination and sporulation activity were ultimately arrested at 200 and 800 ppm, respectively. In conclusion, the propolis extract can be explored as an anti-oomycete substance for treating saprolegniasis in aquaculture. However, an in vivo study is required to assess the safety and efficacy of propolis before application in aquatic animal medicine.