An In vitro and in vivo study on antimicrobial activity of Origanum vulgare extract and its nano form against Streptococcus iniae in rainbow trout (Oncorhynchus mykiss)

This study was performed in order to determine the antibacterial activity of oregano, Origanum vulgare extract (OVE) and its nano extract (NOVE) on Streptococcus iniae in rainbow trout in vitro and in vivo. Extraction was done under standard percolation method. Then OVE and NOVE were screened to identify their antimicrobial activity against Streptococcus iniae by using disk diffusion assay and challenge methods. Also, the MIC and MBC values for OVE and NOVE were determined by micro dilution method. Furthermore, the antimicrobial effect of OVE and NOVE were examined by feeding rainbow trout fingerlings, with O. vulgare extract and its nano extract and florfenicol as a positive control. Then, all fish were injected intraperitoneal with S. iniae suspension in 3×108 CFU ml-1 concentration. Mortality rate was investigated for next 14 days after injection; then the spleen of fish samples removed to determine bacteria total count (SBT). In vitro results showed that OVE and NOVE had antimicrobial activity which was higher in NOVE. The inhibition zone of NOVE (26.066 mm) was significantly greater than OVE (21.7mm) (p<0.05). Also, the MIC and MBC values with high efficacy against S. iniae in NOVE treatment group were 0.25 and 0.5 mgmL respectively, which was lower than OVE treatment values (p<0.05). In vivo results showed that the mortality rate and SBT was significantly lower (p<0.05) in the fish fed with NOVE than OVE and the control group (p<0.05). These results indicated that the Origanum vulgare extract and its nano extract supplement are satisfactory as a prophylactic measure against rainbow trout streptococcosis and also for fish health improvement.