Effect of Rosmarinus officinalis essential oil and nisin on Log P% of Streptococcus iniae in BHI broth

Background and Aim: Streptococcus is one of the most important food-borne diseases and zoonotic that caused by streptococcus species especially S.iniae. The present study was undertaken to evaluate the effects of R. officinalis essential oil, nisin, temperature, pH and storage time (۴۳ days) on the log۱۰ probability percentage of growth initiation (log P %) of S.iniae in brain heart infusion (BHI) broth in a factorial design study. Materials and Methods: The essential oil yield of the air-dried material was analyzed by gas chromatography (GC) (Agilent ۶۸۹۰, UK). Lyophilized cultures of S. iniae obtained from Department of Aquatic Animals Health and Diseases, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran, were used in this study. The log P% was calculated from the number of total tubes (out of ۲۴) for each EO–N–pH–T combination showing visible growth up to a certain observation time, using ۳×۸ most probable number (MPN) method. Results: Concentrations of essential oil with ۰.۷۵ µg/ml nisin can inhibit growth of bacterial and log P% was calculated as -۴.۲۴۱in ۳۷ ºC. The synergistic effect ۰.۲۵ µg/ml of nisin with concentrations of essential oil (۰.۰۰۵ and ۰.۰۱۵%) was observed in ۱۵ ºC, thus no significantly affected (P>۰.۰۱) with ۰.۷۵ µg/ml of nisin. The growth of bacterial was completely inhibited at combinations of ۰.۰۰۱۵% essential oil with ۰.۷۵ µg/ml of nisin, ۴ ºC and pH= ۵.۵ during storage times. Conclusions: The synergistic effect of R. officinalis essential oil and nisin could increase the scope of essential oil usage within the food industry.