Shirazi thyme (Zataria multiflora Boiss) and Rosemary (Rosmarinus officinalis) essential oils repress expression of sagA, a streptolysin S-related gene in Streptococcus iniae

• The changes of sagA expression were studied upon treatment with plant oils.
• The hemolytic activity of S. iniae was decreased by increasing oils concentration.
• Tested essential oils down-regulated the transcription of sagA gene.
• Essential oils could be used to control Streptococcosis in aquaculture.

The pathogenicity of Streptococcus iniae in fish is depending on the secreted virulence factors, such as streptolysin S (SLS). It is demonstrated that some plant essential oils can affect the production of bacterial toxins. In this study, broth macrodilution method was first used to determine minimum inhibitory concentrations (MICs) of Rosmarinus officinalis and Zataria multiflora essential oils against S. iniae. Hemolysin assay and real-time PCR analysis were then conducted to investigate the effect of subinhibitory concentrations of the essential oils on SLS major encoding gene, sagA, in two isolates of S. iniae. The obtained data showed that both R. officinalis and Z. multiflora decreased the hemolytic activity of S. iniae supernatant, dose-dependently. However, SLS was not inactivated when the essential oils were added to the bacterial culture supernatant. The transcription of sagA was significantly down-regulated during treatment of the bacterial isolates with subinhibitory concentrations of the essential oils. This data suggested that both R. officinalis and Z. multiflora could be advantageous for the control of streptococcosis caused by S. iniae, via inhibition the growth and repression the production of SLS.