Shirazi thyme (Zataria multiflora) essential oil suppresses the expression of the epsD capsule gene in Lactococcus garvieae, the cause of lactococcosis in farmed fish

• Changes of epsD expression where studied upon treatment with plant essential oil.
• Z. multiflora essential oil supressed epsD capsule production gene of L. garvieae.
• Z. multiflora could be an advantageous for control of lactococcosis in aquaculture.

The virulent factor of Lactococcus garvieae contains a specific capsule gene cluster including epsD. Some herbal essential oils have antibacterial activities and could negatively affect the virulence of pathogenic bacteria. In this study, 15 strains of L. garvieae were recovered from diseased rainbow trout, and were identified by molecular works. The encapsulated strains were then detected using both staining and molecular studies. The effect of Shirazi thyme (Zataria multiflora) essential oil was then assessed on the expression of the encapsulated positive epsD gene strains with a higher genetic diversity. The phylogenic features of the five identified encapsulated positive epsD gene categorized them into a group, two clusters and three genogroups. Also, the obtained results showed that from the five encapsulated positive epsD strains, only two strains showed a higher genetic diversity. The antibacterial activity of essential oils of Shirazi thyme (Z. multiflora), Cinnamon (Cinnamomum zeylanicum) and Garlic (Allium sativum) to these two encapsulated positive epsD gene strains revealed the highest inhibition effect by Z. multiflora with a minimum inhibitory concentration (MIC) of 0.12 μl/ml. Subinhibitory concentrations of 1/2, 1/4, 1/8 and 1/16 MICs of Z. multiflora on the epsD gene production showed a suppression effect in reducing this gene production. Therefore, this essential oil could be advantageous for the control of lactococcosis in aquaculture.