Shirazi thyme (Zataria multiflora) essential oil suppresses the expression of PavA and Hly genes in Lactococcus garvieae, the causative agent of lactococcosis in farmed fish

• Lactococcus garvieae is the cause of a serious bacterial disease in fish.
• Expression of PavA gene is suppressed by sub-MIC of Zataria multiflora.
• Expression of Hly gene is suppressed by sub-MIC of Z. multiflora.
• These genes are responsible for encoding fibronectin and hemolysin proteins.
• Use of Z. multiflora can reduce losses by L. garvieae in fish.

Lactococcosis caused by Lactococcus garvieae is known as one of the serious economical bacterial diseases in aquaculture species of both freshwater and marine fish at a wide range of water temperature mainly from 13 °C to 37 °C. The PavA gene and Hly gene, the genes responsible for encoding fibronectin and hemolysin proteins, respectively are the well known virulent factors of L. garvieae. The effect of essential oil of Zataria multiflora was evaluated on expression of these genes in two virulent isolates of L. garvieae isolates recovered from diseased rainbow trout. Fresh cultures of the bacterial isolates were exposed to 1/2–1/16 (0.12–0.015 μl/ml) of the minimum inhibitory concentrations (MIC) of Z. multiflora essential oil and the expression of PavA and Hly genes was assessed by real time PCR works. The results showed that under MIC concentration (0.12 μl/ml) of the essential oil, the expression of both Hly and PavA genes was significantly suppressed in both bacterial isolates (P < 0.05). Therefore, it is feasible to use Z. multiflora in forms of oral and/or bath administration to reduce morbidity and mortality by Lactococcus garvieae in fish via the suppression of expression of these virulent genes.