Bacteriocin paracin wx3 biosynthesis improves antagonistic activity of Lactobacillus paracasei WX322 against soft rot of postharvest pepper

Postharvest peppers are susceptible to soft rot caused by Pectobacterium carotovorum during storage and transportation. Lactic acid bacteria (LAB) are promising antagonist to control soft rot. LAB usually exhibit strong in vitro antibacterial activity on MRS agar plates, but weak in vivo disease control activity on fruit/vegetable. This study aimed to improve the biocontrol activity of Lactobacillus paracasei WX322 in vivo. Stress conditions were firstly investigated and the results showed that starvation stress could not induce any enhancement of antibacterial activity in vitro. Acid-base stress only improved antibacterial activity in vitro, while osmotic stress under 1.5 % NaCl improved both antibacterial activity in vitro and disease control activity in vivo. To facilitate the adaptation of L. paracasei WX322 on green pepper, pepper juice was used to replace MRS medium. To facilitate the growth of L. paracasei WX322 in pepper juice, nutrient supplement was studied. The result showed that pepper juice containing 0.4 % Tween 80 and 1.5 % NaCl was the optimal induction condition which improved the antagonistic activity of L. paracasei WX322 by enhancing LAB’s metabolism. Bacteriocin production was significantly improved under the optimal induction condition. The result of RT-qPCR revealed that bacteriocin production related genes were significantly up-regulated under the optimal induction condition, including three-component system. The addition of 1 μM autoinducer peptide (AIP) in MRS significantly increased antibacterial activity of L. paracasei WX322 in vitro. Simultaneously, L. paracasei WX322 cultured in MRS containing 0.01 μM AIP had a significantly increased disease control activity. RT-qPCR further verified an increased expression of bacteriocin paracin wx3 when adding 0.01 μM AIP. These results suggest that the medium of pepper juice containing 0.4 % Tween 80 and 1.5 % NaCl can improve the in vivo disease control activity of L. paracasei WX322, which is achieved by improved bacteriocin biosynthesis regulated by three-component system.