Effect of food-borne antagonistic bacteria-chitosan coating on reactive oxygen metabolism and pathogenesis during the shelf life of postharvest mango

This study aims to explore the effects of Lactobacillus plantarum PC11 and Lactobacillus plantarum NMGL2 on the incidence of anthracnose disease in mangoes during shelf life. PC11 and NMGL2 fermentation liquids were employed at a concentration of 10¹⁰ CFU/mL combined with chitosan to prepare a strain fermentation liquid-chitosan complex solution. The mangoes were immersed in the strain fermentation liquid-chitosan complex solution for 2 minutes, air-dried to form a coating, stored at (25±1)℃ for 12 days, and sampled every 4 days. The indicators were then observed and measured during storage, including the mango respiration intensity, ethylene release, incidence rate, disease index, reactive oxygen, enzyme/non-enzyme substances, and phenolic substances. A correlation analysis was conducted as well. The results showed that the PC11 combined with chitosan coating effectively reduced the accumulation of reactive oxygen species in mangoes, whereas, there was an increase in the enzyme activity of SOD, APX, GPX, and the content of glutathione in the mango reactive oxygen metabolism. On the 8th day of storage, the H₂O₂ and \rmO_2^- \cdot contents in the PC11 treatment group were 3.6% and 22.6% lower than those in the CK group, respectively. The enzyme activities of SOD, APX in the PC11 treatment group were 14.9%, 60.9% higher than those in the CK group, respectively. There was an increase in the disease resistance of mangoes. However, the treatment of mangoes with NMGL2 combined with chitosan was accelerated to accumulate the reactive oxygen species in mangoes, where the mango POD activity was reduced to slow down the accumulation of total phenols. On the 4th day, the H₂O₂ content in the NMGL2 treatment group is 7.7% higher than that of the CK group and the POD activity is 28.01% and 35.07% lower than the CK group on days 8th and 12th, respectively, indicating that NMGL2 treatment reduced the disease resistance of mangoes. In conclusion, the PC11 treatment combined with chitosan coating treatment maintained a higher level of ROS production and clearance in mangoes. Therefore, the mango disease resistance was enhanced to regulate the reactive oxygen/glutathione pathways, thus effectively inhibiting the occurrence of mango anthracnose disease. NMGL2 was accelerated to accumulate the reactive oxygen species in mangoes during storage. The activities of reactive oxygen metabolism-related enzymes and glutathione content were reduced to promote the occurrence of mango anthracnose. The fermentation broth of the strain was composed of both the cells and the metabolites of the cells. Therefore, the anti-disease effect in the PC11 processing group was attributed to the production of metabolites with certain bacteriostatic or fruit resistance-enhancing functions, the nutritional and space competition of the bacteria itself, as well as the hyper parasitism. The side effects of NMGL2 on mangoes resulted from the production of metabolites that damage mangoes or the pathogenic effect of the cells themselves on mangoes. Whether the bacterium PC11 or the metabolites produced by PC11 can greatly contribute to the disease resistance of mangoes. Both bacterium and its metabolites in the combination of PC11 with chitosan coating can be used to enhance the endogenous resistance in fruits. The composition and key inhibitory of metabolites from Lactobacillus plantarum PC11 also require further investigation.