Effect of probiotic on shrimp immune system and protection against Vibrio parahaemolyticus infection

Abstract

Lactic acid bacteria (LAB) are proposed as potential probiotic candidates in aquaculture and also known to be present in the gut of healthy aquatic animals. To isolate LAB from the gut of the white shrimp Litopenaeus vannamei, intestines were collected and the isolation of 89 LAB isolates was carried out using the MRS selective agar plate. The 16S ribosomal DNA (16S rDNA) sequence analysis reveals that most of the bacterial isolates identified from shrimp guts are the strain of LAB. Two candidate probiotics, Lactobacillus plantarum and Lactococcus lactis were selected for in vitro antimicrobial activity assay. The results indicated that cell-free supernatant of two probiotics contained antimicrobial substances actively against Vibrio parahaemolyticus causing Early mortality syndrome (EMS)/Acute Hepatopancreatic Necrosis Disease (AHPND) (VPAHPND). Administrations of two probiotic-supplemented feed enhanced the survival rate of shrimp upon infection with VPAHPND. In order to investigate the effect of the two probiotic strains on the shrimp innate immune system, immune activity and mRNA levels were monitored. The results indicated that oral administration of probiotic increased the level of prophenoloxidase and SOD activities. Moreover, expression levels of LvproPO1 and LvproPO2, but not LvPacifastin-like in midgut are significantly up-regulated after probiotic L. plantarum and L. lactis feeding. To examine the involvement of proPO-related genes in VPAHPND infection, temporal gene expressions were analyzed and showed that the LvproPO1 transcript level was downregulated at 24-48h and then returned to non-stimulated levels at 72 h. These finding suggests that two candidate probiotic LAB could enhance the immune system of shrimp L. vannamei and increase the resistance to VPAHPND infection.