Effects of artificial infection of Litopenaeus vannamei by Micrococcus lysodeikticus and WSSV on the activity of immunity related enzymes

In this study, the activities of 5 immunity related enzymes namely acid phosphatase (ACP), alkaline phosphatase (AKP), phenoloxidase (PO), peroxidase (POD) and lysozyme phosphatase (LZM)) of Litopenaeus vannamei after they have been injected with different concentrations of Micrococcus lysodeikticus and the white spot syndrome virus (WSSV) were examined. The cumulative mortality at 0, 24, 48, 72, 96 h was obtained. Copy numbers of WSSV in L. vannamei after a single infection, secondary infection and concurrent infection were measured. Hemolymph samples of M. lysodeikticus and WSSV injected shrimp were collected at 0, 6, 12 24, 48, 72, 78, 84, 96 and 120 h. The results were: (i) The cumulative mortality of L. vannamei increased as the shrimp were infected with higher concentration of the bacteria; (ii) The most sensitive changes of ACP, AKP and LZM were in the 6.2 × 105, 6.2 × 106, 6.2 × 107 cfu/mL M. lysodeikticus group; (iii) ACP but LZM were more sensitive to M. lysodeikticus than WSSV, and AKP, PO and POD is more sensitive to WSSV; (iv) The copies of WSSV in the co-injected group were higher than WSSV-single infection and WSSV-bacteria-secondary infection group at 48 h. The amount of WSSV in L. vannamei of concurrent infection and WSSV-bacteria-secondary infection groups were higher than that of the WSSV-single infection group. •Few research on the bacteria Micrococcus lysodeikticus that cause disease in shrimp.•Immunity related enzymes were studied to understand shrimp defense against pathogen infection.•The shrimp infected with M. lysodeikticus would carry more WSSV.•Co-infection and repeat infection could cause more serious effect to shrimp.•WSSV outbreak occur only when environmental condition favor viral amplification.