The prophenoloxidase (proPO) system plays a critical role in host
defensive reactions, which could be activated by some microbial lipopolysaccharides
and β-1,3-glucan to generate phenoloxidase (PO), an
important enzyme in the process of melanization (Figueroa-Pizano
et al., 2014; Gai et al., 2008). Gai et al. (2008) suggested that the transcript
levels of proPO generally corresponded to the activity monitored
for the same enzyme. Moreover, low activity levels of PO could be considered
as a decrease in the capacity of the shrimp immune system and
are associated with high mortality in shrimp when challenged with
pathogens (Gai et al., 2008). In our study, the expression of proPO was
up-regulated in the midgut and hepatopancreas of shrimp fed with
L. pentosus HC-2 at all time points as well as in the supernatant of
L. pentosus HC-2-feeding shrimp, except in the hepatopancreas at two
weeks of feeding. Additionally, proPO transcripts in shrimp fed diets
containing E. faecium NRW-2were also increased, except in the hepatopancreas
at the four-week time point. Based on the above results, the
supplementation of probiotics and metabolites might increase the immune
response of shrimp and enhance the resistance to pathogens;
the results in Table 4 confirm our hypothesis.