**5. Conclusion**

the pathogens that have crossed the mucosal barriers and trigger a set of responses that

Also, [27] claims that different species of gut flora could influence the development of key cells of the immune system, by increasing or decreasing the level of tolerance against foreign entities. Once the host immune system is developed, it regulates the bacterial composition in the gut. One of the regulation mechanisms—immune exclusion—is mediated through the

Harmful bacteria species, such as *Pseudomonas aeruginosa* and *Aeromonas hydrophila*, are unable to grow excessively due to competition from helpful gut flora species adhering to the mucosal lining of the intestine; thus, fish without/with low gut flora are infected very easily [28]. The barrier effect protects hosts from both invading species and species normally present in the gut at low numbers, whose growth is usually inhibited by the gut flora [29]. Helpful bacteria prevent the growth of pathogenic species by competing for nutrients and attachment sites to the epithelium of the colon. Also, symbiotic bacteria are more at home in this ecological niche and are thus more successful in the competition. Probiotics also produce bacteriocins, which are proteinaceous toxins that inhibit growth of similar bacterial strains, substances that kill harmful microbes and the levels of which can be regulated by enzymes produced by the host. Also, the process of fermentation produces lactic acid and different fatty acids, which lowers the pH in the colon, preventing the proliferation of harmful species of bacteria and facilitating

Immune responses of the prawn fed fortified diets were higher than the control. There higher SOD, CAT RBA, THC, PO, ROI and lysozyme activity in prawn fed the *Lactobacillus acidophilus*based diets than in those fed control diet. Similar observations were made by [22, 31] in fish fed probiotics diets. The SOD, CAT RBA, THC, PO, ROI and lysozyme activity have important roles in the innate immune defence system. For instance, it was reported that lysozyme activity plays a significant role in innate humoral immune factors that control intrusiveness of pathogens by destroying the cell walls of the bacteria [1]. The SOD catalyses the dismutation of the superoxide anions to molecular oxygen and hydrogen peroxide, which further catalyses to water and oxygen. The SOD and CAT play an important role as cellular antioxidants against reactive oxygen species. In this study, prawn fed fortified diets had increased values of SOD, CAT, RBA and lysozyme activity, which could be answerable for the increased concentration of hydrogen peroxide in the oxidation process. The chemicals are released from immune cells and enzymes, as they come in contact with pathogens cumu-

The investigation into the resistance level of *Macrobranchium vollenhovenii* fed diets fortified with *Lactobacillus acidophilus* revealed that the prawn were protected that the group fed control diet. In this study, prawns were significantly protected against *Pseudomonas aeruginosa* and *Aeropmonas hydrophila* infection with relative protection up to 96%. Similarly, [32–33, 7] reported that there was significant increase in the survival rate of fish fed *Lactobacillus acidophilus* fortified diets. The protection of prawn against the pathogens could be attributed to the gut serves as an entry point through feeding and thus inhibit the growth and survival

take action against the pathogen [26].

102 Aquaculture - Plants and Invertebrates

that of helpful species [30].

lating into improved immunity.

of the pathogens.

neutralisation of secreted immunoglobulin A (IgA) [28].

*Macrobranhium vollenhovenii* fed diets supplemented with *Lactobacillus acidophilus* had reduced total enterobactericeae in the gut, enhanced innate immune response parameters and suggest it usage as immunomodulation and protective agent against *Pseudomonas aeruginosa* and *Aeromonas hydrophila* infection.
