Probiotic Applications in Crustaceans - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 306

all AMP gene transcripts measured in the haemocytes (anti-lipopolysaccharide factor, crustins
and penaeidins) compared to the control (Antony et al. 2011). The study demonstrated varia-
tions in the expression profile of AMP genes resulting in different levels of protection against
WSSV dependent on the probiotic strain.
Interestingly new strategies based on probiotics are also currently being developed in order
to improve the resistance of crustaceans to WSSV. Even though crustaceans lack a true adap-
tive immune system, it is now reported that memory responses could be induced by applying
inactivated pathogens or recombinant proteins against WSSV (Namikoshi et al. 2004). Can-
didate 'vaccines' based on structural proteins of the virus have been evaluated. For instance
VP28, an envelope protein from WSSV, has shown promising protective effects (Witteveldt
2004). However the application of this 'vaccine' would require injection which is not feasible
at the industrial level. Therefore the use of live bacteria, such as B. subtilis , has been explored
as a vehicle for oral delivery of VP28. Successful attempts using a recombinant B. subtilis
strain able to secrete VP28 have been conducted in shrimp and crayfish (Fu et al. 2010). This
study reported a lower susceptibility of F. chinensis sub-adults to WSSV infection when fed
for 20 days a diet coated with the recombinant B. subtilis strain (20% mortality compared
to 100% in the positive control group) and suggested that the enhancement of the activity of
inducible nitric oxide synthase (iNOS) and NO observed could play an important role in the
antiviral defence of the shrimp. Interestingly the unmodified B. subtilis fed group was able to
marginally reduce the mortality compared to the positive control group.
In addition to Bacillus spp., a number of investigations have studied the immunomodulatory
capacity of other bacterial genera. Specifically, Chiu etal. (2007) were among the first to assess
the effect of an LAB probiotic on the immune system of shrimps. This study demonstrated that
an Lb. plantarum strain, administered at 10 7 CFU g -1 of feed, enhanced the immune response
of L. vannamei from 48 hours after its introduction to the diet, and subsequently increased
shrimp resistance to V. alginolyticus . They suggested that Lb. plantarum may enhance the
antioxidant status of the shrimp based on the increased SOD activity and peroxinectin mRNA
transcription levels. The antioxidant defence system of the organism acts to maintain the lowest
possible levels of reactive oxygen species in the cell, and is recognized as an essential com-
ponent of an organism's self-maintenance. Recent studies have demonstrated the involvement
of the antioxidant system following bacterial infection or virus challenges in marine shrimps
(Liu et al. 2007; Mathew et al. 2007). Castex et al. (2009; 2010) showed that the administra-
tion of the dietary probiotic Ped. acidilactici MA18/5M to L. stylirostris was able to support
the antioxidant system during an infection by a highly virulent Vibrio nigripulchritudo strain.
Even if the exact mechanisms behind it remain unknown at this stage, one of the hypotheses
formulated to explain this effect was that the probiotic may have contributed to increase the
overall physiological status of the shrimp, including antioxidant status and immune response.
Other authors also reported recently the effect of a live yeast based probiotic (the red yeast,
Rhodosporidium paludigenum ) on the performance and the antioxidant enzymes activities in
the serum, muscle and digestive gland of L. vannamei , supporting again the enhancement of
the antioxidant status as another possible explanation for the probiotic effect in crustaceans
(Yang et al. 2011).
In conclusion, the information available to date demonstrates that probiotics are potent
immunomodulators in crustaceans. Future studies are required, specifically to identify direct
and indirect (through a modulation of the microbiota) interaction of probiotics on the localized
immune status of crustaceans. However, with their diverse mechanisms of action (competitive