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possible mechanism (Gram
et al.
1999). However, reports or suggestions of such mechanisms
in crustaceans are presently absent.
11.3.5 Modulation of the host immune response
There are many reports regarding the immunostimulatory properties of microorganism frac-
tions (e.g. lipopolysaccharides from bacteria cell walls, β-glucans from the yeast cell walls
etc.) in crustaceans (Smith
et al.
2003; Sajeevan
et al.
2009; Bai
et al.
2010). As extensively
detailed in the previous chapters, many studies have also revealed that live microorganisms
can stimulate the non-specific immune system of fish; in particular, LAB are often reported
as efficient immunomodulators in fish. However despite the large number of studies avail-
able, the immunomodulatory effects of probiotics in fish systems are, at present, not fully
understood. The same observation can be made for crustaceans with many studies reporting
the effect of probiotics on immunity even though relatively little information is available to
explain the mechanisms behind these observations (Rengpipat
et al.
2000; Gullian
et al.
2004;
Alavandi
et al.
2004; Sajeevan
et al.
2006; Rodríguez
et al.
2007; Chiu
et al.
2007; Li
et al.
2008; 2009; Hai
et al.
2009; Hai and Fotedar 2009; Zhang
et al.
2009; 2011; Peraza-Gomez
et al.
2011). Balcázar
et al.
(2006) suggested that bacteria that did not colonize the gut would
not help in improving the immune system of shrimp. This seems to be in accordance with
the results of Gullian
et al.
(2004) who associated the enhancement of the global immunity
index conferred by
Vibrio
sp. (P62 and P63) and
Bacillus
probiotic strains in shrimps to their
degree of colonization. However, this aspect has not been further documented and more stud-
ies are required to identify whether colonization is a prerequisite for immune stimulation or
modulation in crustaceans.
In crustaceans this mode of action is mainly investigated with probiotics from the
Bacillus
genus. Rengpipat
et al.
(2000) suggested that
Bacillus
S11 provided protection against vib-
riosis by activating both cellular and humoral defences in tiger shrimp. This effect was char-
acterized by improved immunity indices such as total haemocyte count, phagocytic activity,
PO activity and a higher haemolymph antibacterial activity. They attributed this effect to the
peptidoglycan present at the surface of the probiont wall which can stimulate granulocytes and
enhance phagocytosis (Itami
et al.
1998). Other authors have confirmed such effects on PO
activity, phagocytosis, pathogen clearance efficiency and increased resistance to pathogenic
bacteria with other
Bacillus
strains (Tseng
et al.
2009). Interestingly these authors pursued
their investigation and further explained this effect by an up-regulation of the expression of
two proPO isoform genes (Liu
et al.
2011b).
Balcázar (2003) reported that the immune modulation conferred by a mixture of
Bacillus
spp. and
Vibrio
spp. was associated with a better resistance to WSSV. Interestingly a recent
study from Antony
et al.
(2011) showed, for the first time, that dietary probiotics (
Micrococ-
cus
sp. and
Bacillus
sp. alone or in combination) were able to up-regulate the expression of
specific antimicrobial peptide (AMP) genes (ALF, penaeidin-3 and -5) in haemocytes and sev-
eral tissues of the giant tiger shrimp; previous experiments had shown similar priming effects
with lipopolysaccharides (Okumura 2007). AMPs are a primitive first-line innate host defence
mechanism, displaying activities against bacteria, fungi and viruses. Today, AMPs are well
described in crustaceans and are known to be involved in the resistance of shrimps to bacterial
and viral infection (Bachère
et al.
2004). When shrimps were challenged by WSSV, a general
down-regulation of the AMP genes was reported (with the exception of ALF); however, probi-
otic treated shrimps showed a greater level of protection together with enhanced quantities of
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