Probiotic Modulation of the Gut Microbiota of Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 203

as GI acids and competition with the indigenous microbiota. Another suggested factor was
salinity, which may partially explain the contrary results later observed in freshwater trout
(Balcázar et al . 2007a; 2007b; 2009) and the marine species turbot (Villamil et al . 2002) and
grouper (Sun et al . 2012b). Balcázar et al . (2007a) studied the effect of feeding Lc. lactis
subsp. lactis at 10 6 CFU g -1 to brown trout for 2 weeks before reverting to non-supplemented
feeds for a further 2 weeks. A clear relationship was evident between the duration of feeding
and the recovery of the Lc. lactis within the intestinal contents. Peak levels (approaching log
8CFUg -1 ) were observed after 2 weeks feeding. Lc.lactis displayed an ability to persist within
the gut for at least 2 weeks after reverting to non-supplemented diets. A later study by the same
research group reported that Lc. lactis populations colonized the intestinal mucosa of brown
trout (Balcázar et al . 2009), and Balcázar et al . (2007b) reported that feeding rainbow trout
Lc. lactis at 10 6 CFU g -1 for two weeks led to the successful recovery of the probiont from
the intestinal contents at ca. log 7.5 CFU g -1 . Sun et al . (2012b) assessed the autochthonous
microbiota in the fore-, mid- and hindgut of juvenile grouper after the dietary administration
of Lc. lactis MM1, previously isolated from the whole intestine of juvenile grouper (Sun et al .
2009), at 10 8 cells g -1 for 60 days. None of the DGGE bands sequenced were identified as
Lc. lactis , and no unidentified bands were present only in the probiotic group, which may be
suggestive that Lc. lactis does not colonize the mucosa of this marine fish species under the
experimental conditions used. Despite this, bacterial richness and diversity were significantly
elevated in all gut regions of the probiotic fed fish, and dendrogramatic analysis revealed that
the bacterial profiles of the probiotic fed fish were generally clustered into one group, dis-
tinctly different from the corresponding control replicates, in each GI section. In addition, the
abundance of some unidentified bacteria was elevated in the probiotic fish and some potential
harmful species, like Staphylococcus saprophyticus , were decreased.
The topic of salinity and its potential impact on probiotic colonization, particularly of Lc.
lactis , is worthy of further investigation; to the authors' knowledge no data are currently avail-
able regarding the effect of salinity on the efficacy of probiotics to modulate the gut microbiota
of fish. The direct effect of salinity on the probiont itself is not the only consideration; the
indigenous gut microbiota of marine and freshwater fish are often distinctly different (Sakata
1990; Ringø and Strøm 1994; Hansen and Olafsen 1999). For example, Ringø and Strøm
(1994) demonstrated the effect of salinity on the indigenous gut microbiota of Arctic charr
( Salvelinus alpinus ) when reared in freshwater and seawater. Arctic charr reared in seawater
presented lower (by 1-2 log scales) cultivable bacterial populations, a lower proportion of
Aeromonas spp. and a higher proportion of Vibrio spp. Future studies should include probiotic
feeding administered pre-, during and post-transfer to seawater of anadromous species to help
to elucidate the extent of the effect of salinity on the gut microbiota of fish and the subsequent
interactions with probiotics.
8.3.5 Leuconostoc spp.
To the authors' knowledge the only Leuconostoc spp. which has been investigated in vivo with
regards to the intestinal modulation and colonization of fish are Leuconostoc mesenteroides
strains (Balcázar et al . 2007a; 2007b; 2009; Vendrell et al . 2008; Pérez-Sánchez et al . 2011;
Askarian et al . 2011; Table 8.2).
The studies of Balcázar et al . (2007a; 2007b) both demonstrated the positive effects of Leu.
mesenteroides in terms of intestinal presence and subsequent health benefits for trout. Vendrell
et al . (2008) demonstrated that the presence of high gastric levels of Leu. mesenteroides ,after