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It is important to note also that the higher dietary carbohydrate level reduced the frequency
of carnobacteria isolation within replicate fish (Ringø and Olsen 1999). In a later study, it
was observed that the presence of dietary inulin tended to lower culturable autochthonous
carnobacteria levels (by ca. 90%) in the hindgut of Arctic charr, which seemed to benefit
C. maltaromaticum at the expense of C. divergens (Ringø et al . 2006b).
6.3.2 Lactobacillus
Although a great deal of effort has focused on the assessment of Lactobacillus spp. as probi-
otics for salmonids, fewer studies have identified Lactobacillus as indigenous components of
the intestinal microbiota. Additionally, it has been suggested that the identification of intesti-
nal Lactobacillus spp. in older studies based on culture-dependent identifications probably
erroneously describe some strains of ' Lactobacillus spp.' which may belong to the Carnobac-
terium genus (Ringø and Gatesoupe 1998). The first study showing that fish contained lacto-
bacilli in the GI tract was conducted by Kraus (1961) who isolated lactobacilli from herring
( Clupea harengus L.). In their study of several freshwater fishes (Cyprinidae, Escoidae and
Percidae), Kvasnikov et al . (1977) reported several cultivable lactobacilli in the intestinal con-
tents of the fish. More recently, several studies have identified members of the Lactobacillus
genus in the GI tract of trout (Heikkinen et al . 2006; Skrodenyte-Arbaciauskiene et al . 2008;
Balcázar etal . 2008; Azizpour 2009; Navarrete etal . 2010; Pérez-Sánchez etal . 2011a; Navar-
rete et al . 2012; Desai et al . 2012; Vlková et al . 2012), Atlantic salmon (Ringø et al . 2000;
Moffit and Mobin 2006; Hovda et al . 2007; 2012; Askarian et al . 2012; Ringø et al . 2014) and
Arctic charr (Ringø et al . 1998; 2000).
A clear limitation, in most cases, is the lack of identification of such isolates to species level.
Confirming the need to use either selective media or preferably culture-independent meth-
ods, Hovda et al . (2007) failed to isolate Lactobacillus spp. in the gut of rainbow trout using
standard culture based protocols, but using DGGE and subsequent sequence analyses Lacto-
bacillus spp. were identified from the PI, MG and DI of Atlantic salmon. These were identified
as Lactobacillus spp., Lb.fermentum and Lb.fermentum -like. Recently, Lb.curvatus , Lb.sakei ,
Lb. plantarum and Lb. fermentum have been isolated from the gut contents of salmonids (Bal-
cázar et al . 2007a; 2008). Balcázar et al . (2008) further characterized the Lb. plantarum and
Lb. fermentum isolates and assessed their properties with regards to pathogen antagonism and
epithelial colonization. Even though Lb. plantarum and Lb. fermentum did not show antibac-
terial activities against three fish pathogens tested in a well assay, Lb. plantarum reduced the
mucus adhesive capacity of A.hydrophila and A.salmonicida , whereas Lb.fermentum reduced
the adhesion of A.hydrophila , A.salmonicida and Yersiniaruckeri . These findings suggest that
indigenous Lactobacillus populations have the potential to retard pathogenic colonization of
the epithelial mucus layer within the intestine of fish, which is important because adhesion to
epithelial mucus is thought to be an important step in intestinal infection, providing a foun-
dation for mucosal interaction and translocation (Ringø et al . 2010; Sica et al . 2012; Chapter
3 ). How effective these strains are in vivo , under different environmental conditions, is yet
to be ascertained. However, the efficacy of probiotic applications of Lactobacillus spp. for
salmonids is clear, particularly in regards to immunity and disease resistance (Merrifield et al .
2010a), and thus is indicative that natural Lactobacillus spp. are probably of some impor-
tance to the host. In fact Lb. plantarum , isolated from the intestinal mucosa of rainbow trout,
was administered to the same fish species to investigate the effects on the immune response,
and it was observed that this strain was able to up-regulate pro-inflammatory cytokine gene
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