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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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