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strains have demonstrated that these changes can often lead to the improvement of general
animal welfare in terms of survival, immune status, growth performance and/or stress response
(Merrifield
et al
. 2010a; Dimitroglou
et al
. 2011).
8.3.3
Enterococcus
spp.
To the authors' knowledge the investigation of enterococci for probiotic applications in
fish is restricted to
Enterococcus faecium
(previously
Streptococcus faecium
: Schleifer and
Kilpper-Balz 1984) strains (Bogut
et al
. 1998; 2000; Chang and Liu 2002; Panigrahi
et al
.
2007; Shelby
et al
. 2007; Wang
et al
. 2008; Merrifield
et al
. 2010b; 2010c; 2010d; Avella
et al
. 2011; Palermo
et al
. 2011; Gopalakannan and Arul 2011; Sun
et al
. 2012a) and an
unidentified
Enterococcus
sp. (Del'Duca
et al
. 2013) (Table 8.2). Several of these studies
have investigated the interactions with the indigenous fish gut microbiota.
Bogut
et al
. (1998) assessed the effect of
Enterococcus faecium
(M74; described in this
study as
Streptococcus faecium
) on the cultivable microbiota of common carp. Relative to the
control, Enterobacteriaceae,
Streptococcus faecalis
and
Staphylococcus aureus
levels were
considerably lower in the probiotic fed group and
Escherichia coli
was no longer present after
14 days of probiotic feeding. Later, Bogut
et al
. (2000) fed sheat fish dietary
E. faecium
at
10
5
CFU g
-1
for 58 days. Probiotic application of
E. faecium
resulted in a clear alteration
of the culturable intestinal microbiota including reductions of Enterobacteriaceae (including
E. coli
),
Staphylococcus aureus
and
Clostridium
spp. Subsequently, significantly improved
weight gain was observed in
E. faecium
fed fish compared to the control group.
Later, Chang and Liu (2002) fed European eel diets supplemented with
E. faecium
SF68
for 14 days and investigated the gut microbiota every second day, up to day 14, following a
24 h starvation period.
E. faecium
began to successfully populate the intestine after 4 days;
levels generally increased with feeding duration and maximal levels were recovered between
10 and 14 days (log 5.1-5.25 CFU g
-1
).
E. faecium
was not detected in the control fed eels but
at day 14
E. faecium
represented the dominant cultivable intestinal population of the probiotic
fed eels, accounting for 73% of the total culturable microbiota. Modulation of the indige-
nous microbiota was demonstrated by alterations of both the levels and the composition of
Aeromonas
spp. Furthermore, the level of
Vibrio
spp. was reduced from 6% in the control
to non-detectable levels in the probiotic fed fish, and
Pasteurella multocida
and
Plesiomonas
shigelloides
levels were also considerably lower in the probiotic fed fish (at 2% and 4%, respec-
tively) than in the control fed fish (at 37% and 12%, respectively). Chang and Lui (2002)
demonstrated
in vitro
antagonism of
E. faecium
against
Edwardsiella tarda
; subsequently
in
vivo
protection was demonstrated in fish fed
E. faecium
(compared to both the control group
and a group fed
B. toyoi
) during challenge experiments. As no detectable levels of
B. toyoi
were observed in the intestine of
B. toyoi
fed fish, the study demonstrates the importance of
successful establishment of gastric probiotic populations for protection against infection.
More recently the effect of dietary
E. faecium
(10
8
CFU g
-1
diet) was observed on intesti-
nal microbiota of rainbow trout with (Merrifield
et al
. 2010d) and without (Merrifield
et al
.
2010c) an antibiotic pre-treatment (Figure 8.1). In the initial study without antibiotic treatment,
GI tract levels of
E. faecium
were ca. 2 × 10
2
CFU g
-1
(accounting for 45% of the cultured
population) and 3.26 × 10
7
CFU g
-1
(89% of the cultured population) on the mucosa and in the
digesta, respectively, after 10 weeks of dietary administration (Figure 8.1A) (Merrifield
et al
.
2010c). In a similar study Merrifield
et al
. (2010d) investigated the effects of
E
.
faecium
col-
onization of the GI tract of rainbow trout pre-treated with dietary oxolinic acid at 20 mg kg
-1
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