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mucus, and adhesion to and proliferation within mucus, could provide a platform for subse-
quent probiont interaction and attachment with the host epithelium. The adhesion of probiotic
bacteria to fish intestinal epithelial cells has been frequently studied but the mechanisms by
which this process is achieved have received little direct attention. According to Ringø
et al
.
(2007), bacterial adhesion to epithelial cells involves a number of different types of interac-
tions which depend on the proximity of bacteria to the host cell surface; attraction is initially
by van der Waal's forces operating at distances
50 nm but electrostatic interactions become
more significant at closer distances. As the cells are usually negatively charged, electrostatic
repulsion can hinder and prevent closer association. A closer interaction may occur in regions
of lower ionic strength, allowing hydrophobic interaction and specific receptor-ligand bind-
ing. Information on specific adhesins and receptors of probiotics in fish is partly described;
that is, a number of adhesins have been identified on potential fish probiotics and receptors
or receptor analogues have been identified on the cell surface of fish epithelial cells. The
interactions of these molecules and their importance in initiating bacterial-host interactions
and the subsequent epithelial (and lymphoid) cell responses are inferred from studies with
bacterial cells and higher vertebrates. This topic is further discussed elsewhere (Hansen and
Olafsen 1999; Ringø
etal
. 2007). Although further research into the mechanisms is warranted,
a number of studies report that probionts can populate the intestinal mucus or epithelium
of fish.
Can population of the mucus or mucosa be described as colonization?
In order to be sure,
extensive investigations in the absence of dietary probiotic provision, to assess probiotic per-
sistence, must be conducted. This has been the topic of several studies which have shown that
some dietary probiotics have a temporal ability to persist within the GI tract of fish after dietary
administration of the probiont ceases (Jöborn
etal
. 1997; Robertson
etal
. 2000; Panigrahi
etal
.
2005; Kim and Austin 2006; Balcázar
et al
. 2007a; Ferguson
et al
. 2010; Abid
et al
. 2012;
Ran
et al
. 2012; Table 8.5). To our knowledge, the first study to address this topic was that of
Jöborn
et al
. (1997), where culture-dependent methods indicated that
C. inhibens
persisted in
the intestinal tract of juvenile rainbow trout for
>
4 days after reverting to non-supplemented
diets. Robertson
et al
. (2000) reported that the same
C. inhibens
strain could be re-isolated
from the intestine of rainbow trout fingerlings and fry for up to 4 and 10 days, respectively,
after reverting to feeding with control diets. A more recent study using a culture-independent
approach, DGGE, demonstrated that
C. divergens
(and
C. maltaromaticum
) could persist in
the intestine of rainbow trout for at least 3 weeks (Kim and Austin 2006).
It is clear that the efficacy of probiotics to persist within the GI tract of fish differs among
fish species, with differing host selective pressures, and different rearing conditions. For
example, Ferguson
et al
. (2010) reported that
P. acidilactici
could remain present in the GI
tract of tilapia for at least 17 days after the cessation of dietary administration (Figure 8.4),
whereas the same probiotic strain persisted in the intestinal tract of rainbow trout for less
than 3 days after reverting to a control diet (Abid
et al
. 2012). This large discrepancy is likely
to be due to a range of factors, including the different microbial communities present in the
GI tracts of the respective species and the variation in the GI selection pressures, but the
most obvious factor is the large difference in rearing temperatures (15
∘
C in trout versus ca.
25-26
∘
C in tilapia); optimum growth for pediococci is at 25-40
∘
C (Holt and Bergey 1994)
and so lower temperature applications are likely to reduce the bacterial multiplication rates
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