Gastrointestinal Pathogenesis in Aquatic Animals - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 57

intestinal mucus. This fact strongly suggests that the turbot alimentary tract may serve as a site
of amplification of V. anguillarum (Olsson et al. 1992). In another study by the same group,
turbot were infected by intragastric or rectal intubations. Both methods revealed translocation
of viable bacteria into the spleen, and it was proposed that the intestinal tract of juvenile turbot
is a portal of entry for V. anguillarum (Olsson et al. 1996).
In a study by Spanggaard et al. (2000) rainbow trout were bath infected by V. anguillarum .
The onset of mortality occurred 48 h after infection. The skin contained significantly higher
numbers of bacteria than all other sample sites, including the GI tract. V. anguillarum was
detected in just a few intestinal samples and not until an advanced stage of infection, indi-
cating that the GI tract is of minor importance in rainbow trout. Recently a new method, in
vivo bioluminescent imaging (BLI), was used to analyse the colonization of V. anguillarum
in rainbow trout (Weber et al. 2010). The fish were challenged by immersion in biolumines-
cent bacteria. At 24 and 48 h post infection fish were analysed for bioluminescent bacteria. V.
anguillarum colonized the skin in 95% and 100% of fish 24 and 48 h post infection, respec-
tively. To visualize bacteria in the intestine, the faecal content was stripped. V. anguillarum
was detected in 80% and 95% of the intestines 24 and 48 h post infection, respectively. The
bacterial number in the intestine did not reach the same level as on the skin.
The initial stage of bacterial infection of the GI tract is likely to include a chemotactic
response to, adherence to and growth within intestinal mucus before bacteria can interact with
epithelial cells. Indeed, O'Toole etal. (1999) investigated the chemotactic motility of V.anguil-
larum to isolated rainbow trout skin mucus and intestinal mucus. The main conclusion from
this study was that V.anguillarum moves towards both types of mucus, with a higher chemotac-
tic response for intestinal mucus (O'Toole et al. 1999). Mucus is rich in nutrients that bacteria
may utilize for growth. This subject was the focus of a study by Garcia et al. (1997), which
investigated the growth of V.anguillarum in Atlantic salmon intestinal mucus, and showed that
the bacterium was capable of rapid growth in the intestinal mucus. In another study, Larsen
et al. (2001) observed that V. anguillarum was significantly more chemotactic to mucus from
the skin and intestine of rainbow trout than to the gill mucus. It has been observed that V.
anguillarum is able to bind to a neutral glycosphingolipid (glucosylceramide) receptor on the
epithelial cell surface of rainbow trout intestine (Irie et al. 2004) which may explain some of
the mechanisms of attachment observed in previous studies. In a previous study, Chen and
Hanna (1992) recognized that V. anguillarum , V. ordalii and V. parahaemolyticus were able to
attach to cultured rainbow trout cells of gonads, smears of gills, intestine, buccal mucosa and
skin and cryostat sections of whole fish. Among the V. anguillarum strains, serotypes O1 and
O2 showed the greatest attachment. Numerous virulence factors (Toranzo and Barja 1993)
have been identified in V. anguillarum strains and the mechanisms for intestinal enterocyte
destruction and translocation are inferred from experiments using other host cell types. Once
in close association with host cells V. anguillarum strains produce a wide range of cytotox-
ins, including those from the repeat-in-toxin family (RTX; RtxABCHDE) and haemolysins
(Vah1-5), which in combination with other extracellular products increase membrane perme-
ability and up-regulate apoptosis (Li et al. 2011). A number of ex vivo studies, inoculating
V. anguillarum cells into the lumen of intestinal sacs, have depicted the detrimental effects
of V. anguillarum cytotoxins and other extracellular products on the intestinal epithelium of
rainbow trout (Figure 3.2; Harper et al. 2011), Atlantic salmon (Figure 3.3; Ringø et al. 2007)
and European sea bass (Figure 3.4). Effects at the brush border level include disorganized
microvilli and necrotic enterocytes.