Microbial Degradation of Seafood - Aquaculture Microbiology and Biotechnology - page 48

Aeromonas , Vibrio , Moraxella , Psychrobacter , Photobacterium , etc. and to a
lesser extent the CFB (Cytophaga-Flavobacter-Bacteroides ) group (Huber et
al., 2004; Wilson et al., 2008) . Nevertheless, Gram-positive bacteria such
as Micrococcus, Bacillus, Lactobacillus, Clostridium or Coryneforms , may
also be present in variable proportions (Shewan, 1971, 1977; Hobbs, 1983;
Mudarris and Austin, 1988; Gram and Huss, 1996a; Gennari et al., 1999;
Wilson et al., 2008) . Some genera like Vibrio, Photobacterium and Shewanella ,
require the presence of salt to multiply and are thus typically found in
seawater while Aeromonas is more common in freshwater even though it
is often isolated from marine products (Hanninen et al., 1997). In tropical
fi sh, the fl ora has the same composition overall (Al Harbi and Uddin, 2005;
Emborg et al., 2005), but often with a greater proportion of Gram-positive
bacteria ( Micrococcus, Bacillus, Coryneforms) and enterobacteria (Devaraju
and Setty, 1985; Liston, 1992; Huss, 1999).
The indigenous microfl ora of the gastro-intestinal tract of fi sh have
been studied much more than those of the skin or the mucus due to their
importance in digestion, nutrition and growth and in disease control in
aquaculture (Ringo et al., 1995; Spanggaard et al., 2000). Although this
environment is partially anaerobic, most researchers have observed a
predominance of aerobic bacteria which is also present in the surrounding
water and capable of surviving and multiplying in the particular medium
of the gastro-intestinal tract (Cahill, 1990). This predominance of aerobic
bacteria could be due to the collecting techniques, which are not always
suitable for strict anaerobes (Burr et al., 2005). Nevertheless, Huber et al.
(2004) have shown by molecular methods that the aerobic fl ora of rainbow
trout intestine usually represents 50-90% of the total fl ora. Gram-negative
bacteria dominate the intestinal fl ora. In general, Aeromonas , Pseudomonas
and members of the Flavobacterium / Cytophaga group are most often
found in the intestine of freshwater fi sh while Vibrio , Acinetobacter , and
Enterobacteriaceae are more common in marine fi sh (Ringo et al., 1995;
Ringo and Birkbeck, 1999). These are fermentative bacteria that develop
rapidly in the gastro-intestinal tract due to the low pH, the lack of oxygen
and the abundance of nutrients. Staphylococci have also been found to
be the dominant fl ora in the intestine of the Arctic char (Ringo and Olsen,
1999) . Although not predominant, lactic acid bacteria (LAB) (Lactobacillus ,
Carnobacterium , Streptococcus , Leuconostoc, Lactococcus, Vagococcus ) have
often been isolated from the gastro-intestinal tract of fi sh (Ringo and
Gatesoupe, 1998).
Both the number and diversity of the microfl ora are probably widely
underestimated because the majority of studies carried out so far have
used classic microbiological methods involving growth on agar media.
The cultivability of bacteria has been estimated to be sometimes less than
2% of the intestinal fl ora of rainbow trout (Huber et al., 2004), or even