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New strategies for the control of bacterial infections in marine fish larval rearing 5
After mouth opening and the onset of exogenous feeding, it is relatively easy to
manipulate the bacterial diversity in the gut by the use of bacteria bioencapsulated in
prey as rotifers or Artemia metanauplii (Makridis et al., 2000a). This phase is critical in
many aspects for the successful rearing of marine fish larvae as pioneer species, i.e.
bacterial populations that firstly colonize the gut, have a competitive advantage
compared with bacteria that reach the gut at a later stage, when available adhesion sites
have been reduced and environmental conditions are influenced by the bacterial
populations already established (Hansen and Olafsen, 1999; Skjermo and Vadstein,
1999).
A main characteristic of the type of colonization in the gut of larvae is that species
established during this period are mainly opportunistic, taking advantage of the input of
nutrients in a “new” environment (larval gut). The establishment of excessive numbers
of bacteria in weakened larvae may result in high mortalities, but mortalities in healthy
larvae, not exposed to environmental stress and fed on high quality prey, are low and
not dependent on the numbers of bacteria present in the gut (Makridis et al., 2008;
Salvesen et al., 1999).
At the stage of weaning, the numbers of bacteria decrease and relatively few
bacteria are measured in the fish gut per unit of gut weight compared with the earlier
stages (Makridis et al., 2008). The development of stomach and its acid barrier makes
more complicated the direct influence of species established in the intestinal microbiota
by addition of probiotics in the feed.
1.2. New strategies for bacterial infections control in fish larvae
The development of new strategies is based mostly on the study and knowledge of
microbial diversity and ecology in marine fish larviculture systems (Figure 1.1).
Strategies can be classified as based on:
i) Acting and modifying the microbiological communities of the system,
including or not the introduction of live microorganisms.
ii) Acting on host-microbe interactions, which include the use of
immunostimulants.