18 Aquaculture Microbiology and Biotechnology
involved remains unknown. It is also important to consider the specifi city
of immunostimulants, since the stimulation may be too intense, which may
become detrimental or even lethal to the host. Moreover, the knowledge
of the functions of different immunostimulators may be used to stimulate,
more specifi cally, those parts of the immune system that may be more
relevant in certain situations (Vadstein, 1997). The recent introduction of
genomic and proteomic tools might be of special interest in this context.
Strategies Based on the Food Chain
Large scale use of probiotics in the rearing of marine fi sh larvae involves
cultivation of bacteria in artifi cial medium, preparation of a stable form of
these bacteria and development of a process for the transfer of probiotics
to the larval gut either by direct addition to water in the rearing tanks or by
bioencapsulation in live feed. The approach of addition of bacteria to the
water is simpler, but implies that the added bacteria remain in the water
for some time, accumulate in rotifers or Artemia and thereby get introduced
in the larval gut. A far more effi cient approach is the bioencapsulation
of bacteria in live feed. This is the only possible route in the case of
allochthonous bacteria of tellurian origin tested in larviculture, such as
lactic acid bacteria and bacillus strains (Gatesoupe, 1994). These bacteria
are not normally encountered in marine systems, but when introduced in
the larvae may have positive effects by excreting antimicrobial compounds
or by other ways of limiting the growth of harmful bacteria.
Bioencapsulation of bacteria is a relatively predictable process in the case
of rotifers, but more variable in the case of Artemia metanauplii, as the size
of bacteria may be decisive for the effi ciency of the process (Makridis et al.,
2000b). Artemia metanauplii have a fi ltering device which enables them to
fi lter large bacterial cells or aggregates of bacteria, although the effi ciency
of this process is not so high. Bioencapsulation has a dual purpose as on
one hand it provides probiotic bacteria to the larvae and on the other hand
replaces the microbiota originally found in the live feed, acting as a control
agent of opportunistic or pathogenic bacteria in live prey.
New Molecular Tools
The elucidation of the underlying mechanisms of action and the
development of new strategies for microbial control requires the
identifi cation and characterization of the bacterial communities associated
with the different parts of the rearing system. If probiotic or pathogenic
bacterial strains are introduced during in vivo challenges, the specifi c
detection and quantifi cation of introduced strains is also required.
Less than 1% of total bacteria in natural seawater systems are culturable
(Hansen and Olafsen, 1999; Kjelleberg et al., 1993) and thus the use of culture
dependent methods leads to underestimate microbial diversity. Moreover,
involved remains unknown. It is also important to consider the specifi city
of immunostimulants, since the stimulation may be too intense, which may
become detrimental or even lethal to the host. Moreover, the knowledge
of the functions of different immunostimulators may be used to stimulate,
more specifi cally, those parts of the immune system that may be more
relevant in certain situations (Vadstein, 1997). The recent introduction of
genomic and proteomic tools might be of special interest in this context.
Strategies Based on the Food Chain
Large scale use of probiotics in the rearing of marine fi sh larvae involves
cultivation of bacteria in artifi cial medium, preparation of a stable form of
these bacteria and development of a process for the transfer of probiotics
to the larval gut either by direct addition to water in the rearing tanks or by
bioencapsulation in live feed. The approach of addition of bacteria to the
water is simpler, but implies that the added bacteria remain in the water
for some time, accumulate in rotifers or Artemia and thereby get introduced
in the larval gut. A far more effi cient approach is the bioencapsulation
of bacteria in live feed. This is the only possible route in the case of
allochthonous bacteria of tellurian origin tested in larviculture, such as
lactic acid bacteria and bacillus strains (Gatesoupe, 1994). These bacteria
are not normally encountered in marine systems, but when introduced in
the larvae may have positive effects by excreting antimicrobial compounds
or by other ways of limiting the growth of harmful bacteria.
Bioencapsulation of bacteria is a relatively predictable process in the case
of rotifers, but more variable in the case of Artemia metanauplii, as the size
of bacteria may be decisive for the effi ciency of the process (Makridis et al.,
2000b). Artemia metanauplii have a fi ltering device which enables them to
fi lter large bacterial cells or aggregates of bacteria, although the effi ciency
of this process is not so high. Bioencapsulation has a dual purpose as on
one hand it provides probiotic bacteria to the larvae and on the other hand
replaces the microbiota originally found in the live feed, acting as a control
agent of opportunistic or pathogenic bacteria in live prey.
New Molecular Tools
The elucidation of the underlying mechanisms of action and the
development of new strategies for microbial control requires the
identifi cation and characterization of the bacterial communities associated
with the different parts of the rearing system. If probiotic or pathogenic
bacterial strains are introduced during in vivo challenges, the specifi c
detection and quantifi cation of introduced strains is also required.
Less than 1% of total bacteria in natural seawater systems are culturable
(Hansen and Olafsen, 1999; Kjelleberg et al., 1993) and thus the use of culture
dependent methods leads to underestimate microbial diversity. Moreover,