antibiotics and other therapeutic agents. The drawback of this approach
is that the ability of bacteriophages to reduce the numbers of bacteria is
limited by its high specifi city. Production of virus particles specifi c for the
pathogenic bacterial strains in question at each time is therefore required.
In addition, resistance to bacteriophages is quickly developed after a
relatively short period of time due to short generation time of bacteria.
The following are the advantages of bacteriophage therapy against
i) The high specifi city results in protection of intestinal microbiota and
ii) The ability of propagation.
Once susceptible bacteria are present in the fi sh, then few additions of
bacteriophage may be suffi cient for protection against pathogens (Nakai
and Park, 2002).
In larviculture, there is a limited application of bacteriophages as
mortalities are seldom attributed to specifi c pathogens. However, in a
particular hatchery it can occur that problems with mortalities in larval
rearing tanks are related to a specifi c pathogen. In such cases, a tailor-made
bacteriophage therapy could be an alternative solution. Bacteriophages
have shown encouraging results in the case of protection of shrimp larvae
against Vibrio harveyi (Karunasagar et al., 2007).
Interference with Quorum Sensing
Quorum sensing is a mechanism by which bacteria coordinate the
expression of certain genes in response to their population density by
producing, releasing and detecting small signal molecules, inducing a
physiological response which implies an ecological advantage (Defoirdt
et al., 2004). Gram negative bacteria use acylated homoserine lactones
(AHLs) and/or autoinducer 2 (AI-2) as signal molecules (Fuqua et al.,
2001; Miller and Bassler, 2001; Whitehead et al., 2001), whereas gram
positive bacteria use secreted peptides (Dunny and Leonard, 1997; Miller
and Bassler, 2001).
Different bacterial phenotypes are controlled by a quorum sensing
system, the most important being the expression of virulence factors
and biofi lm formation (De Kievit and Iglewski, 2000; Miller and Bassler,
2001; Whitehead et al., 2001). An AI-2 mediated system, is responsible
for the virulence of the fi sh pathogen Vibrio harveyi (Gómez-Gil et al.,
2004) toward gnotobiotic Artemia franciscana (Defoirdt et al., 2005), being
also involved in the growth-retarding effect of this bacterium toward
gnotobiotic Brachionus plicatilis (Tihn et al., 2007). Although these two
models of gnotobiotic Artemia and gnotobiotic rotifers are diffi cult to
compare directly, it seems that V. harveyi quorum sensing systems might
operate in a host-dependent way (Deforidt et al., 2005; Tinh et al., 2007).