more ideas to control disease in aquaculture including quorum sensing

Defoirdt, T., Sorgeloos,
P., Bossier, P. (2011) Alternatives to antibiotics for the control of
bacterial disease in aquaculture. Current Opinion in Microbiology, 14:251–258

As we know,
aquaculture is the fastest-growing food-producing sector worldwide and disease
outbreaks are considered to be a significant constraint to the development of
the sector. This paper reviews some ideas for disease control that are needed
because the use of antibiotics has been constrained due to concerns of resistance
and contamination. The main problem is resistance of useful antibiotics passing
to human pathogens.

An alternative strategy is the use of bacteriophages and
there have been many blogs on this subject. The advantages of using
bacteriophage therapy include their specificity (although this paper mentions
that some phage are effective to different family microbes) and therefore the
reduced risk of compromising the natural microbiota of the aquaculture
ecosystem. A further concern, that will be important to test,
is whether they carry any virulence genes and whether they would be safe to
use.

Another potential strategy is to target specific genes of
pathogenic bacteria by inhibiting their replication. This paper has one example
indicating the idea has not been fully researched yet but has potential.
Bacteria of Vibrionaceae all contain RctB as an initiator of
replication of their smaller second chromosome, which lacks any similarity to
characterised initiators. In a recent study, Yamaichi and colleagues identified
a compound, vibrepin, that inhibited RctB and had potent cidal activity against
V. cholerae and inhibited growth of all Vibrio species tested, including the
aquaculture pathogens Vibrio parahaemolyticus and Vibrio vulnificus.

Inhibiting growth of pathogens rather than killing them
may reduce the risk of resistance to a treatment. Short-chain fatty
acids are currently used in commercial (terrestrial) animal diets to control
pathogens such as Salmonella. They decrease the
intestinal pH, resulting in a more beneficial microbial community in the
digestive tract such as lactic acid bacteria. Recent research showed that these
compounds also inhibit the growth of pathogenic Vibrios. A major disadvantage
of using short-chain fatty acids in aquaculture feeds is that they would leach
into the culture water and consequently, high doses would be needed to retain
sufficient activity. To combat this, this paper hypothesizes that the
poly-b-hydroxybutyrate (PHB) polymer from Brachymonas bacteria can be added to
the rearing water. This is then degraded into b-hydroxybutyrate in the gut of
the aquacultured organisms and the release of this short-chain fatty acid
protects the host from pathogenic bacteria.

Another disease
control method is to reduce the virulence of pathogens by inhibiting virulence
gene expression. Quorum sensing is involved in the activation of virulence of
pathogens. Detection of signal molecules can be disrupted and stopped using
compounds found in nature from algae such as
D. pulchra, corals and sponges. Bacteria, such as Bacillus species, can degrade signal molecules in
order to control the levels. Results obtained so far indicate that organisms
with quorum sensing-inhibitory activity can be isolated from the aquatic environment
and that they show promise as novel biocontrol agents. In addition to
quorum sensing, other regulatory mechanisms control the expression of virulence
genes as well. So far, and not yet tested, is a molecule, called virstatin, that has been reported to
inhibit the transcriptional regulator ToxT, which controls the expression of
virulence genes of the ToxR regulon in Vibrios.

Overall this paper concludes that a number of different strategies must
be developed and used in rotation to prevent resistance development. The paper
is rather brief but puts forward further research directions to aid disease
control and probiotic choice.