Controlling Bacterial diseases in aquaculture

Antibiotics are widely used medicines for controlling diseases both for us and our food. Nowadays, studies worldwide show problems concerning a higher number of pathogenic bacteria strains becoming resistant, increasing health risks. We have high dependency on aquaculture, it is the fastest-growing food-producing business. However, it costs around several billion US$ per year to prevent disease outbreaks and denature those already occurring. Resistant pathogen studies are important in determining how resistance spreads. Defoirdt et al., (2011) states resistance genes, transferable plasmids and integrons are passed via horizontal gene transfer to bacteria, causing diseases such as Salmonella and Cholera. These studies are also important as diseases limit aquaculture development. As a high abundance of untreatable, harmful bacteria grow in waters around shoals, are consequently ingested by fish, they then become treatable. In the absence of diseases antibiotics are still used, causing resistance. Thailand’s 76 shrimp farms received daily doses of 10 antibiotics.

This is an easy to follow review paper detailing other methods, currently tested, for potentially replacing antibiotics. However, methods were not written but referenced other studies, this could have enhanced my understanding. Novel biocontrol strategies have reported success in certain fish and shrimp species, preventing disease. Defoirdt et al , (2011) critically details benefits and constraints of each technique.

1)    Killing one specific pathogen. Using antibacterial agents, bacteriophages specific to infecting target strains, results in decreased resistance and beneficial bacteria harm. Shivu isolated strain-specific phages from shrimp hatcheries and V. harveyi strains, he found lysed phages could not infect other Vibrio sp. Therefore, tiger shrimps (P. mondon) survival increased from 17-86%. However, virulence genes can transfer resistance to phage attachment. Vibrionaceae genomes contain rctB homologues, a perfect target for RctB to inhibit virulence gene replication.

2)    Growth inhibitors. Using short-chain fatty acids in animal diets controls Salmonella and Vibrio’s growth by decreasing intestinal pH, thus, increasing useful microbial communities. One limitation is excess leaching, therefore, high doses are needed. Recently, polyhydroxyalkanoates are added to gastrointestinal tracts producing similar effects, once degraded, to short-chain fatty acids, increasing survival rate.

3)    Inhibition of virulence gene expression. Beneficial as pathogen’s rate of developing resistance is extremely low, however, there is environmental selective pressure. Quarom sensing has two systems in Vibrio’s, first; use N-acylhomoserine lactones (AHLs) as signal molecules in the cytoplasm of Gram-negative bacteria. The second; multi-channel quorum sensing at the cell surface. Both systems regulate virulence gene expression, therefore, interfering with signal molecule detection is a focus. In turbot, lactonase enzymes in Bacillus sp inactivate AHLs. Halogenated furanones also disrupt gene expression preventing vibriosis. Reports however, show it is toxic in slightly higher concentrations than when effective, therefore, a food additive cinnamaldehyde is used. Other quarom sensing inhibiting compounds are produced by algae, corals, sponges, epidiotic bacteria and marine bacterial secondary metabolites. Antivirulence therapy also inhibit specific virulence factors. The cholera toxin in V. chloerae is controlled by the ToxR regulon, expression correlating to its harmfulness, reports of virstatin inhibits gene expression, yet, untested.

                    

This review, in my opinion, suggests antibiotics are becoming outdated. Further work is needed regarding combining new biocontrol strategies for disease prevention, as resistance development still occurs individually. This paper mainly focusses on shrimp aquaculture, including a wider range of species would have benefitted. Therefore, future aquaculture research on farmed species would aid disease prevention. Better or more biocontrol strategies could result, be combined, reducing antibiotic addiction. My knowledge was limited on aquaculture. Thus, novel biocontrol strategies, plus learning the molecular agents are obtained from aquatic organisms, attracted my interest as they link to modern day human medicinal research and species evolution.

Reference

Defoirdt, T., Sorgeloos, P., & Bossier, P. (2011). Alternatives to antibiotics for the control of bacterial disease in aquaculture. Current Opinion in Microbiology, 14(3), 251-258

https://s3.amazonaws.com/academia.edu.documents/45238802/Alternatives_to_antibiotics_for_the_cont20160430-7911-muvuev.pdf?AWSAccessKeyId=AKIAIWOWYYGZ2Y53UL3A&Expires=1508923193&Signature=4MphVgjkwT9GrJAJGCpYHTjsm%2BA%3D&response-content-disposition=inline%3B%20filename%3DAlternatives_to_antibiotics_for_the_cont.pdf