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November 25, 2013
Washington State—Dr. Stephen Newman on Vibrio parahaemolyticus
Vibrio parahaemolyticus, which has virulent and benign strains, causes acute hepatopancreatic necrosis disease (AHPND). The AHPND strain colonizes the stomachs of shrimp by the formation of a biofilm, which protects it from antibiotics and other potential treatments. V. parahaemolyticus tolerates a range of salinities, pH and temperatures and readily piggybacks on marine plankton and may be spread by ocean currents.
A relatively small number of Vibrios cause disease in farmed shrimp. Most are opportunistic pathogens that are able to cause disease because the host animals are stressed and impaired in their ability to fight them off. Only a few are obligate pathogens that cause disease in healthy and impaired animals just by being present. The most virulent of these kill shrimp at very low levels of exposure in the water or when consumed.
V. parahaemolyticus is a common inhabitant of most marine estuarine and some freshwater ecosystems. Most strains are not pathogenic and are harmless to ingest, however, toxin-producing strains of this bacteria, typically found as natural inhabitants in many fish species, are a major cause of seafood poisoning.
Vibrio species can also be spread by broodstock and postlarvae. Clearly the etiologic agent of AHPND occupies many niches. This could explain how it seems to move so easily and why eradication and/or control will not be a simple matter.
A biofilm is an assemblage of organisms that have attached to a surface, like the detritus on shrimp pond bottoms or, in the case of AHPN, the stomachs of shrimp. The biofilm protects the bacteria within from the action of antibiotics and other bacteria that would seek to occupy the niche themselves.
In the formation of biofilm, the bacteria first attach to the chitinous stomach and gastric mill surfaces in the shrimp. The bacteria then form sticky exopolymers that “glue” the bacteria to the surface. As the biofilm matures and forms micro colonies, exopolysaccharides protect the bacteria against antibiotics, disinfectants, herbal extracts and other treatments while still allowing normal metabolic cell activity.
Given the complex nature of the Vibrio genus, a number of things make AHPND a less-than-straightforward problem to address. V. parahaemolyticus produces a toxin that does the damage. Most other pathogenic Vibrios invade the animal and overwhelm the ability of it to defend itself, with ensuing decline and death. The strain of V. parahaemolyticus responsible for AHPND does not appear to be invasive in the sense of finding its way into the hemolymph (blood) of animals through injuries or other mechanisms. This explains why antibiotics do not stop the AHPND infection. If the antibiotics are not able to come in contact with the pathogen at sufficient levels to impact it, then they will not work. Although there is room for the appropriate use of some antibiotics in shrimp farming, this is one example where the use of antibiotics would be inappropriate. Hiding in a biofilm can protect V. parahaemolyticus from the action of many other compounds that could theoretically kill it. This will pose a serious challenge to those trying to develop treatments.
This article was based on a longer paper by the Dr. Newman. To read the full text, visit http://www.sustainablegreenaquaculture.com.
Information: Stephen G. Newman, Ph.D., President and CEO, AquaInTech, Inc., Lynnwood, Washington 98037, USA (phone 1-425-787-5218, email email@example.com, webpages www.bioremediationaquaculture.com and www.sustainablegreenaquaculture.com).
Source: The Global Aquaculture Advocate (The Global Magazine for Farmed Seafood). Editor, Darryl Jory (firstname.lastname@example.org). AHPN Inferences Based on Behavior of Vibrio Bacteria. Stephen G. Newman. Volume 16, Issue 6, Page 34, November/December 2013.
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