When studying bacteria, human pathogens always get a lot of interest and free press. Pathogens of smaller and less important seeming animals, such as shrimp, tend to generate less press interest. After all, what is so exciting about shrimp?
Since the 1970s commercial shrimp farming has been expanding rapidly to meet the demands of a growing population and an expanded western palette. Bacteria that kill off shrimp can severely affect the industry, particularly as there are limited species being grown in highly dense populations. A recent disease of farmed shrimp known as “early mortality syndrome” (i.e “the shrimp are dying!”) has cost the Asian shrimp farming sector around 1 billion US dollars and a bacteria has recently been isolated that is capable of causing the disease.
The bacteria thought to be responsible is Vibrio parahaemolyticus. While the bacteria is capable of causing the disease in shrimp it’s not clear or confirmed whether it actually is leading to all the instances seen. However removing it from the shrimp culture will only help to make the shrimps healthier, not to mention the people eating the shrimp! There are two ways to remove the bacteria, the first is via disinfectants – trying to expunge all V. parahaemolyticus from the water before the shrimps are added. The other is by adding microbiotica cultures to try and out compete the pathogenic bacteria.
This isn’t the first Vibrio bacteria that has caused illness in shrimp, and lessons can be learnt from previous strategies. In the early 1990s Vibrio harveyi was infecting shrimp with luminescent vibriosis. The practice of decontaminating ponds prior to introducing shrimp meant that the natural bacterial flora of the pond was being destroyed, allowing opportunistic pathogens such as the Vibrios to flourish. Addition of antibiotics (often misused and applied incorrectly) only increased the antibiotic resistance of the pathogens.
It’s been observed that the disease is less prevalent in ponds colonised by certain small crustaceans which indicate a mature and stable ecosystem. Rather than clear-water systems (i.e clean water with shrimp growing in) decreased shrimp death has also been seen in ‘greenwater’ systems, where natural algae and bacteria are allowed to grow and spread. It’s thought that these natural strains out-compete the opportunistic pathogens, preventing them from spreading. It’s also likely that natural bacteria and algae produce antibiotics of their own to keep out invaders.
There are also some interesting bio-control strategies proposed. Rather than blitzing the ponds with disinfectants and antibiotics designed to kill the bacteria, treatments can be used which prevent the bacteria from infecting the shrimp without actually killing the bacteria. In the case of the Vibrios, targeting their cell-to-cell communication system (quorum sensing) has been shown to decrease pathogenicity in Vibrio harveyi. There’s also development of antibacterial treatments that are active within the injured shrimp – essentially personalised shrimp medicine.
As antibiotic resistance increases it’s always exciting to see new therapies proposed that use alternatives. Even if they’re only relevant for shrimp!
Reference 1: De Schryver P, Defoirdt T, Sorgeloos P (2014) Early Mortality Syndrome Outbreaks: A Microbial Management Issue in Shrimp Farming? PLoS Pathog 10(4): e1003919.
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