Friday, 30 June 2017

Seafood Poisoning, Shrimp Economies and Bacteria

Food poisoning caused by eating raw or undercooked seafood is very common. The Centers for Disease Control and Prevention (CDC) estimates that there are around 80,000 cases of seafood poisoning in the U.S. every year. What actually causes this is a bacteria called Vibrio parahaemolyticus - which sounds a bit like a Roman Gladiators name in my opinion.


It was previously thought that the bacteria stayed outside the cells of your stomach and intestines and shot molecules at them, called effectors, which like the name suggests affects the cells and damages them. The damage to these cells then causes what we normally associate with food poisoning - vomiting and diarrhoea for a few days before getting back to normal. A very unpleasant experience for most people but actually, food poisoning can be a very serious problem if you have weakened immune system like in conditions such as HIV, diabetes or liver disease. If the bacteria get into the bloodstream of these people, it can cause blood poisoning (also called septicemia) which can be fatal.

However, a new finding by researchers at UT Southwestern Medical Centre shows that shooting molecules is not how the bacteria works at all. The bacteria actually stabs cells with a needle called a Type III Secretion System (T3SS) which then injects toxins into your gut and intestine cells.

The study was undertaken by Dr Kim Orth, Professor of Molecular Biology and Biochemistry at UT Southwestern and an investigator at Howard Hughes Medical Institute and her colleagues, and was published in the journal PLOS Pathogens.

In 2012, the Orth Laboratory identified how V. parahaemolyticus tricks cells into taking the bacteria in with open arms. In this study, the scientists followed the bacteria while it was infecting cells by freezing the bacteria every 15 minutes in the middle of whatever it was doing very quickly (known as flash freezing), and tracked what happened over time. The scientists found that when the bacteria gets into the cell, one effector molecule that comes from the needle T3SS, called VopL, disarms the cell’s defence mechanisms by breaking up all the “roads” so the bacteria-killing machines cannot get to where they need to be in the cell and the bacteria survives.

The “roads” are the cell’s skeleton and without it, the bacteria-killing machines cannot create reactive oxygen species (ROS) which are like missiles that kill the bacteria by damaging its DNA.

“By hijacking the cell skeleton, VopL prevents the cell from launching one of its major weapons, ROS,” said Dr Orth.

The scientists wanted to make sure that it was VopL that was causing the cell's skeleton to dysfunction and not something else, so they made two different types of bacteria - ones that use VopL and one that doesn’t. This confirmed the scientist's theory that it was VopL that was breaking up the cell’s skeleton, as the bacteria that didn’t use VopL was killed by ROS molecules from the bacteria-killing machines in the cell, suggesting that the cell skeleton was still working. The experiment also showed that VopL not only breaks up the cell's skeleton but then gathers the pieces together in bundles called filaments which have no function.

So what is Vibrio parahaemolyticus?

V. parahaemolyticus is a bacteria that lives in warm oceans or estuaries throughout the world. There are around 80 known Vibrio strains, but only a dozen can infect humans. The CDC has estimated that 45,000 people get food poisoning from V. parahaemolyticus every year and “is the reason for the old saying that you shouldn’t eat oysters in months without an “r” in them, meaning the summer months” says Dr Orth.

However, the risk of getting food poisoning from this type of bacteria is increasing with global warming. As the ocean's temperature is rising, it means that the bacteria is spreading around the world at an increased rate and is becoming active earlier in the year (from May - October). This was shown in practical terms when Alaska reported its first ever outbreak of V. parahaemolyticus infections in 2004.

What have Shrimp got to do with it?

As well as affecting humans, V. parahaemolyticus has been identified as the cause of the shrimp illness acute hepatopancreatic necrosis disease (AHPND).

AHPDN was discovered in China in 2009 and since then it has caused a huge economic burden on the shrimp industry with worldwide production down by up to 20%. It wasn’t until 6 years later (2013) that scientists actually identified V. parahaemolyticus as the bacteria causing the disease. It causes early deaths in shrimp (around the 35-day old mark) with a death rate of 40-100%.

In the Philippines, the export value of shrimp is the second most important aquaculture commodity. They mostly cultivate black tiger shrimp (Penaeus monodon) and Pacific white shrimp (Penaeus vannamei). AHPDN has caused massive economic losses for shrimp farmers in some of the major production provinces in the country which has threatened production growth and export expansion of the Philippine shrimp industry. This has led to around 100,000 job losses.

Recently, new technologies for the molecular detection of AHPDN have been developed which is helping to speed up diagnosis efforts and is also speeding up research into the disease for potential treatments.

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