A kelp patch twice the size of the United States that’s sweeping through the Caribbean and carrying loads of seaweed to Florida beaches is also carrying some unwelcome tourists: disease-causing bacteria. And the plastic debris we pour into the oceans could make it worse.
Bacteria belong to a genus called Vibrio, which includes more than 100 species. About a dozen of these can harm humans, typically by infecting open wounds or causing food poisoning when consumed. In a new study published May 3 in the journal Water researchscientists have analyzed the genomes of Vibrio found on samples of plastic debris and Sargasso, a brown seaweed, fished in the Caribbean and Sargasso seas in the North Atlantic Ocean. Their goal was to understand whether rafting species could cause disease in humans.
“It’s actually quite difficult to prove that something is a pathogen,” says Linda A. Amaral-Zettler, a marine microbiologist at the Royal Netherlands Institute for Sea Research and one of the paper’s lead authors. “But effectively, we’ve shown that the genome itself contains all the ingredients, if you will, for a pathogen.”
This is an important new step, says marine biologist Valerie Michotey of the University of Aix-Marseille in France, who was not involved in the new research. “Previous studies have just demonstrated the presence of Vibrio, but they didn’t analyze whether they were pathogenic or not,” he says.
The most dangerous species of Vibrio I am V. cholerae, causing the diarrheal disease cholera, e V. vulnifico, often called “flesh-eating bacteria” because of the sometimes fatal damage it can cause if it infects an open wound. Amaral-Zettler and his colleagues found none of these species in the samples they studied. Instead they found four other known species: V. alginolyticus, V. campbellii, V. strong AND V. parahaemolyticus.
Because bacteria can swap genes and have high genetic diversity, identifying a species isn’t as helpful in determining how dangerous a bacterium is to humans as studying which genes are present in its DNA. In all of the samples they analyzed, the researchers found a gene for a protein that V. vulnifico it uses to attach itself to human intestinal cells and genes that produce compounds that break down red blood cells. In several of the samples, they also found a gene that produces a toxin that weakens the connections between the cells lining the intestines, all evidence that the bacteria could harm humans.
“I think it’s one of the first studies of its kind that has been able to piece together the genomes of that type of environmental sample,” says Craig Baker-Austin, a microbiologist at the UK’s Center for Environment Fisheries and Aquaculture Science, who was not involved in the study.
The researchers also performed laboratory tests which showed that most of the Vibrio found in samples attached very well to plastic and could rupture sheep blood cells, a telltale sign of potential pathogenicity. “They’ve combined genetic approaches and also biochemical approaches, so they bolster the findings,” Michotey says. She notes, however, that researchers don’t know how quickly pathogenic bacteria occur on plastic or debris sargasso, which will be an important step in understanding how big a health risk it is Vibrio represent bacteria.
However, Amaral-Zettler says the new research is conclusive enough to be “a small wake-up call” about the complicated relationship between Vibrio, Sargasso and humans. While Sargasso creates a natural ecosystem that helps sustain fish, sea turtles and other animals in the open ocean, over the past decade it has begun to regularly form huge blooms that spill onto beaches. Scientists say these blooms are fueled in part by the runoff of nutrients, including fertilizers, into the oceans. Humans are also responsible for the recent flood of plastic-giving marine debris Vibrio multiple footholds, as well as rising water temperatures that scientists have shown are creating a ripe environment for bacteria to thrive.
Despite all this, why Vibrio are naturally marine bacteria, they can only accidentally pose a threat to humans. “It’s not that their entire evolutionary mission is to be pathogenic to humans; humans are sort of unintentional intruders into the pathways of these organisms,” says microbiologist Rita R. Colwell of the University of Maryland, who was not involved in the study. However, Colwell says the new research adds to the evidence for the human exposure to Vibrio it’s increasing in ways that people need to respond to.
For beach goers, bacteria are a reason to stay away from Sargasso which could accumulate in the coming months. “In general, if you have open wounds, it’s not a good idea to hang out on the beach,” says Amaral-Zettler.
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