Oysters, bacteria, and parasites, oh my!

Bienlien, L.M., Audemard, C., Reece, K.S., and R.B. Carnegie. 2021. Impact of parasitism on levels of human-pathogenic Vibrio species in Eastern Oysters. Journal of Applied Microbiology 00: 1-12. doi: 10.1111/jam.15287


Oysters are considered a delicacy by many people around the world. However, wild populations of oysters have suffered due to overharvesting, diseases, destruction of oyster habitats, and climate change. One solution to our shortage of oysters is oyster farming, or aquaculture. Oyster aquaculture in particular has grown rapidly in the past few decades, especially in the Chesapeake Bay, where farming the Eastern oyster is more than a $30 million dollar industry.

A bucket of eastern oysters, native to Chesapeake Bay. By U.S. Army Corps of Engineers Norfolk District from United States – 141001-A-OI229-004, Public Domain

Unfortunately, it’s not all good eating when it comes to farming oysters for people to consume. Oysters can contain a certain genus (or group) of bacteria that causes disease called vibriosis. While some species of Vibrio are harmless, one specific bacteria, called Vibrio vulnificus, can cause serious disease in people with compromised immune systems. Furthermore, another species of bacteria, Vibrio parahaemolyticus, can cause uncomfortable stomach flu. Scientists aren’t sure why, but these two strains of Vibrio can be present at different levels in oysters from the same place and at the same time. Because these bacteria can cause disease in humans, it’s important to understand why it can be present at different levels in some oysters and not others. In other words, why do some oysters contain more Vibrio than other oysters?

Researchers from the Virginia Institute of Marine Science hypothesized that these differences in whether (or how badly) oysters are infected with Vibrio bacteria might be linked to a different type of pathogen, specifically parasites. Parasites are organisms that live on or in hosts and get their food from the host, often at the expense of the host’s health. Parasites can affect oyster biology, and may even change how susceptible oysters are to Vibrio infections. These scientists set out to investigate whether two specific parasites had any influence on how oysters were infected with the two species of Vibrio bacteria. They looked at two singled-celled eukaryotic parasites, one called Perkinsus marinus, and one called Haplosporidium nelsoni. Both of these parasites are common in the Chesapeake Bay and cause disease in oysters, but Perkinsus marinus is the more common of the two.

A rack-and-bag oyster aquaculture setup found at the Acuaculture Genetics and Breeding Technology Center (ABC) at the Virginia Institute of Marine Science. Photo credit: VIMS ABC.

Oysters were sampled from the York River, a part of the Chesapeake Bay, during 2014 and 2015. Oysters were collected once every 2 weeks from August to October. Parts of the oyster were cut out and stained with a special dye to look for parasites under the microscope. The rest of the oyster was ground up and used to look for Perkinsus marinus, and the two species of Vibrio bacteria using their specific DNA.

Surprisingly, the researchers found no connection between Perkinsus marinus infection and Vibrio levels. Yet, there were still differences in Vibrio levels between oysters collected at the same time and from the same place. There was a potential correlation between Haplosporidium nelsoni and Vibrio vulnificus infections. Oysters that were infected with Haplosporidium nelsoni contained more Vibrio vulnificus than oysters without an H. nelsoni infection. However, oysters that were the most infected with Haplosporidium nelsoni didn’t necessarily contain the most Vibrio vulnificus. This may be due to individual differences in how each oyster responds to infection, but more research needs to be done. One factor potentially complicating the results of this study is that the oyster’s reproductive state might play a role in Vibrio levels. There is some evidence from past studies that oysters that are more developed in terms of their reproductive state had higher levels of pathogenic Vibrio parahaemolyticus than less developed oysters.

Overall, the researchers concluded that Perkinsus marinus parasitism is unlikely to have a big effect on levels of Vibrio species in oysters that could cause disease in humans. Haplosporidium nelsoni is a less common parasite, and oysters are in fact becoming more resistant to it, so the potential connection between this parasite and Vibrio levels may not be cause for concern. However, more research needs to be done that incorporates H. nelsoni infection into studies that look at levels of Vibrio vulnificus in oysters.

This research is important because oysters play a crucial role in many parts of an ecosystem. Oysters filter particles out of the water, improving water quality and keeping their habitat healthy. They also attach to the bottom of the Chesapeake Bay and other bodies of water, forming large oyster reefs that can be home to other species. In addition, unhealthy oysters eaten by humans can cause disease. To keep oysters, humans, and the ecosystem as a whole healthy, it’s important to understand why and how oysters can contain different levels of bacteria Vibrio.


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