You’ve likely seen it: the warning at the bottom of a sushi menu alerting you that consuming raw or undercooked seafood carries a risk of foodborne illness. It’s not a warning that stops the average person from ordering a tuna roll, and in most cases that tuna roll does not lead to hospitalization – otherwise the sushi industry would be in trouble. But have you ever stopped to wonder why this warning exists, and how the seafood industry can continue to put that raw fish on your plate?
Foodborne illnesses are a significant challenge facing public health, especially when it comes to meat and seafood. Salmonellosis, caused by the bacteria Salmonella enterica, is one of the most well-known of these illnesses and can be deadly, especially in immunocompromised populations, according to the World Health Organization. Humans generally get the disease by eating contaminated food, hence that warning at the bottom of your sushi menu, and there are an estimated 1.35 million cases in the US each year, making the disease the leading cause of hospitalizations and deaths from foodborne illness.
Salmonella enterica is a species of bacteria that can infect animals and humans by invading the GI tract. It is the causative agent of Salmonella infection, or salmonellosis, and mostly causes infection when contaminated food is consumed. The bacterium is able to survive for long periods of time in environments that are hot and dry, which is why it prefers the environments of the GI tract and farms. In general, extra fluids and antibiotics are enough to treat the disease, but complications can lead to more severe outcomes and even death.
Current sanitation practices for preventing salmonella contamination are numbered, and include restrictions on temperature, exposure to certain surfaces, and movement between different areas. A lot of care and time is put into attempting to maintain a clean and sanitary environment, but bacteria are hardy and can still slip through unnoticed. One of the ways salmonella can endure sanitation measures and continue to spread is through biofilm formation, which poses a major challenge that still needs to be addressed in the food industry.
Biofilms are three-dimensional groups of numerous individual bacteria that are held closely together in a matrix by extracellular polymeric substances, or EPS. EPS consists of molecular strands of various biological components such as polysaccharides, proteins, lipids, and DNA. It has many roles in biofilm development, including providing structural integrity, allowing biofilms to stick to surfaces such as seafood, and offering protection against the environment and antibacterials. This makes bacteria better at adapting to diverse environments and harder to eliminate. Biofilms are formed by many types of bacteria and thus can be found almost anywhere. For instance, when your dentist tells you to brush and floss, it is so the bacteria in your mouth are prevented from forming a common biofilm – dental plaque. (Learn more about biofilms and their life cycle here!)
Along with EPS, biofilm stability is aided by quorum sensing, a form of cell-cell communication that aids in gene regulation. Bacterial cells are able to “communicate” by measuring total cell population density through the secretion of signaling molecules called autoinducers. As cell density increases, so does the production of these autoinducers because all of the cells are secreting this molecule at a constant rate. The cells are able to sense the concentration of autoinducer in the environment, and when the concentration reaches a certain threshold level it leads to a change in gene expression in the bacteria. In the case of S. enterica, the genes affected include those associated with biofilm development.
Because biofilm formation is such a problem in the seafood industry, researchers seek to find ways of limiting biofilm growth. This was the goal of researchers Anamika Roy, Pantu Kumar Roy, Sung Rae Cho, and Shin Young Park when they decided to test the anti-biofilm ability of a substance called fucoidan against S. enterica on crab and shrimp surfaces. Fucoidan is a natural antibacterial agent found in brown algae, and is thought to have properties that prevent bacteria from getting the nutrients they need to survive as well as play a role in the suppression of bacterial pathogens by altering bacterial cell wall or membrane integrity. The scientists treated groups of crab and shrimp with fucoidan and then observed S. enterica biofilm development in this treatment group and a control group of crab and shrimp not treated with fucoidan. What they found is that fucoidan was able to effectively inhibit biofilm growth and development in a dose-dependent manner: as the concentration of fucoidan increased, the rate of inhibition of biofilm increased as well.
The above figure is a graphical representation of the effective and dose-dependent inhibition of biofilms by fucoidan. Here, MIC defines the lowest fucoidan concentration where no bacterial growth was observed. Therefore, as you move across the X axis from left to right, the concentration of fucoidan is increasing from none, to fractions of the MIC, to the complete MIC being the highest tested concentration. There were significant differences between all the experimental groups in both crab and shrimp, represented by the different letters above each bar in the graph. In Figure 2, there was not a significant difference between the control group and the 1/8MIC group, hence the ‘a’ over both of those bars. The graph makes it quite clear that as the concentration of fucoidan goes up, the amount of biofilm formation is decreasing.
Not only did fucoidan reduce biofilm formation, but it also disrupted the development of pre-formed biofilms. This means fucoidan has the potential to act as an antibacterial agent even after S. enterica has contaminated a surface! Along with this, in fucoidan-treated populations the bacterial cells appeared irregular and rough in appearance compared to the control populations, providing evidence of EPS breakdown since EPS offers structural integrity for the bacterial cells. These findings pose exciting possibilities, but just how exactly does fucoidan decrease biofilm growth and development? What the researchers found is that fucoidan breaks down the EPS matrix by controlling gene expression of virulence genes, disrupting quorum sensing, and interfering with EPS production by altering important regulatory pathways.
Much like quorum sensing, virulence genes are important for bacterial communication and biofilm control. Outside of biofilm development, these genes have many other important pathological functions that you can learn more about here! These genes are downregulated in the presence of fucoidan, meaning that this communication and control decreases as well. This combined with a disruption of quorum sensing leaves little opportunity for the communication and regulation needed to effectively form the EPS matrix necessary for biofilm formation, and when you add in the decrease in EPS production due to interference in regulatory pathways, it’s easy to understand why the bacteria struggle so much to make a complex biofilm structure.
But when all is said and done, why does this matter? How many people have eaten sushi and been completely fine? The problem is that even as our technology and sanitation measures evolve, so do bacteria. Many subtypes of S. enterica have become antibiotic resistant, meaning that old treatment measures won’t work on them and they will continue to contaminate surfaces. These types of salmonella are becoming more common, and rates of human infection are not going down but rather remaining stagnant. Therefore, using fucoidan as a natural antibacterial would enhance food safety and minimize contamination in seafood processing. Fucoidan is a better alternative to chemical disinfectants for many reasons, including its affordability and minimization of adverse side effects. It is a naturally-derived, food-safe substance that offers minimal toxicity and is much more environmentally friendly as well. Finally, using fucoidan would lessen the danger of antibiotic resistance in S. enterica because it inhibits biofilm development even at minimal concentrations. With so many people becoming more concerned about “unnatural chemicals” in everything from vaccines to drinking water (despite these fears being unfounded), offering a natural alternative to chemical treatment could also be effective in supporting the seafood and fishing industries by addressing concerns of salmonellosis and toxic chemicals in one fell swoop.
About the Author:
Audrey Brogna ‘26 is a senior biology and French major at MHC. She loves anything molecular and microscopic, and when she’s not studying you can find her skiing, hiking, reading, or playing ultimate frisbee!
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