Sharks and rays – not the first thing that often comes to one’s mind when it comes to staples on a menu. Yet, they are frequently consumed in many coastal communities such as along the coast of Rio de Janeiro, Brazil! But although these two species are a dietary staple, they is a notable lack of research on the bacterial risks associated with these marine species (especially given the potential health threats posed by antibiotic-resistant bacteria). A recent study sought to fill this gap by conducting a bacteriological assessment on sharks and rays landed at one of the region’s most significant fishing ports. The results revealed a troubling presence of various pathogenic bacteria, some of which are known to pose substantial public health risks.
Led by Dr. Maíra Duarte Cardoso of Fundação Instituto de Pesca do Estado do Rio de Janeiro, the team began by isolating bacteria from the cloacal swabs of several sampled elasmobranchs. The bacteria were cultured, and thus began the identification process, which involved biochemical assays, antimicrobial susceptibility testing, and Polymerase Chain Reaction (PCR) assays. It seems like a lot of steps – and it is! – but this allowed for the researchers to determine not only the types of bacteria present but also their potential resistance to antibiotics, a critical factor in the spread of infection. A range of strains were identified, including Vibrio, Aeromonas, and Enterobacterales. Known to play a role in human infections, these bacteria are of significant concern in the One Health context, where the health of humans, animals, and the environment is considered interconnected.
Among the most prevalent bacteria found were Morganella morganii and Citrobacter freundii, both of which are Enterobacterales, a family of bacteria that includes various pathogens known to cause gastrointestinal illnesses. Vibrio harveyi and Vibrio fluvialis were the dominant species of Vibrio found, while Aeromonas allosacharophila and Aeromonas veronii bv. veronii were the most frequent Aeromonas species; these bacteria are often associated with waterborne diseases and can lead to severe infections in humans, particularly those with compromised immune systems. The fact that these species were found in sharks and rays is concerning, especially since they can be transmitted to humans through direct contact or consumption of undercooked seafood.
What raised further alarms in the study were the signs of antimicrobial resistance found in several of the bacteria. For example, 10% of the Vibrio strains isolated were resistant to trimethoprim-sulfamethoxazole, an antibiotic commonly used to treat infections. Additionally, 40% of the strains exhibited intermediate resistance to cefoxitin, a drug used to treat bacterial infections in humans. Troubling, especially because they suggest that some of the bacteria present in sharks and rays may be more difficult to treat, especially in the case of infection. Similarly, Salmonella enterica strains isolated from the elasmobranchs showed intermediate resistance to ciprofloxacin, nalidixic acid, and streptomycin – all of which are important antibiotics used to treat various bacterial infections.
Duarte Cardoso and her team also performed a serological assessment for Salmonella enterica subsp. enterica, specifically looking for the Typhimurium serovar, which is known to be zoonotic (this means it can be transmitted from animals to humans, potentially through the consumption of contaminated seafood). The detection of this serovar is of important because it highlights the potential role of sharks and rays in the epidemiological chain of salmonellosis – a type of foodborne illness that can cause severe gastrointestinal issues in humans. These findings, the authors argue, emphasize the need for more comprehensive studies and surveillance of the bacteria found in elasmobranchs, particularly in areas where their consumption is high.
Interestingly, while the study did identify various potentially harmful bacteria, it did not find pathogenicity genes in the Escherichia coli strains sampled, suggesting that these particular strains may not be a direct threat in terms of causing disease. Still, the overall presence of bacteria with the potential for pathogenicity – especially those exhibiting resistance to antibiotics – is of great concern. The detection of antimicrobial-resistant bacteria in these predators is a reminder that our planet is currently facing a growing problem: antibiotic resistance. A problem that, if left unchecked, can complicate the treatment of infections and lead to more severe public health outcomes.
With the high consumption rates of elasmobranchs in Brazil, especially in coastal communities, the potential for zoonotic transmission of bacteria cannot be ignored. Public health authorities and researchers have been alerted to the discovery of pathogenic and antibiotic-resistant bacteria in this sun-drenched country, but the warning should be heeded wherever these species are consumed.
