A novel chromosomally encoded OXA-12-like β-lactamase variant (bla OXA-1433 ) in a new Aeromonas veronii sequence type (ST3591) from ready-to-eat oysters

Bacteria are masters of adaptation, constantly evolving new ways to survive in changing environments. One of their most powerful survival tools is the ability to resist antibiotics, the medicines designed to kill them. This resistance is not just a problem found in hospitals; it circulates freely in rivers, oceans, and the food we eat. Seafood, particularly shellfish like oysters, lives at the intersection of the natural world and human consumption. Because oysters filter vast amounts of water to feed, they can concentrate bacteria and genetic material from their surroundings. This makes them a critical window into understanding how resistance genes move through the environment and potentially reach people. Scientists are increasingly interested in the bacteria found in ready-to-eat seafood, not just to ensure food safety, but to track the natural evolution of these microscopic defenders before they become a threat to human health.

In a recent study, researchers examined ready-to-eat oysters from a commercial source in Florianópolis, Brazil, to see what bacteria they carried. Among the samples, they found a specific type of bacterium called Aeromonas veronii. While this bacterium is common in aquatic environments, the one found in these oysters was unusual. It belonged to a previously unknown genetic family, or sequence type, which the researchers designated as ST3591. More importantly, this specific strain showed resistance to a powerful class of antibiotics known as carbapenems. These drugs are often considered a last resort for treating serious infections, so finding a bacterium that can withstand them in food is significant. The team set out to understand exactly how this bacterium achieved this resistance and where the genetic instructions for it came from.

To solve this mystery, the scientists used advanced genetic sequencing to read the entire instruction manual, or genome, of the bacterium. They combined two different sequencing technologies to ensure they captured every detail of the organism's DNA. This high-quality genetic map allowed them to identify the specific genes responsible for the antibiotic resistance. They discovered that the bacterium carried a new version of a gene called blaOXA-1433. This gene produces an enzyme, a type of protein that acts like a molecular scissors, capable of cutting open and destroying carbapenem antibiotics. The researchers named this new enzyme variant OXA-1433. It is a close relative of a known enzyme called OXA-12, sharing nearly 95 percent of its amino acid building blocks, but it differs in 13 specific spots. Despite these small differences, the new enzyme retained the essential structural features needed to function as a carbapenemase, effectively neutralizing the drug.

The study went beyond just identifying the gene; it investigated where this gene lived within the bacterium's DNA and whether it could easily jump to other bacteria. The researchers found that the blaOXA-1433 gene was embedded directly into the bacterium's main chromosome, the central library of its genetic information. It was surrounded by stable, essential genes that the bacterium needs to survive, such as those involved in building its cell wall and generating energy. Crucially, the area around this resistance gene lacked the mobile genetic elements, like jumping genes or plasmids, that usually help resistance spread rapidly between different bacteria. This suggests that the resistance is an intrinsic, built-in feature of this specific bacterial lineage rather than a recently acquired trait that could easily be passed around. The gene appears to be a stable part of the bacterium's evolutionary history, maintained over time within this group of aquatic bacteria.

To confirm that this new gene was indeed the cause of the resistance, the team performed laboratory tests. They exposed the bacterium to imipenem, a type of carbapenem antibiotic, and watched to see if the bacteria could break it down. The results showed that the bacterium successfully hydrolyzed, or broke apart, the antibiotic. They also tested whether the resistance depended on metal ions, which is a characteristic of a different class of resistance enzymes. The test showed that the resistance remained strong even when metal ions were blocked, confirming that the OXA-1433 enzyme, which does not rely on metals, was the active agent. This finding ruled out the possibility that another type of enzyme, which the bacterium also carried but which requires metals to function, was the primary cause of the resistance.

The discovery of this new sequence type and its unique resistance gene highlights the hidden diversity of bacteria living in our food supply. The researchers found that this specific Aeromonas veronii strain, with its new genetic identity and its stable, chromosomally encoded resistance, represents a previously unknown combination of traits in the environment. While this particular bacterium is not currently a major cause of human disease, its presence in ready-to-eat oysters serves as a reminder that aquatic ecosystems are dynamic reservoirs for genetic innovation. The study suggests that the evolution of antibiotic resistance is an ongoing natural process occurring in the food chain, driven by the constant adaptation of bacteria to their environments. By monitoring these changes in seafood, scientists can better understand the origins and stability of resistance genes, providing a clearer picture of how these microscopic defenders evolve before they ever encounter a human patient.