First detection of porcine associated bacterium Inconstantimicrobium in a reptilian host: gastrointestinal microbiome characterisation of healthy and unhealthy green turtles (Chelonia mydas) using full-length 16S rRNA sequencing

Green sea turtles are gentle giants of the ocean, grazing on seagrass and algae that are tough to break down. To survive on this high-fiber diet, they rely on a hidden workforce inside their guts: billions of bacteria that ferment the plant matter, turning it into usable energy. This internal ecosystem is as vital to the turtle as the lungs are to a human, yet it remains a mystery in many ways. Scientists have long known that when these animals fall ill, their gut bacteria often change, but the tools used to study them have been too blunt to see the specific players involved. For years, researchers could only see the broad strokes of this microbial world, missing the fine details that might explain why a turtle is sick or how to help it recover.

A team of researchers from the University of the Sunshine Coast and partner organizations recently decided to look closer. They turned to a powerful new way of reading genetic code that captures the entire length of a bacterial gene, rather than just a tiny fragment. This allowed them to identify specific types of bacteria with a precision never before achieved in sea turtles. They focused on three green turtles found in Hervey Bay, Queensland: one healthy adult, a sick juvenile, and a sick sub-adult. By comparing the bacterial communities inside these three very different animals, the team uncovered a story of how gut health can unravel in two distinct ways, and they discovered a bacterium in a turtle that had never been seen there before.

The researchers collected samples from the cloaca, the single opening at the rear of the turtle where waste exits, which serves as a window into the gut. They sequenced the DNA of the bacteria present, aiming for a deep look at the community. The results revealed that the healthy adult turtle hosted a balanced mix of bacteria, dominated by groups known for helping digest plants. This animal had a diverse and stable community, much like a well-managed garden with many different species working together. In contrast, the sick juvenile turtle showed a very different picture. Its gut was overwhelmed by a single group of bacteria that can cause trouble when they take over, while the overall variety of species dropped significantly. This suggests the animal was suffering from a severe imbalance where harmful bacteria had crowded out the helpful ones.

The third turtle, a sub-adult with a different set of symptoms, presented a third, surprising scenario. This turtle was suffering from a severe blockage in its intestines, a condition where waste stops moving. One might expect a sick animal to have a poor or simple bacterial community, but this turtle actually had a very rich and diverse mix of bacteria, similar to the healthy adult. However, the types of bacteria were different. Instead of the usual mix, its gut was flooded with a specific group of bacteria that thrive in stagnant, oxygen-free environments. These are the kinds of microbes that ferment food and produce gas, which fits perfectly with the turtle's condition of a stopped gut. The bacteria were essentially feasting on the food that was no longer moving, creating a fermentative environment that likely contributed to the animal's distress.

The most striking discovery came from this third turtle. Among the flood of fermenting bacteria, the researchers found a specific type called Inconstantimicrobium porci. This bacterium was previously known only from the intestines of pigs and from agricultural soil. Finding it in a sea turtle was a first for science. The researchers detected it in significant numbers in the sick sub-adult, while it was barely present in the healthy turtle and absent in the other sick one. They suspect that recent heavy flooding in the region may have washed this pig-associated bacterium from the land into the coastal waters where the turtles feed. The turtle likely ingested it, and because its gut had stopped moving, the bacterium was able to multiply rapidly, joining the other fermenting bacteria in a bloom that the healthy turtle's moving gut would have naturally flushed away.

This study highlights that sickness in sea turtles does not always look the same on a microbial level. Sometimes it looks like a collapse of diversity, where harmful bacteria take over a weakened system. Other times, it looks like a shift in the type of bacteria, where a healthy-looking variety of species rearranges itself to suit a blocked gut. The ability to see these differences clearly is a major step forward. By using a method that reads the full genetic code of the bacteria, the researchers could identify the specific pig-associated bacterium that shorter, older methods would have missed. This level of detail is crucial for understanding how environmental changes, like flooding, might introduce new organisms into the marine environment and how those organisms interact with wildlife health.

The findings also point to the complex relationship between the environment and the animals that live in it. The presence of a pig-associated bacterium in a sea turtle suggests that the boundaries between land and sea are more porous than previously thought, especially when extreme weather events occur. While the study involved only three animals, the results provide a new lens for veterinarians and conservationists. It suggests that looking at the specific types of bacteria inside a turtle could help diagnose the nature of its illness, distinguishing between a general infection and a specific blockage. As the researchers note, this is just the beginning of understanding these internal worlds, but it offers a promising path toward better care for these ancient mariners as they face a changing planet.