Microplastics in seabirds linked with increase in illness-causing bacteria

Microplastics aren't just affecting large animals, they're also affecting microscopic species too.

Evidence of the widespread and damaging impacts of plastics has been building in recent years, with studies showing that it can contribute to starvation, change blood chemistry and cause multi-organ damage in animals. Now, a new study reveals these particles are also affecting microbial gut health.

An international team of researchers found that the diversity of bacteria in the gut microbiome was significantly greater in seabirds which had consumed a greater amount of plastic. The new arrivals are thought to arrive as hitchhikers on the microplastics before colonising the gut.

These microorganisms may make the animals more vulnerable to illness, with birds that were more contaminated with microplastics tending to have more bacteria associated with disease.

Dr Gloria Fackelmann, the lead author of the study, says, 'The seabirds in this study ingested microplastics as part of their daily lives in their natural home ranges, showing that current environmental concentrations of these particles are having an impact on the gut microbiome.'

The findings of the study were published in Nature Ecology & Evolution.

What is the microbiome?

The microbiome is the term given to the range of different microorganisms that live in a particular environment. While these microbiomes can be anywhere, it is the gut microbiome of different species that has been of particular interest in recent years.

As well as aiding in digestion, the gut microbiome has been found to have a wide range of effects on the body, such as the ability to fight disease or adapt to a new environment. These effects can change over time, such as the bacteria that allow pandas to bulk up becoming more prevalent during the bamboo shoot season.

However, research on the gut microbiome of birds has tended to lag behind that of other animals. It's only in the past decade or so that this has begun to change, with studies finding that gut bacteria can help birds to ferment plant matter or break down toxic compounds.

In seabirds, gut bacteria are associated with defending against diseases and building up fat. Their microbiomes tend to be dominated by bacteria from groups including Firmicutes, Actinobacteriota, Proteobacteria and Bacteroidetes, and the two bird species in the current study, northern fulmars and Cory's shearwaters, were no different.

However, when the researchers compared the number of plastic pieces in the gut of each bird with bacterial swabs, they found that the diversity of these bacteria was being affected.

The increase in diversity was most obvious in the first chamber of the bird's stomach, the proventriculus, and had a much smaller impact in the cloaca, which sits at the other end of the digestive system.

Conversely, the mass of plastic in the birds was associated with bacterial diversity falling. While there isn't a definitive answer at the moment, this may be due to the different properties of the plastic being eaten.

'Microplastics may be able to act as vectors for microbes, so seabirds ingesting more of these particles could have more microbes being shuttled into their body,' Gloria explains. 'Mass, on the other hand, can be influenced by the density of the plastic's polymer, be that PVC or PE.'

'This loss of diversity might be linked more to the chemical composition of the plastic than by any microbes adhering to it, but further research will have to dive deeper into this to find out if this is the case.'

How do microplastics affect gut bacteria?

After looking at bacterial diversity more broadly, the researchers turned their attention to see which microbes were most affected by plastic consumption. In total, ten groups of bacteria changed their abundance in response to an increasing amount of microplastics.

They found that while bacteria associated with a healthy gut declined, such as Pseudoalteromonas, some bacteria associated with disease increased.

For example, Clostridium perfringens, a species associated with food poisoning and gangrene in humans, was more prevalent in birds with a higher mass of microplastics, while Corynebacterium xerosis, which can cause abscesses, was associated with more microplastic pieces.

More heavily contaminated birds were also likely to have increased levels of plastic degrading bacteria in their gut, as well as those associated with antibiotic resistance.

But linking these bacteria to disease which is directly affecting the birds is difficult. The gut microbiome of the Cory's shearwater has never been sequenced once before, while that of the northern fulmar has only been analysed once.

As the gut microbiome naturally varies between individuals, picking apart what effects the changes have, if any, is a difficult task. That said, while having more disease-causing bacteria in the gut doesn't necessarily mean these birds will suffer from illness, it might predispose them to it.

Further research could help to explain some of these uncertainties, and give scientists a better idea about the changes plastics cause in the gut.

'We hope that this avenue of research will be explored more in the future, as there are still many questions to answer,' Gloria says. 'Knowing the polymer types of the microplastics, for example, would help to figure out if changes in the gut microbiome are being driven by certain polymers, or if the mixture of different types contributes more.'