Why birds might be better off without gut bacteria

First, birds (and perhaps all animals) might have to spend a lot of their energy fending off microbes in their intestines by preventing them from entering the blood stream. Yes, bacteria can be beneficial when they’re kept restricted to the gut environment, but they can wreak havoc when they’re set loose. (Adamo/Wikimedia Creative Commons)

Probiotic consumers are probably aware of that fact that bacteria living inside of their guts have nutritional benefits, but a new study on the gut bacteria of house sparrows suggests the opposite. In fact, bacteria may be setting these birds back.

When researchers gave house sparrows antibiotics to wipe out their internal bacteria, the sparrows grew much larger than they would have without the antibiotics. This is exactly what happens in chicken breeding – farmers have long known that they could make their chickens gain more weight by knocking back the bacteria that live in their guts.

But does that mean bacteria are in some way connected to how animals digest and process their food? In house sparrows, it turns out that gut bacteria have little to do with digestion at all.

Researchers collected these sparrows in Wisconsin and nurtured them in the lab, feeding them 15 times a day and lacing antibiotics into the diets of some. After treating some birds with regular diets and some with antibiotics-spiked diets, they looked into the various enzymes being produced and the bacteria living inside of the intestinal tract.

What the scientists got was essentially a negative result, but an answer that paves the way for alternative propositions. While there wasn’t a connection between digestion and the microbes living inside of their guts, the biologists who conducted the study gave two possible explanations.

First, birds (and perhaps all animals) might have to spend a lot of their energy fending off microbes in their intestines by preventing them from entering the blood stream. Yes, bacteria can be beneficial when they’re kept restricted to the gut environment, but they can wreak havoc when they’re set loose. In the medical world, this is considered sepsis and often results from infected wounds.

Animals, including us, must spend a lot of energy building and continually replacing a critical layer of mucus that lines the inside of our intestines. The mucus acts like a barrier to millions of microbes that invade the gut through our food. When the microbes living inside of the house sparrows were decimated by the antibiotics spiked into their diets, the energetic grip microbes had within the sparrow could have been loosened. In other words, the sparrow wasn’t required to spend its resources on bolstering its immunity.

The second explanation is that antibiotics made it possible just a few beneficial bacteria to dominate the intestinal environment. In this scenario, antibiotics wipes out bacterial competitors, allowing one or a few bacteria with nutritional benefits to dominate. More beneficial nutrients would then be produced, boosting the growth of the sparrow.

Whichever explanation is correct, animals and their gut bacteria are interacting in complex ways that require both to make tradeoffs. While animals might be better off with bacteria that provides nutrients, they suffer by having to host and fend them off. Bacteria may find an escape from bacterial competition and viral predators, but living inside of an animal means changing lifestyles.

For sparrows, bacteria seem to be inhibitory because they prevent the birds from growing larger than they may otherwise grow. Bird size plays an important role in bird survival and reproduction, making microbes more than just intestinal squatters.


Diler Haji is a campus correspondent for The Daily Campus. He can be reached via email at diler.haji@uconn.edu.