by Quinn Eberhard
Of the great variety of animals on Earth, few are considered more repulsive than the vulture. Often seen feeding on the remnants of old roadkill, these birds have managed to make meals out of what most animals would have considered spoiled leftovers, yet they don’t seem to experience symptoms of food poisoning as a result. The secret to their successful adaptation relies on another life form altogether: bacteria.
The mutually beneficial relationships between bacteria and other organisms has become better understood in recent years. Researchers have estimated that humans have between “10-100 trillion microorganisms” living in our digestive systems that help us break down and process nutrients from our food. Microorganisms are equally important for maintaining the health of our skin, organ function, and immune systems. We call this complex internal ecosystem our “microbiome.” The average person has around 23,000 genes in their DNA, yet there are an estimated ~600,000 bacterial genes in our bodies; how might all these bacterial genes be impacting our health? This codependency with microbes is observed in other animals as well, making it possible to study a wide range of microbial impacts. A group of researchers from Denmark were inspired to ask: could it be that the bacteria living in vultures is what lets them eat the otherwise inedible?
To find what allows vultures to eat decaying and spoiled carcasses without getting sick, the researchers took swabs from the facial skin and the ‘hindgut’ (the tail end of the bird’s digestive tract) from 26 black vultures and 24 turkey vultures, all from Tennessee. The samples from the face swabs revealed the presence of a wide variety of bacteria, many of which produced toxins that could cause food poisoning, that was highly similar between the two species. Since vultures feed directly out of carcasses, the facial bacteria would be mostly representative of the bacteria present in the vultures’ food. And yet, the samples from the hindgut swabs revealed a lesser variety of bacteria, a little more than 85% of the bacteria found on the face was not present in their guts.
Figure 1: (left) a turkey vulture and (right) black vulture.
Ultimately there were two main types of bacteria found in the hindgut: Clostridia and Fusobacteria, both of which are known to be pathogenic, with Clostridia causing food poisoning and Fusobacteria skin decay. The researchers were uncertain as to why so many different types of bacteria could be found on the facial skin and food and yet so few in the intestines. They proposed two possibilities: one was that the Clostridia and Fusobacteria were simply the best fit bacteria for surviving in the vultures’ intestines. The second was that these particular bacteria may somehow provide a positive benefit for the vultures that increases their chance of survival, allowing this mutualistic relationship to continue. When studying the genes detected in the hindgut samples, the scientists found many from both Clostridia and Fusobacteria that formed toxins which would break down tissues and help with the vulture’s digestion. Could it be possible that vultures benefited from these bacteria and therefore evolved over time to become better hosts?
The researchers also compared the bacterial species between these wild vultures with those held in captivity back in Denmark. They once again obtained swabs from the face and hindgut from two turkey vultures and several other species of birds kept in the Copenhagen zoo, all of which were fed similar diets. Amazingly, the microbiomes of the captive vultures were highly similar to the wild vultures from Tennessee and yet were significantly different from the gut bacteria found in the other captive birds, despite having been fed the same diet. This supports the idea that vultures uniquely evolved to be better host environments for Clostridia and Fusobacteria, and the presence of these bacteria cannot be explained by the vulture’s diets alone.
The impact of diet on health has been a long studied part of human medicine, but the microbiome adds a new level of complexity. As we continue to learn more about the microbiome’s influence on our health, we expect that imbalances in these symbiotic relationships may lead us to the causes of different diseases, as well as reveal potential treatment options. We are only at the tip of the iceberg when it comes to truly understanding the nuance of both ourselves and our microscopic friends.
