Biology Prof’s New Paper Eyes Microbes, Hosts

SOCORRO, N.M. June 30, 2015 – Biology professor Dr. Tom Kieft has published a new paper that details his discovery of a consistent mathematical relationship between the abundance of microbes that live in animal (including human) digestive tracts and the weight of the animals  – from the smallest to the largest animals.

Kieft Thomas Kieft copy

Dr. Tom Kieft


Kieft’s new study also included a global census of animal-associated microbes and appears in the Proceedings of the Royal Society of London B’s latest issue, dated June 23, 2015.

“There has been a lot of work characterizing what’s in the human gut and linking those microbes to various diseases and other health-related conditions,” Kieft said. “We are interested in a new scaling relationship involving animals and their microbes and the implications and the underlying principles”

Kieft and co-author Karen Simmons, a master’s student, gathered information from existing literature in which other scientists have published the quantity of microbes in the gut.

Most animal-microbe studies focus on microbial community structure; in other words, most studies ask, “Who’s there? instead of “How many are there?”  For this research, Kieft applied allometry (scaling of animal attributes with animal size) to animal-microbe relationships across a broad range of species. From nematodes to whales, the study spans 12 orders of magnitude in animal mass.

Kieft said that scientific research has shown that about 10^30 prokaryotes live on the planet. Kieft estimates that only 10^25 of those microbes lives in animals.

“How those microbes are distributed among animals is revealing,” Kieft said. “We as humans are hugely successful macro-organisms. Our numbers have increased exponentially and other large animals have gone extinct. Other successful animals are domesticated animals – those that are successful because of humans.”

The Earth’s population of about 7 billion people harbors about 3.5 percent of animal-associated microbes, while domesticated animals harbor 14 to 20 percent. Kieft’s novel allometric approach also hints at underlying mechanisms involving the transfer of energy and materials between the microorganisms and their animals hosts.

Kieft’s study is the first allometric study of the relationship between animal body mass and abundance of microbes-per-individual animal across a range of vertebrate and invertebrate species and of the animal-associated microbial abundance in the biosphere estimated from this relationship. 

Microbes can affect an animal’s well-being, behavior and functioning. By understanding the relationship between animal mass and microbial mass, scientists can get a better understanding of the importance of microbes. Kieft said this sort of study is a first step to further understanding the effects of antibiotics on microbial communities, food supply and the general health of the larger environment.

“This is a new way of looking at antibiotic resistance,” he said. “We select for antibiotic-resistant organisms.  The sheer numbers of microbes we are exposing to antibiotics is enormous. In this country, most animals are given antibiotics in feed.  Putting antibiotics into animal feed is banned in Europe, but common elsewhere. This provides a new perspective on the problem of antibiotic-resistance.”

Kieft said his estimates further underscore the impact of the human species – and our domesticated animals – on the biosphere.

“The phenomenal success of humans has favored many thousands of associated microbial species. The scales of human manipulation of those microbes, by dosing humans and domestic animals with antibiotics, may have an even greater impact than previously considered,” he wrote.

Kieft and Simmons compiled many previous studies of various animals and their microbial populations. They also conducted surveys of their own – including zebrafish bought at Walmart in Socorro.

This sort of study also sheds light on the volume of methane produced by animals. Kieft said about 20 percent of the planet’s methane – a greenhouse gas 20 times more powerful than CO2 – is produced in the guts of animals.

“This provides another way of looking at the greenhouse gas problem,” he said. “Methane production is increasing as domestic animals increase. And it follows a pattern – an allometric relationship.”

Kieft said that demonstrating the allometric relationship of microbes and animals provides a new way of considering human impact on the global biome. He said the current, on-going mass extinction is certainly bringing about a great loss of microbial species as well – species of microbes that had adapted to their host animals.

“There’s been so much work on the make-up of microbial communities in animals,” Kieft said. “This research begs the question: how does the composition (who’s there?) scale with animal size?” One measure of community make-up is diversity.  A second part of Kieft’s and Simmon’s study looked for a consistent relationship between microbial diversity and animal size but found none.  

Kieft and Simmons are continuing their research with the use of a simple organism, a nematode, as a model for animal-microbe interactions.

– NMT –

By Thomas Guengerich/New Mexico Tech