A post-microbial apocalypse

In the December issue of <em>PLOS Biology</em>, Prof. Jack Gilbert (Argonne National Laboratory, Chicago) and&nbsp; Prof. Josh Neufeld (University of Waterloo) explored whether life would be possible, let alone worth living, in a microbe-free world. They considered the question from several angles. What if all microbes were magically erased?&nbsp;</p>

“Actually, we wouldn’t notice at first,” said Neufeld. “It would take a day or two … but then the world would be turned upside down.”

Microbes do many things for us without us even realizing: they train our immune system, and can also protect us from pathogens by squishing them out. Microbes are important in digestion, especially in animals like cows with an organ entirely dedicated to microbes digesting their food. Clearly, with such dependence for vital things like immunity and nutrition, there is no way any animal could survive without microbes, right? Wrong. It turns out there’s an entire field of research that uses animals raised microbe-free; these are called gnotobiotic organisms.

Gnotobiotic animals can survive and thrive when fed a special diet consisting of essential nutrients that may have previously been obtained from their gut micro-organisms. Organs of gnotobiotic animals are reported as being smaller and more susceptible to lethal complications. Barring complications, these animals seem to have slightly longer lifespans, provided they remain protected from microbes within their sterile environment. 

Tom Curtis, a professor of environment at theUniversity of Newcastle once wrote, “If the last blue whale choked to death on the last panda, it would be disastrous but not the end of the world. But if we accidentally poisoned the last two species of ammonia-oxidizers, that would be another matter.” 

Gilbert and Neufeld pointed out that microbes play a dominant role in nutrient cycling, especially when it comes to the nitrogen cycle; they are the only organisms that perform certain steps. Microbes break down decaying organic matter, an important part of the carbon cycle — they also produce as much as 50 per cent of the atmosphere’s oxygen and are a part of the sulfur and phosphorous cycles. Without microbes, humans would need to find ways to compensate for these losses in order to survive. Without nitrogen supplied to plants from microbial activity, we wouldn’t be able to sustain our agricultural activities, or maintain Earth’s plant biodiversity. Decreases in agriculture could lead to starvation and war. 

“It’s very difficult to know how long humans might survive in a microbe-free world,” said Neufeld. “The vast majority of humans would die under such conditions.”

It is hard to predict how long the food supply could hold out until mass starvation began, or how much of a toll fighting over resources would take on our population. Nonetheless, it’s clear that most humans and animals would suffer and die. Survivors would need to find ways to continue nutrient cycling without microbes or face eventual extinction.

As Neufeld said, “One of the reasons we wrote this article was to promote the beneficial roles that micro-organisms play in our lives. The handful of pathogens that harm us are a drop in the bucket compared to the myriad of microbes and metabolisms that engineer our planet for the survival of all life.” 

Next time you wish that microbes didn’t exist, you should think twice.