Stoy Hedges

ou might know that nearly all animals — including us humans — have a unique, discrete and distinctive set of gut microbes that help us digest food, fight diseases and control hormones. These are just a handful of critical functions driven by an entire ecosystem inside our gut. We tend to forget about this bacterial community until it gets invaded, disrupted or distorted — this concept is known as dysbiosis. It’s something you might be miserably familiar with if you’ve been a victim of irritable bowel syndrome, Crohn’s disease or a myriad of other intestinal disorders.

German cockroaches, just like humans and other animals, use their own unique gut microbes to perform a variety of metabolic functions. Researchers around the world have begun to study these microbes to determine how they interact with each other, how they interact with their cockroach host, and whether they have the potential to detoxify toxic compounds. It goes without saying that German cockroaches are notorious for their ability to resist insecticides, as this insect has received a lot of attention in the scientific community over the past several decades, but we are still learning ways in which its bacteria can influence insecticide tolerance. If these microbes play a role in insecticide degradation, we might be able to develop safer, more effective and more creative insecticides targeted at gut dysbiosis in the German cockroach.

GUT BACTERIA RESEARCH. What do we already know about gut bacteria in cockroaches? Thanks in part to research done by Dr. Wada-Katsumata et al., we know that each colony of German cockroaches has its own unique set of gut microbes that is passed between individuals in the cockroach community through coprophagy. When the roaches are nymphs, they feed on the feces of the adults in the colony, which allow them to ingest the colony’s unique gut microbiome. It is thought that many of the volatile carboxylic acids (VCAs) that partially create the distinctive “roach poop” smell in infestations are byproducts of the metabolic processes performed by gut bacteria. Through in-depth research on microbial chemistry, these researchers demonstrated that gut microbes play an important role in communication between cockroaches.

Dominguez-Santos et al. published an article this year that confirms that there exists a stable core in gut microbiome composition in colonies of lab-reared German cockroaches. Just two years prior, Pietri et al. discovered an indoxacarb- resistant laboratory strain of German cockroach that harbors a gut microbial community differing significantly from that of susceptible cockroaches. Both research teams show that antibiotic treatment affected the gut microbial community significantly; this disruption may alter the dynamics of insecticide resistance at the host population level. Together these results suggest that gut microbiota are important symbionts for the survival and fitness of German cockroaches and are resilient enough to recover from disturbances. Since the cockroach gut microbiome is remarkably efficient at detoxifying foreign compounds, the industry has to be aware of these intricacies when developing orally administered insecticides.

PURDUE UPDATE. The O.W. Rollins Urban Entomology lab at Purdue University specializes in urban insect pest science and has been studying gut microbiota in the eastern subterranean termites for more than a decade. Termites and cockroaches evolved from very similar ancestries; this is made clear when observing their gut physiology. Not only do these insect groups share many anatomical features, but they also have diverse gut microbiomes. Dr. Michael E. Scharf, along with his colleagues and students, examine the interactions between insect gut microbes and how these relationships can be enhanced, deteriorated and exploited for the purposes of pest management.

“Both termites and cockroaches host bacteria and protozoa, but they are far less diverse in cockroaches, and more diverse and specialized in termites,” Scharf said. “The big theme that is emerging in termites is how ‘consortia’ of microbes work together, for example, individual protist species have many bacterial species associated with them, both inside them and stuck to the outside of their bodies.” Since Eastern subterranean termites have a more specialized diet than German cockroaches, it’s logical they also have more specialized gut microbiota. That doesn’t mean that cockroach gut microbiomes lack these unique consortia between microbial species, however, as researchers have yet to explore the cockroach gut microbiome to the same depth as many pest termite species.

The gut of a German cockroach.

Scharf also shared some insights about how these bacteria might affect host insect fitness and insecticide resistance. “We are just starting to learn how microbes of all kinds contribute to life processes in pest insects. They can potentially use insecticides as a source of nutrition and will partially digest them in ways that either make them more toxic (activation) or less toxic (detoxification) to the host insect. By specifically targeting these microbes, or processes they affect, this can lead to novel ways of controlling pests.”

He also pointed out that pest researchers have been brainstorming microbial- targeted strategies for nearly a century. “One way to inhibit microbial function in the gut is to deprive microbes of key nutrients or environmental conditions that they require, like pH or oxygen levels. Regarding protists in termite guts, in 1923 L.R. Cleveland showed that simply heating termites to 28°C could eliminate protozoa (and bacteria) and the cellulose digesting capabilities they provided.”

There is a potential to create exponentially more control products for cockroaches aimed at gut dysbiosis and the inhibition of gut bacterial functions. First though, we must learn more about how the individual species of the gut microbiome interact with each other and how they act with foreign compounds like insecticides. Controlling gut microbes and their functions will add yet another tool to the mode of action toolbox and, over the long run, could help to mitigate insecticide resistance in this notorious urban pest.

The authors are both Ph.D. students at Purdue University, West Lafayette, Ind.