Insects comprise over half of all described species excluding bacteria and around 90% of the animals on Earth. They are so prevalent that we encounter them every day without thinking twice, and have evolved to live in nearly every imaginable habitat, from mountaintops to the depths of caves. How do insects survive in such harsh environments?
Most people are familiar with one of the inhospitable environments inhabited by insects – we’ve all seen flies buzzing around a carcass. Animals generally can’t eat spoiled meat, yet insects such as the burying beetle thrive on carrion. These beetles are aptly named because the adults bury dead vertebrates and raise their larvae inside them. So how do the beetles keep the carcass from decaying? The answer is that they need help. Who are these mysterious allies?
The beetles’ helpers are microscopic, yet critical for the success of their eccentric lifestyle. When the adult burying beetles carve out a space in the carcass for their larvae, they introduce microorganisms that displace those responsible for decay, keeping the meat fresh for the larvae to eat. One of these microbes, a yeast called Yarrowia, breaks down proteins and fats in the meat, making it more digestible for the larvae. Without these bacteria and fungi, the carcass is not palatable to the larvae and they cannot subsist on it.
All multicellular organisms interact constantly with microorganisms, and in many cases host microbial communities that are critical to the host’s health. In animals, the richest communities typically reside in the gut and help digest food. They also protect the host from infection because pathogens have difficulty colonizing niches that are already occupied by other microbes – like people trying to access a crowded elevator. But what makes organisms like the burying beetle unique is that they can manipulate their external environment using microbes that they cultivate. These “microbe farmers” typically form obligate partnerships with specific microbial species, meaning the two organisms depend on one another for survival. This contrasts with facultative relationships, in which the two organisms can either independently or as partners, such as those that form when an animal acquires microbes from its environment.
Another microbe farmer, the leaf-cutter ant, chews pieces of leaves off of plants and carries them back to their den. However, these ants cannot digest the leaves they harvest, but rather culture fungi on them that they then eat. In addition to culturing fungi that provide nutrition, the exoskeletons of these ants are peppered with Streptomyces bacteria that serve a protective role. Streptomyces species are prolific antibiotic producers and, in the case of those that grow on the leaf-cutter ants, synthesize antimicrobials that kill pathogens of the fungal cultures. Due to this advantageous property, Streptomyces bacteria are cultivated by several other insect species to fend off pathogens. Despite the leaf-cutter ants and Southern pine beetles not being closely related, these beetles also grow fungi to feed their larvae and Streptomyces bacteria to defend the fungi from pathogens. European beewolf larvae incorporate Streptomyces that they acquire from their mother into their cocoon to protect them from infection as they pupate.
So what can we humans learn from these microbe farmers? While we don’t biologically have the means to emulate the examples above, we can leverage these associations for our own advantage, or culture bacteria in the laboratory to serve similar purposes. For example, food scientists are always looking for ways to extend the shelf life of perishable foods. Recent studies have found that inoculating produce with some bacterial species readily cultured in the lab may prolong spoilage – like how burying beetles preserve their food.
Another critical problem facing our society is the rise of antimicrobial resistance. Antibiotic-resistant infections are a serious public health concern as antibiotic resistance rapidly spreads in microbial communities. Discovering new antibiotics would be a big step in solving this problem because human pathogens would not have been exposed to them previously. Novel antibiotics with promising activity against antibiotic-resistant human pathogens have been discovered in associations like that between leaf-cutter ants and Streptomyces. Maybe some of humanity’s greatest problems can be solved by taking a leaf out of the insects’ book.