Antibiotics could potentially disrupt the health-promoting microbes that make up the microbiomes in humans and other animals. But pinning down exactly how microbiomes and antibiotics affect human health is complicated by the sheer number of microbes involved.
Evolutionary biologist Nancy Moran’s answer to this challenge: the honeybee. Seeking microbiome insights, Moran and colleagues at the University of Texas at Austin report that antibiotics can dramatically alter a honeybee’s gut microbial community and result in higher mortality. Their work appeared in a recent issue of PLOS Biology.
“We’ve been developing the honeybee gut microbiota as a model that has many similarities to humans, and mammals in general, but is simpler,” Moran says. Like humans, honeybees get their microbial residents through social contact. And both honeybee and human microbes are specialized to their hosts. But the honeybee microbial community has only eight core bacterial species. For humans, the number of core bacterial species is in the hundreds, says Moran.
Honeybees and humans are also both regularly exposed to antibiotics. For half a century, beekeepers in the United States have used these drugs to prevent bacterial infections in honeybee larvae. Although these treatments may help protect honeybees, past work by Moran and others has also suggested antibiotics could contribute to declining hive health.
To test how antibiotics affect bees and their microbiomes, the team first collected about 800 adult worker honeybees from a hive. They marked bees from each group with different colored paints to tell them apart. For five days, they fed half of the bees a mixture of sugary syrup and tetracycline, an antibiotic commonly used by beekeepers. They fed the other half only the syrup. The team then returned the bees to the hive.
Over the course of one week, the researchers repeatedly sampled bees from each group to analyze their microbiomes. After harvesting the bee guts, the team used a technique called quantitative PCR to estimate the total number of bacterial cells in the guts, and gene sequencing to determine which bacterial species were present.
After three days in the hive, only about 30% of the antibiotic-treated bees survived while twice as many control bees did. The antibiotic also affected the bees’ microbiomes. Over the course of a week, the control bees had an average of five times more bacterial cells in their guts than the antibiotic-treated bees. Five of the antibiotic-treated bees’ core bacterial species were hit especially hard and did not show clear signs of recovery by day seven, the duration of the study. Meanwhile, the control bees’ gut microbiomes remained relatively constant.
Moran and her team also performed complementary studies on bees kept in the laboratory. One showed that the antibiotic-treated bees were less able to survive the common insect pathogen Serratia than the control bees. “Disrupting the gut microbial community with antibiotics opens the door to the invasion by these other pathogens,” explains Moran.
“This is a very robust study,” says microbiologist Irene Newton of Indiana University, who was not involved in the work. “They’ve linked the presence of the microbial community and its composition to survival. In many model systems, it’s difficult for people to do because they have a hard time manipulating the symbiosis – separating the partners.”
Moran stresses that while the results suggest that antibiotics may harm honeybees, these drugs are only one of many factors. (Viruses and habitat loss, for example, potentially contribute to honeybee decline as well.) Plus, antibiotic treatments—which beekeepers administer by a different means than did Moran’s team—can help bees in some cases by protecting against infection.
The team’s next step is to figure out exactly how the honeybee gut microbial community offers protection against pathogens. “Is it a single member of the microbiota?” Moran asks. “Does the microbiota interact with the immune system and that leads to the protection? Or does the microbiota interact with the pathogen directly? Or is it both?” With the honeybee system, she says, testing these questions is possible.