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Gut bacteria influence behavior in worms

The roundworm Caenorhabditis elegans, which has long been used to study different aspects of basic biology, has emerged as a model organism to study the microbiome. HeitiPaves / iStock / Getty Images Plus

The gut teems with bacteria and other microorganisms. This collection of gut microbes, known as the gut microbiome, may play a role beyond the stomach. Studies in animals suggest that gut bacteria could influence the brain and affect their host’s behavior.

Some gut bacteria have been shown to produce neurotransmitters, chemical messengers that allow communication between brain cells. However, the way in which gut microbes might influence a host animal’s behavior isn’t well understood. 

To explore the influence of gut microbes on behavior, a research team, led Drs. Michael O’Donnell and Piali Sengupta of Brandeis University studied Caenorhabditis elegans, a type of worm. They investigated how gut bacteria affected the worms’ food selection.

Bacteria are C. elegans’ primary food source. The researchers fed the worms different bacteria that are known to influence how worms perceive odors. They then tested whether the worms moved toward or away from specific odor molecules.

The study was funded by NIH’s National Institute of Neurological Disorders and Stroke (NINDS), National Institute on Deafness and Other Communication Disorders (NIDCD), and National Institute of General Medical Sciences (NIGMS). Results appeared in Nature on June 17, 2020.

The team found that worms grown on Providencia bacteria were less likely to avoid octanol—a large molecule secreted by some bacteria—than those grown on other bacteria. Worms normally avoid high concentrations of octanol.

Live Providencia bacteria were present in the guts of the worms that moved toward octanol. This suggested that the worms’ behavior could be determined in part by a substance produced by these bacteria. 

The researchers uncovered a pathway involving the chemical tyramine, which is produced by Providencia bacteria in the gut. Tyramine is transformed by the worms’ body into octopamine, which acts as a neurotransmitter. Octopamine targets a receptor on sensory neurons that controls the worms’ aversion to scents. This made the worms more tolerant of octanol.

The researchers went on to identify the genes required for the bacteria to produce tyramine. In addition, they found that worms grown on Providencia preferred eating Providencia over other bacterial food sources. When the team used genetic engineering to remove the ability of the bacteria to produce tyramine, the worms lost their preference for them.

The team found that the bacteria produced odor compounds related to octanol. Providencia were able to survive being eaten by the worms and thrived in their guts. These findings show how a neurotransmitter produced by gut bacteria can influence behavior. In this case, the change benefits both the worms and the bacteria.

“We were able to connect the dots, all the way from microbe to behavior, and determine the entire pathway that could be involved in this process,” Dr. O’Donnell says.

“Here, the gut bacteria are influencing how the animal senses its environment and causing it to move toward an external source of the same bacteria,” says Dr. Robert Riddle, NINDS program director. “The gut bacteria are literally making their species tastier to the animal.”