Aggression shapes the gut microbiome; a study in rats.

The gut-brain axis is regarded as a bidirectional communication system that integrates signals from the gut microbiome into behavioral aspects and vice versa. The aim of the present study was to investigate the gut microbiome-behavior interaction in relation to aggression. For that, male rats from a group-housed colony were individually housed with a female to become territorial. Next, a coping strategy was assigned to them, by evaluating their aggression levels against an intruder, during the Resident-Intruder test (RI). To investigate if their microbiome would change as a consequence of the developed coping strategy, fecal samples were collected before and after the RI test. We found that the relative abundances of Ruminococcaceae UCG-5 and Gram-negative bacterium cTPY-13 in rats sampled before the RI test were negatively correlated with the aggression that was demonstrated during the RI test. After the RI test, several bacterial taxa could be assigned to each coping strategy, with Clostridium sensu stricto 1 being strongly associated with less aggressive rats and higher abundances of Bifidobacterium. Furthermore, the family of Lachnospiraceae was not only associated with more aggressive rats, but functional prediction analysis found it to be the main contributor of betaine reductase; an enzyme catalyzing betaine production that was indicative of aggressive rats. This amino acid derivative, which has been connected with higher energy and testosterone levels, could potentially explain the connection of Lachnospiraceae with demonstrated aggression. Overall, our data revealed that the gut bacterial communities are responsive to the imposed social challenge of building and defending territoriality in co-habitation with a female. At the same time, predisposing microbiome characteristics may have predictive value for the development of a coping strategy.