Obese people have a less diverse array of bacteria living in their guts than do thin people—and the same holds true for dogs. In a new study, researchers fed seven beagles unrestricted amounts of food for 6 months, during which each dog gained an average of 4.93 kilograms—about 67% of their initial average weight of 7.37 kilograms. The investigators fed another seven beagles controlled food portions, and, as expected, this group did not gain weight. When the researchers examined the fecal samples collected from both groups after 6 months, they found that the guts of obese beagles contained a smaller diversity of bacteria than those of the other dogs. What’s more, microbes from the phylum Firmicutes were the predominant group in the lean dogs, whereas Gram-negative bacteria called Proteobacteria were prevalent in the obese group, the team reported online this month in the Journal of Veterinary Internal Medicine. The researchers speculate that an abundance of Proteobacteria may lead to an increase in lipopolysaccharide, a major component of the cell wall of Gram-negative bacteria, which has been linked to weight gain in mice. More research is needed to examine what role gut bacteria may play in the development of obesity, however, the authors say.