The study, published in mBio, investigated the role of different human gut bacteria in obesity and metabolism - noting that other research has linked one particular species of gut bacteria called Clostridium ramosum with obesity.
Led by Michael Blaut from the German Institute of Human Nutrition Potsdam-Rehbruecke, Germany, the team used a model of the human microbiota in mice to identify that when coupled with a high-fat diet C. Ramosum can help to drive weight gain and the risk of obesity.
"Our results indicate that Clostridium ramosum improves nutrient uptake in the small intestine and thereby promotes obesity," Blaut said – revealing that mice harbouring human gut bacteria including C. ramosum gained weight when fed a high-fat diet, while mice that did not have C. ramosum were less obese even when consuming a high-fat diet.
Mice that had C. ramosum but consumed a low-fat diet also stayed lean, the team added.
"We were surprised that presence or absence of one species in a defined bacterial community affected body weight and body fat development in mice," commented Blaut.
Blaut and colleagues investigated the role of C. ramosum in three groups of mice: some harboured a simplified human intestinal microbiota of eight bacterial species including C. Ramosum (group 1); some had simplified human intestinal microbiota except for C. Ramosum (group 2), and some had C. ramosum only (group3). Mice were fed either a high-fat diet or low-fat diet for four weeks.
After four weeks eating a high-fat diet, the mouse groups did not differ in energy intake, diet digestibility, and selected markers of low-grade inflammation.
However, there were significant differences in body weight, said the team, revealing that mice in group 1 and those in group 3 (both containing C. Ramosum) gained significantly more body weight and body fat - which implies that they converted food more efficiently to energy than the mice in group 2 which did not contain C. Ramosum.
In contrast, all groups of mice fed a low-fat diet stayed lean, indicating that the obesity effect of C. ramosum only occurred on high-fat diets, they said.
Indeed, Blaut and his colleagues also revealed that obese mice in group one – containing simplified human microbiota plus C. Ramosum - also had higher gene expression of glucose transporter 2 (Glut2), a protein that enables absorption of glucose and fructose, and fat transport proteins including fatty acid translocase (Cd36).
“Upregulation of small intestinal glucose and fat transporters in these animals may contribute to their increased body fat deposition,” said the team.
However the team noted that associations between obesity and increased levels of lipopolysaccharides causing inflammation, or increased formation of short chain fatty acids, reported by other researchers, were not found in this study.
"This possibly means that there is more than one mechanism underlying the promotion of obesity by intestinal bacteria,” said Blautm, who added that through additional studies he hopes to learn more about how C. ramosum affects its host's energy metabolism and whether similar results occur in conventional mice given the bacteria.
"Unravelling the underlying mechanism may help to develop new strategies in the prevention or treatment of obesity," he said.
Volume 5, Number 5, e01530-14, doi: 10.1128/mBio.01530-14
“Clostridium ramosum Promotes High-Fat Diet-Induced Obesity in Gnotobiotic Mouse Models”
Authors: Anni Woting, Nora Pfeiffer, Gunnar Loh, Susanne Klaus, Michael Blaut