Another Weight Loss Jab? Soil Microbe Injections Prevent Weight-Gain in Mice

Just as semaglutide products like Ozempic revolutionize the world of weight loss treatment, another fat-fighting injection emerges on the horizon.

Researchers at the University of Colorado Boulder have discovered that weekly inoculations of the bacteria Mycobacterium vaccae prevent mice from gaining any weight when on a high-fat diet.

They say the bacterial injections could form the basis of a “vaccine” against the Western diet.

Their findings were published in Brain, Behavior and Immunity.

Treat-fighting jab

M. vaccae is naturally found in soils and has shown promising medical properties in several prior studies.

One paper published in 2007 found that, when injected into mice, heat-killed M. vaccae stimulated neurons that increase serotonin and decrease anxiety and immobility.

To follow-up on this research, the team at the University of Colorado Boulder set out to test if the bacteria could help counter the anxiety-like behavior that can come with a poor diet.

One set of adolescent mice was fed a standard, healthy diet of chow for 10 weeks while another set was fed a high-fat diet consisting of 40% fat, 40% carbohydrates and 20% protein.

Half of the mice in each group received weekly injections of M. vaccae.

As expected, the mice that ate the “junk food diet” and weren’t inoculated gained significantly more weight than the mice fed the healthy chow. By the study’s end, these mice weighed 16% more, on average, than the healthy mice and carried significantly more visceral fat.

What did surprise the researchers, however, was that the mice that ate the junk food diet and were inoculated weighed the same by the end of the study – and had similar levels of visceral fat – as the mice fed the healthy chow.

“This finding suggests that M. vaccae effectively prevents the excessive weight gain induced by a Western-style diet,” Luke Desmond, a PhD candidate at the University of Colorado Boulder, said in a statement.

Having an old friend for dinner

While Desmond and his colleagues don’t know how the injected bacteria helped the mice stay lean, they posit that immune system regulation and a reduction in inflammation may be at play.

“M. vaccae is considered to be an ‘old friend’, i.e., microbes that humans co-evolved with but have largely lost contact with during the process of urbanization,” Dr. Christopher Lowry, a professor of integrative physiology at the University of Colorado Boulder and lead researcher of the study, told Technology Networks.

“What these ‘old friends’ have in common is their ability to interact with our innate immune system, including dendritic cells, in a way that biases them toward an anti-inflammatory phenotype,” he explained.

“There is a complex, bidirectional relationship between obesity and inflammation, in which increases in body fat can lead to chronic low-grade inflammation, which in turn can lead to further weight gain and higher risk for metabolic complications, such as metabolic syndrome and type 2 diabetes. Our studies demonstrate that regular treatment with M. vaccae can, unexpectedly, completely prevent excessive weight gain in response to consumption a Western-style diet.”

Poor diets consisting of ultra-processed foods (UPFs) have been blamed for the recent rise of obesity, heart disease and cancer rates in Western countries. From ice cream to white bread, instant soup to soda, UPFs are thought to account for 57.9% of the diets of US citizens.

To find out whether “old friends” M. vaccae can help treat this rise in disease, Lowry and his colleagues are already planning further, more investigative studies.

“Future studies can determine if the protective effects of M. vaccae against Western-style diet-induced weight gain indeed depend on its anti-inflammatory or immunoregulatory effects. We would also like to know if M. vaccae can reverse overweight or obesity once obesity has become established [and whether] oral administration of M. vaccae is as effective as administration of M. vaccae by injection. Finally, we’d like to know if these effects of M. vaccae can be replicated in humans.”

“If these effects can be replicated in humans,” he added, “this may represent a safe, effective, accessible and affordable approach to preventing overweight and obesity – including in vulnerable populations.”

“There is something inherently attractive about addressing the global obesity epidemic (or other conditions in which inflammation is a risk factor, including stress-related psychiatric disorders) by simply replacing an organism that we have inadvertently lost contact with throughout our transition from hunter-gatherer, to agricultural, to urban lifestyles. We are now exploring commercial applications of this technology through a new CU Boulder spinout company, Kioga.”

Reference: Desmond LW, Dawud LM, Kessler LR, et al. Protective effects of Mycobacterium vaccae ATCC 15483 against “Western”-style diet-induced weight gain and visceral adiposity in adolescent male mice. Brain, Behav and Immun. 2025. doi: 10.1016/j.bbi.2024.12.029