Researchers at the University of Chicago have now developed a new approach that could help overcome this problem. In a study published in Science Advances, the team used BifidoSumIL-2, an engineered strain of Bifidobacterium longum, a probiotic bacterium naturally found in the gut, to carry an immune-stimulating treatment directly into tumors.

In animal models, the therapy slowed the growth of pancreatic tumors by selectively activating T cells that fight cancer. Its effects became even stronger when researchers combined it with chemotherapy, radiotherapy or immunotherapy. The findings suggest that BifidoSumIL-2 could eventually provide a new way to improve how pancreatic tumors respond to treatment.

Using Bacteria to Deliver Cancer Therapy

“A big unmet medical need has been pancreatic cancer, and so that was going to be our mountain to climb,” said Ralph Weichselbaum, MD, the Daniel K. Ludwig Distinguished Service Professor and Chair of Radiation and Cellular Oncology at the University of Chicago.

BifidoSumIL-2 was created to release a modified version of interleukin-2 (IL-2) once it reaches a tumor. IL-2 is a potent immune signaling molecule that activates T cells involved in attacking cancer. Conventional IL-2 treatment, however, can produce serious side effects and can also stimulate immune cells that actually weaken the antitumor response.

The researchers sought to avoid these problems by using SumIL-2, a modified form of IL-2 engineered to more precisely activate cancer-fighting T cells while reducing stimulation of regulatory T cells. They then placed SumIL-2 inside Bifidobacterium longum so that the therapeutic molecule could be concentrated within tumors instead of throughout the body.

Developing the treatment required scientists from several disciplines to work together, including specialists in microbiology, synthetic biology, oncology, and immunology.

“This was a highly interdisciplinary effort,” said Mark Mimee, PhD, Assistant Professor of Microbiology at the University of Chicago. “We had to bring together people who understand bacteria, people who understand tumors, and people who understand the immune system to make something like this possible.”

Why Bifidobacterium Can Target Tumors

Bifidobacterium offered the researchers an unusual advantage as a delivery system. The bacterium grows in anaerobic environments, meaning places with very little oxygen. Low oxygen levels are common inside many solid tumors, including pancreatic tumors, while healthy tissues generally contain more oxygen and are therefore less suitable for the bacteria.

“Bifidobacterium is an obligate anaerobe, so it doesn’t grow in the presence of oxygen,” Mimee said. When the bacteria are injected systemically, they are cleared from healthy tissues with abundant oxygen. Inside the low-oxygen regions of tumors, however, they can become active.

That preference allows the engineered bacteria to function as microscopic drug factories inside tumors. Once there, they produce SumIL-2 where the treatment is needed rather than broadly throughout the body. Researchers also noted that Bifidobacterium has shown a favorable safety profile in preclinical models and is already well known as a probiotic organism. It is commonly present in yogurt and is generally recognized as a safe, off-the-shelf probiotic.

Engineering the organism was not simple.

“Bifidobacterium is not the easiest organism to work with,” Mimee said. “It’s anaerobic, it grows slowly, and the genetic tools for manipulating it are much more limited compared to model bacteria like E. coli. A lot of the work was just figuring out how to reliably engineer it.”

Stronger Results With Combination Treatments

Tests in animal models showed that BifidoSumIL-2 preferentially gathered inside tumors, stimulated immune activity, and slowed the growth of pancreatic cancer. It also changed the tumor microenvironment in a potentially beneficial way by increasing the activity of cancer-fighting CD8+ T cells.

The results improved further when BifidoSumIL-2 was paired with established cancer treatments. Combining the bacterial therapy with chemotherapy, radiation therapy, or anti-PD-L1 immunotherapy led to better tumor control and longer survival than the individual treatments achieved on their own.

“This combination potential is one of the study’s most important findings; BifidoSumIL-2 not only works by itself — it works with radiotherapy, chemotherapy, and immunotherapy,” Weichselbaum said.

Despite the encouraging findings, BifidoSumIL-2 has not been tested in humans. Future research will need to examine its long-term safety, the possibility of effects outside the intended tumor, how long the immune response lasts, and whether the bacteria might eventually be given orally instead of through injection. The researchers also want to investigate whether the strategy can be combined with newer pancreatic cancer treatments, including KRAS inhibitors.

The Growing “Bugs as Drugs” Approach

The research adds to growing interest in a strategy known as “bugs as drugs.” By engineering probiotic bacteria to seek out tumors and produce therapies directly inside them, scientists may be able to concentrate powerful immune treatments where they are most useful while reducing unwanted effects elsewhere in the body.

The study, “Engineered probiotic Bifidobacterium for tumor-targeted pancreatic cancer therapy” was supported by funds from the Ludwig Foundation and the National Institutes of Health.

Additional authors include Jaehyun Lee, Kaiting Yang, Christina Nowicki, Wei Liu, Emile Naccasha, and Hua Liang from the University of Chicago; Zhichen Sun from the University of Texas Southwestern, Dallas; and Yang-Xin Fu from Tsinghua University, Beijing, China.

UChicago Medicine and the Biological Sciences Division continue to be at the forefront of cancer care and research. In April 2027, UChicago Medicine will open the AbbVie Foundation Cancer Pavilion, Chicago’s first freestanding cancer pavilion, to bring advanced diagnostics, innovative treatments, translational discoveries, and comprehensive support to patients and the community.