Nathan Ni, Ph.D.
In many cases, bacteria cause diseases, not cure them. However, Tal Danino from Columbia University believes that bacteria may be the key to conquering cancer. He and his research team recently designed bacteria to deliver anticancer agents to the core of a tumor. Armed with a litany of modern tools and technologies, Danino sees an opportunity for expanding this approach to fight cancer.
As far back as 2600 BCE and the royal court of ancient Egypt, physicians observed a relationship between bacterial infection and cancer progression. From a modern medical perspective, the first intentional inoculation of a cancer patient occurred in 1863 when the German physician Wilhelm Busch infected a cancer patient with Streptococcus pyogenes and noted tumor regression. Unfortunately, the patient also died of the infection, demonstrating the importance of controlling bacterial virulence.1
Later, the American physician William Coley experimented with a heat-inactivated mixture of Streptococcus pyogenes and Serratia marcescens dubbed “Coley’s toxin.” Injecting this mixture into or near tumors brought considerable success,2 but an inability to explain the therapeutic mechanism or to control side effects meant that Coley’s toxin did not garner widespread acceptance.
The development of radiotherapy and chemotherapy further marginalized bacteria-mediated tumor therapy.1 Nonetheless, bacteria continued to be part of cancer therapeutics throughout the 20th century, most notably the Mycobacterium bovis BCG vaccine, which physicians used as an anti-bladder cancer agent.
Today, several developments have fueled a renewed interest in bacteria as an anticancer therapeutic tool. Claudia Gravekamp from the Albert Einstein College of Medicine credited DNA recombination technology with enabling bacterial attenuation to reduce side effects and increasing tumor-targeting capacity.
Additionally, immuno-oncology brought a better appreciation for how molecular agents activate the immune system for cancer treatment. But the most important shift came with microbiome research and the discovery that tumors naturally host bacteria. “Contrary to the dogma that our bodies are sterile, it’s quite natural for us to have transient exposures to bacteria. This helped people accept bacteria as a natural therapeutic vehicle,” said Danino.
The enemy of my enemy
Although the full breadth of bacterial anticancer mechanisms is still unknown, bacteria counter cancer in several ways. The first is direct competition. Bacteria are better equipped for hypoxic environments than human cells. As such, they can compete with cancer cells for nutrients and the limited oxygen available in tumor microenvironments (TMEs). Danino noted that some bacteria have a therapeutic effect against cancer just through competition alone.
The second lies in the toxins that bacteria naturally produce. They exert cytotoxic effects, and bacteria-produced endotoxins, such as lipopolysaccharide (LPS), also trigger inflammation and immune activation. This recruits effector cells such as macrophages and neutrophils to the tumor site, thereby constraining both bacterial and tumor growth. Some bacteria can even recruit adaptive immune system elements such as CD4+ and CD8+ T cells.
Researchers like Danino plan to use these natural advantages to increase efficacy and improve targeting selectivity. For example, they can control bacterial activity by placing drug-producing or immunostimulatory mechanisms under oxygen-sensitive promoters that only turn on in hypoxic environments. Alternatively, bacteria can be controlled through quorum sensing.
“Our group is looking at the idea that bacteria can only hit critical mass in the tumor,” said Danino. “We can then design it so that they’ll only activate once they hit that critical mass.”
Walking a fine line
There is no single optimal design strategy for engineering bacteria for anticancer purposes, according to Danino. One idea is to attenuate bacteria by making them dependent on a particular supplement.3 The bacteria can acquire some of this nutrient while in the tumor, but would eventually starve to death.
“This is a biocontainment feature, to prevent [the bacteria] from mutating while in tumors, wreaking havoc, and potentially migrating to other organs,” Danino said. Indeed, the prospect of losing control is always a concern since most bacteria used to treat cancer are attenuated versions of pathogenic strains such as Listeria, Salmonella, and Clostridium.4
While this strategy is useful in a clinical setting, most researchers working at the basic science level do not use attenuations like this. “I think that [bench scientists] view longevity as an advantage,” Danino said. “[Bacteria that persist] for several weeks can actively sense and respond to changes in the tumor and manufacture different compounds accordingly.”
These long-lasting bacteria still need to be reined in before therapeutic use, lest they cause sepsis. Danino highlighted several methods for this, including LPS modification to reduce sepsis-causing responses, as well as direct intratumoral administration to limit systemic exposure. Limiting sepsis responses can also aid bacterial efficacy. “Listeria is rapidly eliminated by the immune system in the blood, but persists in regions that feature strong immune suppression like the TME,” Gravekamp remarked.
Meeting the locals
Researchers still need to explore how the native microbiome affects exogenously introduced bacteria. “Mice don’t have tumor microbiomes because they live their entire lives in a clean animal care facility, but humans are believed to accumulate bacteria in their tumors over a long period of time through multiple exposures,” Danino said. “For example, bacteria on teeth can enter the bloodstream through trips to the dentist and the act of brushing, and they can populate internal niches that way. So we don’t quite have accurate models to look at how tumor microbiome composition affects bacteria-based therapeutic approaches.”
Another question is how the native microbiome affects the tumor microbiome. Competition between native and tumor microbiomes exists, but native intratumoral bacterial levels are orders of magnitude lower than the concentrations needed for therapeutic use. Researchers recently found that the majority of bacteria in tumors reside within cells rather than as part of the tumor structure,5 which leads Danino to believe that competitive effects will be minor.
Surveying the big picture
From a general standpoint, there are three key questions to be answered when using bacteria for anticancer therapy: what is the best strain; what is the best agent; and what is the best control method? The answers to these questions differ depending on cancer type, specific tumor properties, and any other treatments underway. Bacteria-based therapeutic approaches are compatible with other therapeutic approaches such as immunotherapy, chemo/radiotherapy, and so forth, and can be combined with these modalities. “We developed radioactive listeria,6 which was highly effective against pancreatic cancer, and immunogenic listeria carrying [the chemotherapy agent] gemcitabine,7 which resulted in better T cell responses in the TME,” Gravekamp said.
At the same time, finding the right bacteria for the right cancer can be like finding a needle in a haystack. Danino remarked that animal models are not well-equipped to handle the necessary screening throughput. “This is why we developed an in-vitro system last year8 in order to rapidly test hundreds of different types of bacterial therapy. I think [in-vitro screening] will play a role in how we develop these approaches as we progress.”
Nonetheless, Danino highlighted what he called the “democratic ability” of bacteria-based therapeutics. “I would say that the power of the bacteria approach, relative to more costly personalized methods, is that you’re targeting the general characteristics of a tumor and you don’t need genetic level information for screening,” he stated. “That’s the beauty of it, especially when it comes to applications in the developing world.”
S. Felgner et al., “Bacteria in cancer therapy: Renaissance of an old concept,” Int J Microbiol, 2016:8451728, 2016.
W.B. Coley, “The treatment of inoperable sarcoma by bacterial toxins (the mixed toxins of the streptococcus erysipelas and the bacillus prodigiosus),” Proc R Soc Med, 3(Surg Sect):1-48, 1910.
M.R. Charbonneau et al., “Developing a new class of engineered live bacterial therapeutics to treat human diseases,” Nat Commun, 11:1738, 2020.
S. Zhou et al., “Tumour-targeting bacteria engineered to fight cancer,” Nat Rev Cancer, 18(12):727-43, 2018.
D. Nejman et al., “The human tumor microbiome is composed of tumor type-specific intracellular bacteria,” Science, 368(6494):973-80, 2020.
W. Quispe-Tintaya et al., “Nontoxic radioactive Listeria(at) is a highly effective therapy against metastatic pancreatic cancer,” Proc Natl Acad Sci U S A, 110(21):8668-73, 2013.
B.C. Selvanesan et al., “Tumor-targeted delivery of childhood vaccine recall antigens by attenuated Listeria reduces pancreatic cancer,” bioRxiv, 2019.
T. Harimoto et al., “Rapid screening of engineered microbial therapies in a 3D multicellular model,” Proc Natl Acad Sci U S A, 116(18):9002-7, 2019.