By Laura Tran, C2ST Intern, Rush University

Recently scientists¹ found a potential alternative for managing pain with anthrax². But bacteria, it turns out, have medical applications beyond just pain management. Not only do some bacteria possess natural tumor-targeting and tumor-killing abilities, but bacteria can be engineered to be used in a number of different therapeutic applications. 

Imagine using bacteria as drug delivery vehicles to kill tumor cells. That’s right, researchers have developed a “cloaking” system³ that allows bacteria to temporarily avoid detection by our immune system. This system allows bacteria to effectively deliver drugs directly to tumors and kill tumor cells in mice. Now that’s putting bacteria to work!

Normally, invading bacteria are detected by our immune system. Our white blood cells (i.e., leukocytes) are responsible for patrolling and making quick work of invading pathogens. Much of this is facilitated through our immune cell’s receptors such as toll-like receptor 4 (TLR-4)⁴ which recognize specific components on bacterial surfaces. 

However, the unique properties of certain bacteria allow them to target tumors, making them a great candidate as a drug-delivery vehicle⁵. 

For instance, certain bacteria have capsular polysaccharides (CAP), which are sugar molecules that coat bacterial surfaces to form a protective barrier. With CAP, the bacteria can temporarily evade immune attacks, but without CAP they lose their “cloaking” protection and are subject to detection and immune attacks. 

There are two long-standing challenges to bacterial therapy⁶: 1) to prevent the immune system from rapidly clearing the bacteria, higher doses of bacteria are needed, and 2) to ensure that the bacteria are eventually cleared from the body to prevent toxicity. 

So how can we control CAP to be an effective therapy? 

Scientists engineered a way to balance these concerns with a “cloaking system” that functions as a transient CAP system.

The cloaking system works by manipulating the microbes’ DNA and reprogramming a gene that controls the bacteria’s surface. To do this, researchers altered the CAP system of a probiotic E. coli strain, Nissle 1917. They engineered an inducible CAP system (iCAP) that features an on/off switch. It works by using an external cue, such as a small molecule called IPTG. This molecule is given to the iCAP E. coli to turn on the cloaking system, which ensures bacterial survival and allows the bacteria to successfully deliver its cargo of tumor-fighting drugs and suppress rapidly growing tumors. 

Inducing CAP expression through IPTG before injection allows the bacteria to only temporarily evade immune detection. Over time, since no more IPTG is given to the bacteria, the body will be able to detect the bacteria and clear them out. Additional studies have also shown that they can control bacterial migration within the body. For instance, iCAP E. coli that was injected directly into the tumor (instead of the bloodstream) also traveled to distant uninjected tumors. Now that’s efficiency!

One of the challenges of bacterial therapies is knowing how much bacteria to use. It is important to use enough bacteria so they can successfully complete their drug delivery to tumors. But you can have too much of a good thing since increased bacterial doses can be toxic to our bodies and ultimately do more harm than good (i.e., dose-limiting toxicity). Balance is the key to this type of therapy.

However, scientists were recently able to increase the tolerable bacterial dose seen in a mouse model. Because humans are 250-times more sensitive to bacterial components (i.e., endotoxins), these results may have a more significant effect on humans than on mice. Thus, the “tweak to treat” method may be a solution to a long-standing challenge for bacterial therapies.

There are many promising avenues for this CAP engineering to take, such as 1) exploring different types of CAP in E. coli, 2) utilizing different bacterial species, and 3) modulating other aspects of the bacterial surface for a boosted, synergistic effect. The possibilities are endless for our disguised bacteria!

References:

1. https://www.nature.com/articles/s41593-021-00973-8
2. https://www.c2st.org/managing-pain-withanthrax/ 
3. https://www.nature.com/articles/s41587-022-01244-y
4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4887650/  
5. https://nyaspubs.onlinelibrary.wiley.com/doi/abs/10.1196/annals.1326.031 
6. https://pubmed.ncbi.nlm.nih.gov/20331869/