The transient nature of probiotic bacteria in the gut prevents them from exerting their full health benefits. Persistence and stable colonization can happen when a probiotic bacterium finds a receptor on the host cells to cling on. Surface layer protein or other surface molecules is known to provide an advantage for some Lactobacilli to interact with host cell receptors such as toll-like receptors¹.  Such natural ligand-receptor interaction is inconsistent and is often ineffective. Moreover, probiotics are claimed to compete with pathogens for adhesion sites², however, there is no direct mechanistic evidence of such a phenomenon¹. Therefore, an engineering approach to modify a probiotic to express proteins that specifically target a host cell receptor may be a solution to improve probiotic functionality.

We cloned and expressed a well-characterized Listeria adhesion protein (LAP)^3,4 from a nonpathogenic Listeria on the surface of Lactobacillus casei, a probiotic bacterium⁵. LAP binds to the host cell receptor, heat shock protein 60 (Hsp60), a chaperone protein, and its cell membrane expression is linked to stress or diseased condition^6,7.  The resulting bioengineered Lactobacillus casei probiotic (BLP) strain expressing LAP forms a biofilm-like structure that restricts Listeria interaction with the gut epithelial cells (Figure 1). 

This demonstrates the ability of BLP to pass through the layers of gut microbiota and the loosely and tightly adherent mucus to interact directly with epithelial cells. The BLP restricts Listeria by interacting with Hsp60, the cognate epithelial surface receptor of LAP (Figure 2). 

Intimate probiotic contacts with the epithelial cells not only prevent the interaction of Listeria monocytogenes with the epithelial Hsp60 receptor and lethality (92% survival rate) but also augments immunomodulatory function and maintain epithelial barrier integrity (Figure 3). In contrast, parental Lactobacillus strain is ineffective.

 This study thus offers direct evidence that rational engineering of probiotic strains could outcompete and diminish the colonization of the pathogens by competing for the receptor binding adhesion sites. Because of its specific interaction with Hsp60, this bioengineered probiotic strain has the potential to exert immunomodulatory action to alleviate symptoms associated with a variety of chronic inflammatory diseases of the gut where Hsp60 expression is very high⁶.  

You can read about all details of our study in the full paper here.

References

1. Plaza-Diaz, J., Ruiz-Ojeda, F.J., Gil-Campos, M. & Gil, A. Mechanisms of Action of Probiotics. Adv. Nutr. 10, S49-S66 (2019).
2. Amalaradjou, M.A.R. & Bhunia, A.K. Modern approaches in probiotics research to control foodborne pathogens. Adv. Food Nutr. Res. 67, 185-239 (2012).
3. Drolia, R., Tenguria, S., Durkes, A.C., Turner, J.R. & Bhunia, A.K. Listeria adhesion protein induces intestinal epithelial barrier dysfunction for bacterial translocation. Cell Host & Microbe 23, 470-484 (2018).
4. Drolia, R., & Bhunia, A. K. Crossing the intestinal barrier via Listeria adhesion protein and internalin A. Trends in Microbiology, 27 (5), 408-425 (2019).
5. Drolia, R., Amalaradjou, M.A.R., Ryan, V. et al. Receptor-targeted engineered probiotics mitigate lethal Listeria infection. Nat Commun 11, 6344 (2020). https://doi.org/10.1038/s41467-020-20200-5. 
6. Cappello, F. et al. Hsp60 as a Novel Target in IBD Management: A Prospect. Front. Pharmacol. 10 (2019).
7. Burkholder, K.M. & Bhunia, A.K. Listeria monocytogenes uses Listeria adhesion protein (LAP) to promote bacterial transepithelial translocation, and induces expression of LAP receptor Hsp60. Infect. Immun. 78, 5062-5073 (2010).