Pseudomonas aeruginosa contracts mucus to rapidly form biofilms in tissue-engineered human airways

Context and background: P. aeruginosa is associated with various human infections, for example in wounds and airways, drastically affecting those with underlying conditions and compromised immune systems. Like numerous other pathogens, P. aeruginosa has the ability to form aggregates encased in a self-secreted matrix, called ‘biofilms’. These biofilms are recalcitrant to antibiotics and immune clearance, making the treatment of such infections more difficult. In airways, while mucus is our first line of defense against pathogens, very little is known about the association between mucus and formation of biofilms by pathogens.

In this study, authors develop a new in vitro model to study the association between P. aeruginosa and mucus. This model mimics the airway microenvironment and allows the authors to use of high-resolution microscopy to visualise the cells, mucus and bacteria within the artificial airway.

 

Experimental setup: The in vitro model (‘AirGels’) was developed by allowing human bronchial epithelial cells to grow on the cylindrical cavity of collagen scaffolds, creating a model airway tube. Under a maintained air-liquid interface, the epithelial cells differentiate to form relevant physiological features. This model was first characterised to determine if it reflected features of the human airway epithelium. AirGels were then inoculated with fluorescently-tagged P. aeruginosa to study their biofilm forming capabilities under these relevant conditions.

 

Important Findings:

 

A new airway infection model that replicates human airway epithelium characteristics in vitro – The ‘AirGels’ developed in this study replicate many key features of the human airway epithelium – the tube-like shape, different cell types, presence of mucus and beating cilia that generate relevant clearance velocities. Thick filaments indicative of extracellular mucus were seen in the AirGels and the cilia were seen to beat at frequencies similar to previous ex vivo measurements.

 

1. aeruginosa forms mucus-associated biofilms in AirGels –

The authors explored what happens to P. aeruginosa when inoculated into the AirGel model. Within a few hours, P. aeruginosa formed aggregates resembling biofilms on the mucosal surface of the artificial airway model. The mucosal component of the AirGels was found to be crucial for biofilm formation. Through investigations of different mutant strains, the type IV pili, which are surface-exposed filamentous structures of the bacteria, were found to be play a major role in the biofilm formation on the mucosal surface. The type IV pili perform many functions in P. aeruginosa – motility, adherence, mechanochemical regulation, biofilm formation and many more. Hence it was interesting to see that they were also important for mucus-associated biofilm formation. Despite the role of the pili in adhesion, the formation of aggregates here was attributed to the contracting of mucus by the pili rather than adhesion, resulting in rapid aggregation of the bacteria.

It is fascinating to think that mucus which has a protective function, also serves as a substrate for a pathogen to rapidly attach and form aggregates. Pathogens are great at adapting to the host environment, and this study highlights another way in which P. aeruginosa adapts to the airway, using mucus to adhere, attach and aggregate in that environment.

 

What I found interesting about this preprint: As someone who is fascinated by biofilms, when I first came across this preprint, I was excited to read about it because the study looked at P. aeruginosa biofilms. However, after having read this preprint, I am most excited about the novel airway infection model that the authors have developed!

This preprint really highlights the importance of using human-relevant models in in vitro studies of infections and biofilms. Such models can give us novel insights into how pathogens behave in these specific human microenvironments, thus enabling us to understand these infections better. It would be really interesting to see how traditional antibiotics, as well as other non-traditional treatments, perform against in vitro mucus-associated biofilms in this human-relevant infection model.

 

Questions for the authors:

1. Was the antibiotic tolerance of these mucus-associated biofilms investigated? Do you expect the association with a mucosal surface would change the effects of antimicrobials?
2. What I found valuable in the model was the ability to make the AirGels from diseased cells from Cystic Fibrosis donors. This could be very beneficial in studying such disease using in vitro Were there any significant differences between these and the AirGels from healthy epithelial cells? Was there difference in the biofilm formation and any biofilm characteristics between these two?

 

Further Reading:

1. Burrows, L. L. (2012). Pseudomonas aeruginosa twitching motility: type IV pili in action. Annual review of microbiology, 66(1), 493-520.
2. Malhotra, S., Hayes Jr, D., & Wozniak, D. J. (2019). Cystic fibrosis and Pseudomonas aeruginosa: the host-microbe interface. Clinical microbiology reviews, 32(3), e00138-18.
3. Faure, E., Kwong, K., & Nguyen, D. (2018). Pseudomonas aeruginosa in chronic lung infections: how to adapt within the host?. Frontiers in immunology, 9, 2416.

Tags: biofilms

doi: https://doi.org/10.1242/prelights.32290

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