A tale of two temperatures: Thermoregulation in Pseudomonas aeruginosa biofilms, from environment to host

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Pseudomonas aeruginosa is a ubiquitous microorganism in the environment. It is also a notorious pathogen that commonly infects immunocompromised patients. In our study, we used this pathogen as a model organism to study thermoregulation within biofilms because temperature shift is a major cue when it transitions from the environment into a human host. Moreover, the pathogen experiences this temperature shift when it leaves the abiotic surfaces in the hospital environment at room temperature (23–25°C) to enter the host. In this work, we tried to answer the question of how P. aeruginosa adapts its biofilm formation during the temperature shift when it enters the host (37°C) from the environment (23°C)? We hypothesized that while biofilm formation is known to be important in each of these niches, there would be fundamental structural differences to the biofilm driven at least in part by the dramatic temperature shift.

What did we do?

We used both proteomics and transcriptomics tools to study the differential protein expression level and the gene expression level in the biofilm formed by P. aeruginosa at 23°C and 37°C. We also studied the morphology and the structure of the biofilm using microscopy techniques.

What did we find?

We found that P. aeruginosa (lab strain UCBPP-PA14) forms a higher biofilm mass at 23°C than at 37°C with different proteins being expressed at each temperature. However, the 37°C biofilm appeared to possess a more complex architecture with more apparent extracellular polymeric substance accumulation connecting and containing the cells. One interesting phenotype that we observed was the presence of small colony variants (SCVs) enriched at 37°C. This led us to focus on the importance of a subset of the bacteriophage-related genes identified in our transcriptomics data since the SCV phenotype in P. aeruginosa has been linked to the induction of a particular type of filamentous phage. Most bacteria carry prophages. Previous studies have shown the role of filamentous phages, and specifically the coat B (CoaB) protein, in maintaining the biofilm structural integrity at 37°C. We found that coaB was important for biofilm formation specifically at 37°C, and its absence could also influence the biofilm matrix as seen in the SEM micrographs. Since our transcriptomics data also showed the presence of other phage-related genes being upregulated at 23°C, in the future it will be interesting to study the different roles of phage proteins in either strengthening or targeting the biofilm. Overall, our study shows that temperature is an important environmental cue for this pathogen, which can influence its genetic circuit. Also, the differential protein expression observed is not only specific to P. aeruginosa biofilm but is found in the biofilm of yet another common opportunistic pathogen, Acinetobacter baumannii. Antibiotic resistance is a global problem and needs our immediate attention. Finding biofilm targets specifically for environmentally grown biofilms versus infectious biofilms can help us prevent it further.

Check out our new publication here to read more: https://www.nature.com/articles/s41522-021-00194-8

Karishma Bisht

Graduate Research Assistant, Texas Tech University