Study Finds How Bacterium Grows in Mucus Associated with Cystic Fibrosis
The basis of this study was to understand how the bacterium Pseudomonas aeruginosa thrives within the lungs of cystic fibrosis patients. Notably, the nutrients source that it requires for growth are poorly characterized. In this paper, my postdoc Jeff Flynn used a high-throughput sequencing approach (TnSeq) to understand how Pseudomonas uses mucin glycoproteins, major components of CF mucus secretions. By creating a mutant library of Pseudomonas (in which each cell contains a single mutated gene), we can determine which cellular processes are essential for growth under a given set of conditions (in this case, growth on mucus). Using Illumina sequencing to compare the library composition before and after growth, we discovered that Pseudomonas uses two essential processes to derive nutrients from the host. First, it relies on the glyoxylate pathway, a variation of the Krebs cycle, which allows the use of simple carbon compounds when glucose and other complex sources are not available. These include acetate, which found to be liberated from mucin and rapidly consumed during growth. Second, by generating a series of defined Pseudomonas mutants, we also discovered that two extracellular proteases (elastase and alkaline protease) are also used by this bacterium to break down the mucin glycoprotein.
Yet, it is important to note that growth on mucus is not a particularly efficient strategy for this pathogen. In fact, we know that Pseudomonas does much better when co-colonizing bacteria from the oral cavity are also present. However, in late stages of disease, Pseudomonas predominates and must rely on its own metabolic strategies to survive. We hypothesize that the slow growth of this bacterium on mucins contributes to their ability to withstand antibiotic treatments and the host immune response, and we are currently exploring these interactions in further detail. Our long-term goal is to exploit these processes in an effort to effectively manage chronic airway infections and improve clinical outcomes for the CF population.
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