Emergence of a Pseudomonas aeruginosa Hypermutator Strain During the Course of Ventilator-Associated Pneumonia

Hypermutator lineages of Pseudomonas aeruginosa arise frequently during the years of lung infection seen in patients with cystic fibrosis and bronchiectasis but are rare in the absence of structural lung disease. Since the onset of the COVID-19 pandemic, large numbers of patients have remained mechanically ventilated for extended periods of time. These patients are prone to acquire bacterial pathogens that persist for many weeks and have the opportunity to evolve within the pulmonary environment. However, little is known about what types of adaptations occur in these bacteria and whether these adaptations mimic those described in chronic infections. We describe a COVID-19 patient with a secondary Pseudomonas aeruginosa lung infection in which the causative bacterium persisted for >90 days. During the course of this infection, a hypermutator lineage of P. aeruginosa emerged and co-existed with a non-hypermutator lineage. Compared to the parental lineage, the hypermutator lineage evolved to be more extensively resistant to antibiotics, to change its type III secretion profile, and to grow more slowly. Genomic analyses of the hypermutator lineage identified numerous mutations, including in the mismatch repair gene mutL and other genes frequently mutated in individuals with cystic fibrosis. Together, these findings demonstrate that hypermutator phenotypes can emerge when clearance of P. aeruginosa fails to occur in typically acute infections such as ventilator-associated pneumonia and suggest that hypermutator lineages can affect patient treatments and outcomes.