The effects of subinhibitory antibiotic concentrations on Pseudomonas aeruginosa within model Cystic Fibrosis bacterial communities

Chronic pulmonary bacterial infections are a leading cause of morbidity and mortality among Cystic Fibrosis (CF) patients. Despite traditional classification of infections in terms of the most prominent detected pathogen, CF pulmonary infections are highly polymicrobial. The most prevalent of the bacterial pathogens among patients is Pseudomonas aeruginosa, infection with which results in greatly decreased lung function and increased risk of mortality. Antibiotic treatment is key to managing bacterial infections, but treatment often fails to clear the target bacteria. A contributing factor is likely failure of treatment to deliver sufficient doses of antibiotic to bacterial populations, resulting in exposure to subinhibitory concentrations which are known to select for high-level resistance. In this thesis, I find that, in addition to selecting for high-level resistance, subinhibitory antibiotic concentrations can drastically impact the coculture dynamics between P. aeruginosa and another CF-associated species, Stenotrophomonas maltophilia, such that the usually dominant P. aeruginosa was made to coexist with- or driven extinct by-S. maltophilia in cocultures treated with a subinhibitory concentration of tobramycin. From this observation, I go on to find that increasing the size of the community through the addition of Staphylococcus aureus, also a CF-associated species, magnifies this effect of tobramycin. However, when the viscosity of the liquid lab media was increased to more resemble the thick mucus in the CF lung, treatment with tobramycin resulted in a community where all three species were able to coexist. Further investigation found that this coexistence was stable in the absence of viscosity and tobramycin, suggesting coevolution between the three species. Likelihood of coexistence differed between different selection lines and was influenced differently by individual evolved species. Sequencing of coexisting isolates did not reveal a concrete mechanism, but suggested candidate genes in P. aeruginosa involved with polyamine synthesis that warrant further investigation. Sequencing also found that mutations were constrained in communities of increasing complexity. Together these findings suggest that the results of ineffective antibiotic treatments can be wide ranging and unexpected, and make the case for further investigation into interspecies interactions within the CF lung, which are currently largely unknown.