Effect of Low-Frequency Ultrasound on Antibiotic Susceptibility of Pseudomonas aeruginosa Biofilms

Biofilms are complex communities of microorganisms generally attached to a surface and surrounded by a gel-like matrix of extracellular polymeric substances (EPS). This matrix provides protection and support to the biofilm community, shielding the microorganisms from environmental stresses, including antibiotics and host immune responses. Biofilms play significant roles in several aspects of human life and health. While they can be beneficial in certain environmental processes, biofilms are also associated with chronic infections. They exhibit high levels of antibiotic tolerance, contributing to the escalating antimicrobial-resistance health crisis. The combination of low-frequency ultrasound (LFU) with antibiotics has emerged as a potential strategy to combat antimicrobial resistance. However, the mechanisms underlying this synergistic approach at both the biofilm and cellular levels remain poorly understood. This dissertation aims to elucidate these mechanisms through a series of comprehensive studies. Preliminary studies were conducted to verify the influence of LFU on antibiotic susceptibility of Pseudomonas aeruginosa biofilms, and to guide the subsequent research. Building upon these findings, a thorough analysis was performed to assess the impact of LFU on biofilm characteristics, including biofilm morphology and mechanical properties, and their subsequent influence on antibiotic susceptibility. Additionally, the effects of combined LFU and antibiotics on mucoid biofilms, relevant to cystic fibrosis infections, were explored. Furthermore, a 1-dimensional biofilm inactivation model was developed to probe potential mechanisms underlying the observed enhancement with LFU, suggesting changes in diffusion coefficient and biofilm cohesion. The enhanced inactivation of biofilms was characterized by an increase in antibiotic inactivation constants, resulting in shortened contact times for cell inactivation. This led to an exploration of cellular-level mechanisms, with a particular focus on the effect of LFU on efflux pumps, a key mechanism employed by cells to resist antibiotics. By achieving a better understanding of the combination of LFU and antibiotics, this research provides valuable insights into the management and treatment of biofilm-associated infections.