Effects of Long-Term, Low-Dose Macrolide Treatment on Pseudomonas aeruginosa PAO1 Virulence Factors In Vitro

Pseudomonas aeruginosa (P. aeruginosa) is a common cause of chronic airway infections in patients with pulmonary disorders such as diffuse panbronchiolitis (DPB) and cystic fibrosis (CF). Long-term, low-dose macrolide treatment has markedly increased long-term survival of patients with DPB. Consequently, researchers are interested in using macrolides to treat CF patients. Previous studies have demonstrated that macrolides influence P. aeruginosa virulence. However, most studies evaluated the regulatory effects of macrolides on P. aeruginosa virulence factors after treatment over a short period (<48 hours). In this study, we subcultured P. aeruginosa PAO1 for 2 to 18 months in the presence of lowdose macrolides and evaluated antibiotic minimum inhibitory concentrations (MICs), bacterial growth, and virulence factors, including various motilities, biofilm formation, and production of rhamnolipids, total protease, elastase, and pyocyanin. The production of many virulence factors gradually decreased with macrolide exposure, and some were maximally affected after only 2 months. Despite an initial inhibition after treatment with macrolides, treated bacteria later exhibited increased biofilm formation compared to untreated controls. These findings suggest that low-dose macrolide therapy for chronic airway infection should be administered for at least 2 months to achieve optimal effects against P. aeruginosa.