Anti-Bacterial vs. Anti-Biofilm Effects of Azithromycin on Pseudomonas aeruginosa Isolates from Cystic Fibrosis Patients

Background: Cystic fibrosis (CF) is an autosomal recessive disease characterized by respiratory, gastrointestinal, and reproductive tract problems. Patients with CF often suffer from chronic pulmonary infections due to the Pseudomonas aeruginosa, which frequently ends in pulmonary exacerbations. With their anti-inflammatory, anti-virulence, and anti-biofilm properties, macrolides such as azithromycin (AZM) play a beneficial role in the treatment of CF. Objectives: We evaluated the ability of biofilm formation among P. aeruginosa isolates from CF patients and compared the anti-bacterial and anti-biofilm effect of the AZM on them. Methods: Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of AZM against 21 P. aeruginosa isolates from Iranian CF patients were determined. Then, we investigated the ability of biofilm formation by P. aeruginosa and also anti-biofilm effect of AZM on biofilm producing P. aeruginosa isolates. Results: The results showed a high level of AZM MIC (≤ 512 µg/mL) and MBC (≤ 2048 µg/mL) against P. aeruginosa isolates. Most isolates (70%) were biofilm producers, 20% being strong producers. The AZM anti-biofilm effect showed that sub-MIC concentration (≤ 64 µg/mL) could inhibit biofilm production. Conclusions: P. aeruginosa isolated from CF patients showed a high level of MIC and MBC for AZM that indicated a weak anti-bacterial effect of AZM. However, AZM inhibited biofilm formation in low doses; this might lead to dangerous chronic pulmonary infection by P. aeruginosa.