Effect of subinhibitory concentrations of azithromycin on adherence of Pseudomonas aeruginosa to bronchial mucins collected from cystic fibrosis patients

Sir, Pseudomonas aeruginosa, an opportunistic pathogen, is the major pathogen in the airways of patients with cystic fibrosis (CF) and is currently associated with the morbidity and mortality seen in this disease. Macrolides, such as azithromycin, are normally not included in the anti-pseudomonal therapeutic arsenal because of the absence of bactericidal or bacteriostatic activity. However, several previous investigators have reported that long-term administration of azithromycin is effective in patients with pulmonary P. aeruginosa infections. 1 The clinical benefits achieved could include the effects of an anti-inflammatory 2 and/or modulation of the production of virulence factors of P. aeruginosa, such as bacterial exoproducts, 3 the formation of biofilm, 4 or the synthesis of flagella 3 and some outer membrane proteins. 5 Adherence of P. aeruginosa to respiratory mucins plays an important role in the colonization of the airways of these patients. This adherence process involves flagella and several non-pilus adhesins localized on the outer membrane of P. aeruginosa. Supporting the clinical observations, adherence of this organism to human salivary or airway mucins has been demonstrated in vitro using liquid- or solid-phase adherence assays. 6 In order to investigate the action of azithromycin on the pathogenesis of P. aeruginosa, we evaluated—in a solid-phase adherence assay— the action of subinhibitory concentrations of the macrolide on the adherence of different bacterial strains to respiratory mucins of CF patients. The reference strain PAO-1, and 13 strains (five mucoid and eight non-mucoid strains) freshly isolated from the sputum of CF patients known to be infected by different strains and who were not treated with azithromycin, were tested. MICs of azithromycin were determined by an agar dilution method, following the recommendations of the NCCLS (2002), with powdered azithromycin supplied by Pfizer laboratories (Groton, CN, USA). Adherence assays were performed in 96-well microtitre plates (Linbro, INC laboratories, France), in which wells were coated with 10 µg of purified bronchial mucins prepared from tracheobronchial secretions of CF patients. As described by Vishwanath & Ramphal,6 100 µL of a suspension of ∼1 × 106 bacteria/mL, grown while agitating for 18 h