88 Prevalence and characterization of unusual Staphylococcus aureus strains with a mucoid phenotype recovered from the airways of cystic fibrosis patients

Abstract

Objectives Staphylococcus aureus is frequently isolated from the respiratory tract already in early childhood and often persists for extended periods. Mucoid bacterial colony morphology has been described for Pseudomonas aeruginosa, the most prominent pathogen in CF, but not for S. aureus. Recently, we identified mucoid isolates also for S. aureus. To determine the prevalence and the underlying mechanism of mucoid S. aureus isolates we used S. aureus isolates collected during two independent studies including 371 CF patients. Methods To determine the underlying mechanism of mucoidy, the ica promoter region of all isolates and the entire ica operon of selected strains were sequenced. Biofilm formation was determined by a microtiter plate assay. Mucoid and normal isolates of one patient were characterized in terms of capsule expression and phagocytosis by neutrophils. Results From the respiratory specimens of 8 patients (prevalence of 2.2%) mucoid S. aureus isolates were recovered. M ucoid isolates were strong biofilm producers. Mucoidy was associated with a 5-bp deletion in the ica promoter region. Subsequent non-mucoid isolates, which also carried a 5-bp deletion, harbored compensatory mutations . Transformation of 5-bp deletion carrying normal strains with a vector expressing intact ica restored mucoidy. In contrast to the normal phenotype, mucoid strains were protected against phagocytosis by neutrophils. Conclusion Although mucoid S. aureus strains are rarely identified during chronic airway infection in CF, biofilm hyper-production seems to be an effective strategy for protection against phagocytosis by neutrophils.