Determinants of chronic infection with Staphylococcus aureus in patients with bronchiectasis

Staphylococcus aureus is an uncommon pathogen in bronchiectasis not caused by cystic fibrosis (CF). The object of this study was to identify characteristics that cause patients to be prone to infection with S. aureus. The study population consisted of patients with bronchiectasis attending the authors' unit, excluding those with a diagnosis of overt CF. All patients had a high resolution computer tomographic scan (HRCT) of the thorax which demonstrated bronchiectasis. Cases that mere currently chronically infected with S. aureus (isolated consecutively on more than two occasions >3 months apart) were identified (n=12) and compared with 74 control patients who had not been chronically infected with S. aureus. Patients were carefully evaluated to determine the aetiology of their disease. Odds ratios (OR) and 95% confidence intervals (Cr) as measures of the association between disease characteristics and chronic infection with S. aureus were calculated. The results for patients chronically infected by S. aureus demonstrated significant associations with allergic bronchopulmonary aspergillosis (ABPA; OR=8.8, 95% CI 1.8-41.9), atypical variants of CF (OR=12.0, 95% CI 1.8-81.7) or equivocal sweat sodium values (OR=4.0, 95% CI 1.0-15.3). The associations persisted when the analysis was based on cases (n=28) in whom S. aureus had ever been isolated from sputum. In the latter analysis there was also a significant association with predominant upper zone disease on HRCT. These results suggest that patients with bronchiectasis in whom S. aureus is isolated from sputum should be carefully evaluated to exclude allergic bronchopulmonary aspergillosis or atypical cystic fibrosis.