BACKGROUND Pseudomonas aeruginosa may undergo a phenotypic change from the wild (prototrophic) type to an auxotrophic phenotype in the course of respiratory infection in patients with cystic fibrosis. The clinical significance of this is unclear. A study was undertaken to investigate whether the presence of auxotrophs of P aeruginosa in the sputum of patients with cystic fibrosis correlated with severity of respiratory disease, and whether increased sputum concentrations of amino acids were associated with the emergence of these forms. METHODS Sixty adult patients with cystic fibrosis, colonised by P aeruginosa, were recruited and baseline clinical data including lung function were recorded. Serial sputum samples were obtained before, during, and after infective exacerbations where possible. These samples were used for routine microbiological culture, assessment of auxotrophy of P aeruginosa, measurement of amino acid content, and neutrophil elastase assay. RESULTS Auxotrophy was common in patients with cystic fibrosis and 20 (33%) had a mean percentage auxotroph count of more than 50% total cfu/ml. The mean percentage auxotroph count was inversely correlated with forced expiratory volume in one second (FEV(1); tau = -0.194, p = 0.031). The median sputum amino acid concentration of the group was 12.5 mmol/l (range 0.13-40.6). The mean amino acid concentration in 33 subjects during infective exacerbations was 18.2 mmol/l (95% CI 15.1 to 21.3) compared with 12.3 mmol/l (95% CI 9.8 to 14.8) when well (p = 0.001). The amino acid content of sputum was inversely correlated with FEV(1) (tau = -0.253, p = 0.005). CONCLUSIONS P aeruginosa frequently exhibits auxotrophy in patients with cystic fibrosis, particularly in those with severe underlying pulmonary disease. The sputum amino acid content of patients with cystic fibrosis is high during infective exacerbations and correlates with pulmonary disease severity.
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