A 19-year-old boy has a h...

A 19-year-old boy has a history of repeated chest infections. He had problems with a cough and sputum production in the first 2 years of life and was labelled as bronchitic. Over the next 14 years he was often ‘chesty’ and had spent 4–5 weeks a year away from school. Over the past 2 years he has developed more problems and was admitted to hospital on three occasions with cough and purulent sputum. On the first two occasions, Haemophilus influenzae was grown on culture of the sputum, and on the last occasion 2 months previously, Pseudomonas aeruginosa was isolated from the sputum at the time of admission to hospital. He is still coughing up sputum. Although he has largely recovered from the infection, his mother is worried and asked for a further sputum sample to be sent off. The report has come back from the microbiology laboratory showing that there is a scanty growth of Pseudomonas on culture of the sputum.