Biomarkers for respiratory infection

Disclosed is a method of determining a level of activity of Pseudomonas aeruginosa bacteria in the lung of a patient, the method comprising: making a first measurement at a first time of a level of at least one marker of a bacterial iron scavenging process and of at least one secreted bacterial protein in a sample of sputum from the lung; making a second measurement at a second time of the levels of said marker and said protein in a sample of sputum from the lung; and determining said level of bacterial activity from changes in said measured levels of said marker of a bacterial iron scavenging process and said secreted bacterial protein over time, wherein an increase in levels of either said marker or said protein, or both, are indicative of an increase in bacterial activity.