Protracted Bacterial Bronchitis (PBB) is a type of chest infection in young children. It causes a persistent cough which affects school performance, limits activity and disrupts sleep. It is treated with antibiotics but prolonged and / or multiple courses are often required. Knowing which bug is causing the infection, guides antibiotic choice and reduces the risk of antibiotic resistance. This is normally done by growing (culturing) bugs from a lower airway sample. However, this does not identify some of the bugs which are thought to be important in PBB such as those that grow in the absence of oxygen (anaerobes) and fungi. All the bacteria and fungi in the lung (the lung microbiome) can be identified using laboratory tests based on the detection of their DNA. Investigating the lung microbiome has increased understanding of several chronic lung diseases but little is in known about it in PBB. We aim to analyse the lung microbiome in 40 children with PBB on two types of lower airway sample. The samples will be taken for another study that has already been funded (CLASSIC PBB). Our analysis will use leftover samples that would otherwise be thrown away. We plan to start recruiting children at UHNM to CLASSIC PBB in December 2021. Children and parents will be informed about this extra part to the study and if they want to take part will complete an additional consent form. The results will help our understanding of PBB and lead to further research studies.