The respiratory microbiota of preterm infants at risk of chronic lung disease of prematurity

Abstract

Extreme preterm infants are at risk of developing chronic lung disease of prematurity (CLD). This thesis examines the bacterial colonisation of the respiratory tract in extreme preterm infants. Longitudinal sampling was performed from three anatomical sites within the respiratory tract of preterm infants over the first 28 days of life (Nasopharyngeal aspirates (NPA), tracheal aspirates (TA) and bronchoalveolar lavages (BAL)). Sequencing the bacterial 16S rRNA gene identified bacteria within the samples. IL-6 and IL-8 concentrations were measured using ELISA on TA and BAL samples. Widespread antibiotic use appeared to suppress bacterial load at all sites with most samples appearing sterile. Most samples successfully sequenced showed a dominant bacterial genus, consistent with previous studies of neonatal bacterial colonisation. In all samples, organisms from the Proteobacteria and Firmicutes phyla were most abundant with smaller number of Tenericutes and Actinobacteria. NPA samples demonstrated a clear effect of delivery mode over early colonisation patterns and a progression from Firmicutes to Proteobacteria dominated profiles over time. Tracheal aspirate samples showed evidence of an infectious process or dysbiosis associated with bacterial colonisation, with higher concentrations of IL-6 and IL-8 in samples successfully sequenced. BALs sampling more distally in the lungs verified these results suggesting pulmonary infection in preterm infants may be more common than clinically suspected. Comparing the three anatomical sites sampled revealed significant differences in bacterial community structure suggesting that these are separate bacterial niches. Also presented is work analysing the effect of extracellular apoptosis-associated speck-like protein containing a caspase activation domain (ASC), a protein involved in the initiation and propagation of the innate immune response. ASC was demonstrated to be present and bioactive in BAL supernatant from preterm infants. ASC induced IL-6 and IL-8 secretion in cell culture models and is a potential therapeutic target for preventing CLD in preterm infants