Phenotypic characterisation of pneumococcal serotype-1 variants presenting low haemolytic activity

Background: Streptococcus pneumoniae is a significant human pathogen responsible for life-threating diseases such as pneumonia, septicaemia and meningitis. Of the nearly 100 distinct pneumococcal serotypes, pneumococcal serotype 1 is described as one of main causes of invasive disease worldwide. One highly enigmatic feature of serotype 1 resides in its being rarely detected in human nasopharyngeal specimens. Aim: The aim of this project was to compare serotype 1 lineage A (ST306) and C (ST615) with respect to their immunological and virulence properties using both in vitro and in vivo tools, as well as their differential gene expression profile. Methods: A series of in vitro experiments were performed to assess the ability of serotype 1 to adhere and invade epithelial cells, and to determine its ability to inhibit phagocytosis and gain insight into the mechanisms involved. In parallel, three in vivo standardized mouse models of pneumococcal infection were exploited to examine the virulence properties of ST306 and ST615 and their ability to colonise the nasopharyngeal tissue. Finally, RNA-seq analysis of in vitro cultures of ST306 and ST615 was carried out in an attempt to identify differential expression patterns. Results: ST615 serotype 1 was determined to be highly virulent, causing the death of 80-100% of infected mice by around 48h post-infection, while all mice infected with ST306 survived when using pneumococcal doses inductive of invasive pneumonia model. In a nasopharyngeal carriage mouse model, ST306 serotype 1 was shown to be able to establish colonisation persisting up to 28 day post-administration, although at a much lower density compared to ST615. While ST615 was capable of establishing nasopharyngeal colonization, clearance occurred earlier at day 14 compared to ST306. RNA gene expression analysis focused on virulence factors and critical biological functions determined that ST615 serotype 1 presented a profile consistent with its weak colonisation and its invasive properties. The genes associated with capsule synthesis were not differentially expressed between ST615 and ST306 but that other virulence factors such as psp, pavA, ply and cpbD were differentially expressed. Conclusions: Although ST306 and ST615 possesse a unique and virulent capsule, ST306 was not able to cause pneumonia to mice model