Isolation and Characterisation of Bacteriophages That Infect Capsulated Streptococcus pneumonia

S. pneumoniae (pneumococcus) is a major cause of pneumonia, sepsis, and meningitis, responsible for over 1.2 million deaths per year. Rates of antibiotic resistance are rising, with over one third of isolates in the US and parts of Europe showing a reduced susceptibility to penicillin. Although pneumococcal conjugate vaccines have resulted in a decline in invasive disease caused by the pneumococcal serotypes included in the vaccine, non-vaccine serotypes have been shown to cause replacement disease. To address these important clinical challenges and provide cross serotype protection against pneumococcal infection, alternative therapies are urgently needed such as the exploitation of bacteriophages, which can specifically target and kill pneumococci. Bacteriophages are being developed to treat a range of bacterial pathogens due to their ability to kill pathogens which are resistant to conventional antibiotics, and due to their specificity and ability to access and replicate in difficult micro-environments within the human body. Although the previously isolated pneumococcal lytic phages; Dp-1 and Cp-1 showed promise as a treatment for pneumococcal infection, they could only infect non-capsulated strains, which are attenuated and non-invasive in the human clinical setting. This project describes the isolation and characterisation of a new lytic phage SP-QS1, which can infect and significantly reduce the load of capsulated pneumococcal strains that cause human invasive disease. SP-QS1 is a distinctive new siphovirus with prolated-head, non-contractile tail and tail fibres. The interaction between SP-QS1 and S. pneumoniae in both in vitro and in in vivo assays demonstrated that it is able to significantly reduce the amount of S. pneumoniae in two mouse models of invasive pneumonia; the intranasal and the intravenous model of infections. The genomic sequencing of SP-QS1 revealed that genes with recognisable homologies are often ordered according to the following; genes involved in phage packaging, structural proteins, replication and genes associated with cell lysis. Interestingly, SP-QS1 genome does not encode CRISPR sequences, proteins with trans-membrane domains or regulatory elements. In addition, because the phage genome does not encode integrase genes, it appears to be a genuine lytic phage. Genetic characterisation of SP-QS1 genome illustrated that this phage encodes for glycosyltransferase, and it is suggested to be responsible for capsule degradation. SP-QS1 shows promise to control and treat the infections caused by S. pneumoniae.