There is mounting evidence that probiotic bacteria could play a role in reducing disease in the respiratory tract. Streptococcus pneumoniae (the pneumococcus) is an important paediatric pathogen, causing pneumonia, meningitis, septicaemia and otitis media. Pneumococci commonly colonise the nasopharynx of children, and this carriage is considered a prerequisite for pneumococcal disease. Our previous studies have found that the probiotic Streptococcus salivarius can disrupt pneumococcal adhesion to epithelial cells when added before, during, but not after pneumococcal colonisation is established. S. salivarius strain K12 is known to inhibit pathogenic bacteria through production of megaplasmid-encoded bacteriocin-like inhibitory substances (BLISs) in vitro. In order to further investigate the mechanism by which S. salivarius inhibits S. pneumoniae colonisation, we have developed two duplex quantitative PCR assays. One assay detecting the autolysin gene (lytA) in S. pneumoniae and dextranase gene (dex) in S. salivarius, will be used to quantify these two species. To further investigate the role of the megaplasmid in S. salivarius-mediated inhibition of S. pneumoniae colonisation, the second duplex qPCR assay will detect the chromosomal dex gene as well as a megaplasmid-encoded S. salivarius gene. These assays will be used to investigate the inhibitory activity of plasmid-harbouring and plasmid-cured strains of S. salivarius K12 and the more recently characterised M18 in an in vitro colonisation model.