Inhibitory effect of Dolosigranulum pigrum and Corynebacterium pseudodiphtheriticum on pneumococcal in vitro growth

BackgroundStreptococcus pneumoniae is a nasopharynx coloniser that can invade sterile tissues, causing Invasive Pneumococcal Disease (IPD). Dolosigranulum pigrum and Corynebacterium pseudodiphtheriticum are commensal bacteria commonly isolated from the nasopharynx of healthy children, potentially playing a protective role. This study aims to analyse the effects of D. pigrum and C. pseudodiphtheriticum on S. pneumoniae in vitro growth. MethodsPneumococcal strains were collected from IPD patients and healthy carriers in Catalonia (2016-2023). D. pigrum and C. pseudodiphtheriticum strains were isolated from a healthy childs nasopharynx. S. pneumoniae was co-cultured with each commensal bacterium in triplicate experiments. Pneumococcal growth was quantified using a real-time PCR assay targeting the lytA gene. The effect of commensal bacteria on pneumococcal growth was evaluated using a linear mixed-effect regression model. ResultsTwenty-eight pneumococcal strains expressing 24 different serotypes and 26 clonal types were analysed (18 isolated in blood and 10 in nasopharyngeal aspirate). Pneumococcal growth was decreased by D. pigrum ({beta} = -0.763, 95% CI: -0.94 to -0.59, p < 0.0001) and C. pseudodiphtheriticum ({beta} = -0.583, 95% CI: -0.76 to -0.41, p < 0.0001). The combined presence of both had a stronger inhibitory effect ({beta} = -0.971, 95% CI: -1.15 to -0.79, p < 0.0001). No association was found between isolation site or serotype with pneumococcal growth. ConclusionD. pigrum and C. pseudodiphtheriticum significantly reduced pneumococcal growth, with a synergic effect when combined. This antagonistic effect supports the potential protective factor of healthy nasopharyngeal microbiota against IPD and the development of these microorganisms as probiotics.