Transcriptomic analysis of Streptococcus pneumoniae serotype 1 reveals serotype-specific gene regulation

<i>Streptococcus pneumoniae</i> (<i>Sp</i>) is an opportunistic pathogen that colonizes the mucosal surfaces of the human upper respiratory tract. While transcriptomic studies of <i>Sp</i> have become more common, most have focused on laboratory-adapted strains such as D39 or TIGR4. These strains, though widely used in research, may not fully capture the biology of clinical isolates, particularly the hypervirulent serotype 1 (S1). S1 is clinically significant due to its association with invasive disease, epidemic outbreaks and a distinct global distribution, particularly in regions with a high pneumococcal disease burden. Unlike many other serotypes, S1 is frequently linked to hypervirulence and a propensity for rapid spread, making it a high-priority target for understanding the molecular mechanisms underpinning pneumococcal pathogenesis. In this study, we conducted a comprehensive <i>in vitro</i> transcriptomic analyses of <i>Sp</i> S1 strains, positioning this work as a valuable resource for the pneumococcal research community. Using a straightforward approach, we cultured three distinct S1 strains - ST306, ST217 and ST615, representing European, African and South American S1 lineages, respectively - in Brain Heart Infusion medium and compared transcriptomic profiles during exponential growth to those of the well-characterized laboratory-adapted D39 strain. Our analysis revealed significant differential expression of 292 genes in all three S1 isolates compared to D39. Among these, 151 genes had higher expression, including those involved in competence pathways and purine metabolism, while 141 genes exhibited lower expression, particularly those linked to lactose metabolism and iron/amino acid transport. These findings underscore the distinct molecular features of S1 strains, which likely contribute to the unique pathogenic properties of this serotype. The identification of the distinct transcriptional signatures of hypervirulent S1 strains paves the way for future efforts to design targeted therapeutics against pneumococcal S1 infections.