Anatomical Site-Specific Carbohydrate Availability Impacts Streptococcus pneumoniae Virulence and Fitness during Colonization and Disease.

Streptococcus pneumoniae colonizes the nasopharynx asymptomatically but can also cause severe life-threatening disease. Importantly, stark differences in carbohydrate availability exist between the nasopharynx and invasive disease sites, such as the bloodstream, which most likely impact S. pneumoniae's behavior. Herein, using chemically defined medium (CDM) supplemented with physiological levels of carbohydrates, we examined how anatomical site-specific carbohydrate availability impacted S. pneumoniae physiology and virulence. S. pneumoniae cells grown in CDM modeling the nasopharynx (CDM-N) had reduced metabolic activity and a lower growth rate, demonstrated mixed acid fermentation with marked H2O2 production, and were in a carbon-catabolite repression (CCR)-derepressed state versus S. pneumoniae cells grown in CDM modeling blood (CDM-B). Using transcriptome sequencing (RNA-seq), we determined the transcriptome for the S. pneumoniae wild-type (WT) strain and its isogenic CCR-deficient mutant in CDM-N and CDM-B. Genes with altered expression as a result of changes in carbohydrate availability or catabolite control protein deficiency, respectively, were primarily involved in carbohydrate metabolism, but also encoded established virulence determinants, such as polysaccharide capsule and surface adhesins. We confirmed that anatomical site-specific carbohydrate availability directly influenced established S. pneumoniae virulence traits. S. pneumoniae cells grown in CDM-B formed shorter chains, produced more capsule, were less adhesive, and were more resistant to macrophage killing in an opsonophagocytosis assay. Moreover, growth of S. pneumoniae in CDM-N or CDM-B prior to the challenge of mice impacted relative fitness in a colonization model and invasive disease model, respectively. Thus, anatomical site-specific carbohydrate availability alters S. pneumoniae physiology and virulence, in turn promoting anatomical site-specific fitness.

Sugar-Coated Killer: Serotype 3 Pneumococcal Disease.

Capsular polysaccharide (CPS), which surrounds the bacteria, is one of the most significant and multifaceted contributors to Streptococcus pneumoniae virulence. Capsule prevents entrapment in mucus during colonization, traps water to protect against desiccation, can serve as an energy reserve, and protects the bacterium against complement-mediated opsonization and immune cell phagocytosis. To date, 100 biochemically and serologically distinct capsule types have been identified for S. pneumoniae; 20 to 30 of which have well-defined propensity to cause opportunistic human infection. Among these, serotype 3 is perhaps the most problematic as serotype 3 infections are characterized as having severe clinical manifestations including empyema, bacteremia, cardiotoxicity, and meningitis; consequently, with a fatality rate of 30%-47%. Moreover, serotype 3 resists antibody-mediated clearance despite its inclusion in the current 13-valent conjugate vaccine formulation. This review covers the role of capsule in pneumococcal pathogenesis and the importance of serotype 3 on human disease. We discuss how serotype 3 capsule synthesis and presentation on the bacterial surface is distinct from other serotypes, the biochemical and physiological properties of this capsule type that facilitate its ability to cause disease, and why existing vaccines are unable to confer protection. We conclude with discussion of the clonal properties of serotype 3 and how these have changed since introduction of the 13-valent vaccine in 2000.