Abstract
The exopolysaccharide capsule of Streptococcus pneumoniae is an important virulence factor, but the mechanisms that regulate capsule thickness are not fully understood. Here, we investigated the effects of various exogenously supplied carbohydrates on capsule production and gene expression in several pneumococcal serotypes. Microscopy analyses indicated a near absence of the capsular polysaccharide (CPS) when S. pneumoniae was grown on fructose. Moreover, serotype 7F pneumococci produced much less CPS than strains of other serotypes (6B, 6C, 9V, 15, and 23F) when grown on glucose or sucrose. RNA-sequencing revealed carbon source-dependent regulation of distinct genes of WT strains and capsule-switch mutants of serotypes 6B and 7F, but could not explain the mechanism of capsule thickness regulation. In contrast, 31P NMR of whole-cell extract from capsule-knockout strains (Δcps) clearly revealed the accumulation or absence of capsule precursor metabolites when cells were grown on glucose or fructose, respectively. This finding suggests that fructose uptake mainly results in intracellular fructose 1-phosphate, which is not converted to CPS precursors. In addition, serotype 7F strains accumulated more precursors than did 6B strains, indicating less efficient conversion of precursor metabolites into the CPS in 7F, in line with its thinner capsule. Finally, isotopologue sucrose labeling and NMR analyses revealed that the uptake of the labeled fructose subunit into the capsule is <10% that of glucose. Our findings on the effects of carbon sources on CPS production in different S. pneumoniae serotypes may contribute to a better understanding of pneumococcal diseases and could inform future therapeutic approaches.
Highlights
The exopolysaccharide capsule of Streptococcus pneumoniae is an important virulence factor, but the mechanisms that regulate capsule thickness are not fully understood
S. pneumoniae regulates the thickness of the polysaccharide capsule in response to its environment and this is key in colonization and invasion of epithelial host cells [23]
It has been previously hypothesized that capsular polysaccharide (CPS) production in S. pneumoniae is reduced when grown on fructose as compared with glucose as the carbon source but the mechanism was unclear [10]
Summary
The exopolysaccharide capsule of Streptococcus pneumoniae is an important virulence factor, but the mechanisms that regulate capsule thickness are not fully understood. 31P NMR of whole-cell extract from capsule-knockout strains (⌬cps) clearly revealed the accumulation or absence of capsule precursor metabolites when cells were grown on glucose or fructose, respectively. This finding suggests that fructose uptake mainly results in intracellular fructose 1-phosphate, which is not converted to CPS precursors. Our findings on the effects of carbon sources on CPS production in different S. pneumoniae serotypes may contribute to a better understand-. Recent studies have investigated the host glycan sugar-specific pathways in S. pneumoniae [3, 4] This has provided an accurate map of the biochemical pathways for the pneumococcal galactose, mannose, and GlcNAc catabolism.
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