Abstract
The structure of the exopolysaccharide capsule of Streptococcus pneumoniae is defined by the genetic arrangement of the capsule operon allowing the unequivocal identification of the pneumococcal serotype. Here, we investigated the environment-dependent composition of the polysaccharide structure of S. pneumoniae serotype 6F. When grown in a chemically defined medium (CDM) with glucose versus galactose, the exopolysaccharide capsule of the serotype 6F strains reveals a ratio of 1/0.6 or 1/0.3 for galactose/glucose in the capsule by 1H-NMR analyses, respectively. Increased production of the capsule precursor UDP-glucose has been identified by 31P-NMR in CDM with glucose. Flow cytometric experiments using monoclonal antibodies showed decreased labelling of Hyp6AG4 (specific for serotype 6A) antibodies when 6F is grown in glucose as compared to galactose, which mirrors the 1H-NMR results. Whole-genome sequencing analyses of serotype 6F isolates suggested that the isolates evolved during two different events from serotype 6A during the time when the 13-valent pneumococcal conjugate vaccine (PCV-13) was introduced. In conclusion, this study shows differences in the capsular structure of serotype 6F strains using glucose as compared to galactose as the carbon source. Therefore, 6F strains may show slightly different polysaccharide composition while colonizing the human nasopharynx (galactose rich) as compared to invasive locations such as the blood (glucose rich).
Highlights
S. pneumoniae asymptomatically colonizes the human nasopharynx, but it can invade different body niches and cause diverse diseases such as acute otitis media, bronchitis, sinusitis, pneumonia, sepsis and meningitis [1,2]
The 6F strains contain a bispecific glycosyltransferase within the capsule operon and we hypothesized that the capsule operon may not exclusively account for the capsular structure but that the polysaccharide capsule is dependent on the available carbon sources
It has been broadly accepted that the structure of the capsule of S. pneumoniae is largely independent of the environment and is exclusively determined by the genes of the capsule operon [23]
Summary
S. pneumoniae asymptomatically colonizes the human nasopharynx, but it can invade different body niches and cause diverse diseases such as acute otitis media, bronchitis, sinusitis, (bacteremic) pneumonia, sepsis and meningitis [1,2]. The introduction of pneumococcal conjugate vaccines (PCVs) has largely decreased the incidence of invasive pneumococcal disease (IPD) but may not be the ultimate solution as serotype redistribution has taken place [4,5]. The respiratory tract contains a lot of mucins, which are glycoproteins rich in N-acetylglucosamine (GlcNAc), N-acetylgalactosamine (GalNAc), galactose and N-acetylneuraminic acid (NeuNAc). These carbohydrates are degraded by the glycan-specific metabolic machinery of
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