A soluble host signal drives rapid, brain-predominant capsular thickening in <i>Streptococcus pneumoniae</i> via a putative sodium-dependent transporter (SPD_0642) and capsular prepromoter sequence

The Streptococcus pneumoniae capsule is a major determinant of virulence, yet whether bacteria actively remodel it during infection remains unclear. Studying Swiss and South African clinical isolates (serotypes 1, 6B, 8, 12F, 19F, and 35B), we identified a rapid, tissue-specific response: capsule thickness increased within hours upon co-exposure to host cells and tissues. Only two 12F strains failed to thicken. Thickening was greatest in brain tissue, moderate in serum, and absent on the epithelium. This adaptation occurred independently of cod locus phase variation and nutritional factors, and was instead driven by a soluble, thermostable host signal (&lt;3 kDa). Thickening correlated with neuroinflammation but did not require it, as it also occurred in contact with resting brain immune cells. It exacerbated meningitis in mice and enhanced bacteremia. Once induced, capsule thickening dampened inflammatory responses, coinciding with downregulation of pneumolysin, a major pro-inflammatory toxin. Genetic analysis of the non-thickening 12F isolates, together with targeted mutagenesis, identified two independent determinants of capsule-thickness modulation: a specific promoter-proximal element and SPD_0642, a conserved putative transporter encoded outside the capsule operon. Both contributed to the host-induced thickening phenotype. Pneumococci therefore rapidly remodel their surface in response to tissue-specific cues within the host, in a manner distinct from stochastic phase variation outside it.