Impaired capsular polysaccharide is relevant to enhanced biofilm formation and lower virulence in Streptococcus pneumoniae

Streptococcus pneumoniae has been reported to form biofilms. Many different surface molecules, including capsular polysaccharide (CPS), may play a fundamental role in pneumococcal biofilm development. We designed a CPS mutant, TIGR4cps4D⁻, from the TIGR4 strain and detected enhanced biofilm formation. The pathogenic diversities of the mutant were also investigated with the in vitro expression levels of pavA, lytA, IgA1, piaA, psaA, ply, and spxB. The mean OD₅₉₅ of TIGR4cps4D⁻ biofilm was 1.77 and 1.74, whereas that of TIGR4 was 0.76 and 0.33 on day 1 and day 2, respectively. Scanning electron microscopy and confocal laser scanning microscopy showed TIGR4cps4D⁻ formed a biofilm that was significantly thicker than that formed by TIGR4 (~12.22 vs. ~6.29 μm). Compared to TIGR4, the gene expression of lytA, IgA1, and, psaA in TIGR4cps4D⁻ was 1.9 × 10⁻⁵-, 2.4 × 10⁻⁵-, and 3.2 × 10⁻³ fold lower under the planktonic condition, and 1.9 × 10⁻⁵- and 9.7 × 10⁻⁵ fold lower in biofilms, respectively. Furthermore, TIGR4cps4D⁻ seemed to induce less cell death, compared to the results of TIGR4 (21.38 vs. 33.47 %, after a 5-h exposure; P < 0.05). Our data indicate that impaired pneumococcal CPS may increase biofilm formation and be involved in inhibition of virulence, possibly by influencing the gene expression.