Capsule Enhances Pneumococcal Colonization by Limiting Mucus-Mediated Clearance

Abstract

Expression of a polysaccharide capsule is required for the full pathogenicity of many mucosal pathogens such as Streptococcus pneumoniae. Although capsule allows for evasion of opsonization and subsequent phagocytosis during invasive infection, its role during mucosal colonization, the organism's commensal state, remains unknown. Using a mouse model, we demonstrate that unencapsulated mutants remain capable of nasal colonization but at a reduced density and duration compared to those of their encapsulated parent strains. This deficit in colonization was not due to increased susceptibility to opsonophagocytic clearance involving complement, antibody, or the influx of Ly-6G-positive cells, including neutrophils seen during carriage. Rather, unencapsulated mutants remain agglutinated within lumenal mucus and, thus, are less likely to transit to the epithelial surface where stable colonization occurs. Studies of in vitro binding to immobilized human airway mucus confirmed the inhibitory effect of encapsulation. Likewise, pneumococcal variants expressing larger amounts of negatively charged capsule per cell were less likely to adhere to surfaces coated with human mucus and more likely to evade initial clearance in vivo. Removal of negatively charged sialic acid residues by pretreatment of mucus with neuraminidase diminished the antiadhesive effect of encapsulation. This suggests that the inhibitory effect of encapsulation on mucus binding may be mediated by electrostatic repulsion and offers an explanation for the predominance of anionic polysaccharides among the diverse array of unique capsule types. In conclusion, our findings demonstrate that capsule confers an advantage to mucosal pathogens distinct from its role in inhibition of opsonophagocytosis-escape from entrapment in lumenal mucus.

The Capsular Serotype ofStreptococcus pneumoniaeIs More Important than the Genetic Background for Resistance to Complement

“…Unencapsulated mutants colonized nasal spaces at a density of 10-to 100-fold less than the encapsulated parent strains (33). Increased encapsulation ensures that a portion of the bacterial population overcomes the initial clearance mechanism and progresses to the epithelial surface where stable colonization occurs (33).…”

“…At the early stages of colonization the capsule inhibits trapping of the pathogen to the luminal mucus and clearance by mucociliary flow (33). Unencapsulated mutants colonized nasal spaces at a density of 10-to 100-fold less than the encapsulated parent strains (33).…”

Role of p38 MAP Kinase and Transforming Growth Factor-β Signaling in Transepithelial Migration of Invasive Bacterial Pathogens

“…Bacterial capsules are known to interfere with adherence (29), and the delayed migration of encapsulated bacteria may be due to reduced bacterial-host cell interaction. Because adherence of S. pneumoniae and H. influenzae to epithelial tissue is one of the first steps in colonization and infection of the host, tight regulation of expression of the capsule might be important to colonize and disseminate in the host (1,30). Another prominent virulence determinant expressed by S. pneumoniae is the pore-forming toxin pneumolysin.…”

Pneumococcal Adhesins PavB and PspC Are Important for the Interplay with Human Thrombospondin-1

“…The dissemination of pneumococci occurs via aerosols from person to person, as humans represent the main biological reservoir (5,6). A prerequisite for a stable colonization is the capability to adhere to respiratory epithelial cells either directly or indirectly by targeting the extracellular matrix and the resistance against phagocytosis, antimicrobial substances, and mucus-mediated clearance (7). Here, the polysaccharide capsule of pneumococci represents the major barrier against the innate immunity (8).…”