Capsule Type and Amount Affect Shedding and Transmission of Streptococcus pneumoniae

Abstract

The capsular polysaccharide (CPS) of Streptococcus pneumoniae is characterized by its diversity, as it has over 95 known serotypes, and the variation in its thickness as it surrounds an organism. While within-host effects of CPS have been studied in detail, there is no information about its contribution to host-to-host transmission. In this study, we used an infant mouse model of intralitter transmission, together with isogenic capsule switch and cps promoter switch constructs, to explore the effects of CPS type and amount. The determining factor in the transmission rate in this model is the number of pneumococci shed in nasal secretions by colonized hosts. Two of seven capsule switch constructs showed reduced shedding. These constructs were unimpaired in colonization and expressed capsules similar in size to those of the wild-type strain. A cps promoter switch mutant expressing ~50% of wild-type amounts of CPS also displayed reduced shedding without a defect in colonization. Since shedding from the mucosal surface may require escape from mucus entrapment, a mucin-binding assay was used to compare capsule switch and cps promoter switch mutants. The CPS type or amount constructs that shed poorly were bound more robustly by immobilized mucin. These capsule switch and cps promoter switch constructs with increased mucin-binding affinity and reduced shedding also had lower rates of pup-to-pup transmission. Our results demonstrate that CPS type and amount affect transmission dynamics and may contribute to the marked differences in prevalence among pneumococcal types.

Streptococcus pneumoniae: transmission, colonization and invasion

“…As epidemiological studies show that the prevalence of different serotypes is highly variable, the role of capsule type and amount on shedding and transmission was tested using isogenic serotype-switch and cps -promoter switch mutants 19 . Some serotype-switch mutants colonized at wild-type levels but were shed and transmitted poorly in infant mice.…”

Next-Generation Whole-Cell Pneumococcal Vaccine

“…Serotypes differ in both chemical composition and CPS amount. These characteristics are related to the virulence degree, invasiveness, and the survival ability of the bacteria in the circulation of different pneumococcal strains [21,22].…”

Experimental Evolution in vivo to Identify Selective Pressures During Pneumococcal Colonization

“…This is one of the major selective pressures on the pneumococcus, as invasive disease is typically envisioned as an evolutionary dead-end due to clearance by the immune system or antibiotics or via the death of the host, thereby preventing further transmission of the strain. As such, the pneumococcus has evolved numerous strategies to facilitate efficient transmission and colonization of new hosts [8][9][10][11][12][13] . Mutational screens have identified many pneumococcal genes that are required for successful colonization of the nasal passages as well as for invasive infection of the lungs and the bloodstream 14,15 .…”

“…This has been demonstrated both at a population level and in murine models, where strains 6 encoding more negatively charged capsules, resulting in more negative surface charge potentials, have a competitive advantage during colonization, especially during competition with other pneumococci within the nasal passages 26 . Capsule switching can incur both fitness advantages and disadvantages during colonization and transmission 9,27 . Capsule biosynthesis is linked to a number of additional cellular processes, including metabolism 28,29 .…”