High Levels of Genetic Recombination during Nasopharyngeal Carriage and Biofilm Formation in Streptococcus pneumoniae

Abstract

Transformation of genetic material between bacteria was first observed in the 1920s using Streptococcus pneumoniae as a model organism. Since then, the mechanism of competence induction and transformation has been well characterized, mainly using planktonic bacteria or septic infection models. However, epidemiological evidence suggests that genetic exchange occurs primarily during pneumococcal nasopharyngeal carriage, which we have recently shown is associated with biofilm growth, and is associated with cocolonization with multiple strains. However, no studies to date have comprehensively investigated genetic exchange during cocolonization in vitro and in vivo or the role of the nasopharyngeal environment in these processes. In this study, we show that genetic exchange during dual-strain carriage in vivo is extremely efficient (10−2) and approximately 10,000,000-fold higher than that measured during septic infection (10−9). This high transformation efficiency was associated with environmental conditions exclusive to the nasopharynx, including the lower temperature of the nasopharynx (32 to 34°C), limited nutrient availability, and interactions with epithelial cells, which were modeled in a novel biofilm model in vitro that showed similarly high transformation efficiencies. The nasopharyngeal environmental factors, combined, were critical for biofilm formation and induced constitutive upregulation of competence genes and downregulation of capsule that promoted transformation. In addition, we show that dual-strain carriage in vivo and biofilms formed in vitro can be transformed during colonization to increase their pneumococcal fitness and also, importantly, that bacteria with lower colonization ability can be protected by strains with higher colonization efficiency, a process unrelated to genetic exchange.

Minimal Peptidoglycan (PG) Turnover in Wild-Type and PG Hydrolase and Cell Division Mutants of Streptococcus pneumoniae D39 Growing Planktonically and in Host-Relevant Biofilms

“…Previous characterization of S. pneumoniae cells in biofilms on human epithelial cells indicated considerable release of DNA, likely by the induction of fratricide as part of increased competence of pneumococci in this host-relevant biofilm model (65)(66)(67). The extracellular DNA released is thought to act as a major matrix component in the biofilms (65,68).…”

Novel configurations of type I and II CRISPR–Cas systems in Corynebacterium diphtheriae

“…Alterations in virulence have often been associated with high frequencies of recombination in pathogens (Joseph et al, 2011;Suarez et al, 2004;Wirth et al, 2006) and this also appears to be playing an important role in the evolution of C. diphtheriae (Bolt et al, 2010;Mokrousov, 2009). It is now clear that the primary niche of C. diphtheriae in humans, the upper respiratory tract, is a hotbed of horizontal gene transfer between pathogens (Marks et al, 2012). However, frequencies of recombination can be variable between different strains of the same species (Sangal et al, 2010), which may reflect differences in strain propensities for acquiring foreign DNA that may result in variations in pathogenicity.…”

Neuraminidase A-Exposed Galactose Promotes Streptococcus pneumoniae Biofilm Formation during Colonization

“…Dislodged pneumococcal aggregates in mucus may also act as vehicles for transmission on fomites (20). Of note, nutrient availability within the nasopharynx is thought to be poor, with carbohydrates and other essential metabolites limited in quantity or sequestered by the host (21). Along such lines, in vitro biofilm formation has been reported to be optimal during growth in nutrient-poor conditions (22), suggesting there may be a link between these occurrences.…”