Neutrophil infiltration in co-housed littermates plays a key role in nasal transmission of Streptococcus pneumoniae in an infant mouse model.

Transmission plays an important role in establishing pneumococcal colonization. It comprises three key events: shedding to transmit, entering into a susceptible new host, and adhering to the mucosal surface. Shedding of pneumococci from the respiratory tract of a colonized host is a pivotal step in transmission. Using a co-housed littermate mouse model, we evaluated the importance of the susceptibility to colonization of Streptococcus pneumoniae TIGR4 strain shed from index pups to non-colonized naive contact pups. Despite sufficient pneumococcal shedding from the colonized host, S. pneumoniae was not contagious between littermates. Neutrophils infiltrated the nasal mucosa of contact pups and contributed to susceptibility of pneumococcal colonization during the course of transmission. Rejection of pneumococcal colonization in the contact pups was associated with accumulation of neutrophils in the nasal mucosa. Inflammation, characterized by neutrophil infiltration, prevents newly entering pneumococci from adhering to the respiratory epithelium in contact mice, suggesting that it plays an important role in reducing the rate of transmission in the initial response of naive susceptible hosts to pneumococcal acquisition. The initial response of contact mice may regulate neutrophil and/or macrophage infiltration and control the acquisition of existing pneumococci.