Pneumococcal otitis media: A polyfactorial disease in a complex microbial community

Introduction: Streptococcus pneumoniae asymptomatically colonizes the upper respiratory tract of children and is a major childhood pathogen worldwide. S. pneumoniae are genetically heterogeneous and strains differ in their propensity for colonization and disease. Although the heptavalent pneumococcal conjugate vaccine is effective in preventing invasive pneumococcal disease, it has not provided the same level of protection against pneumococcal otitis media (OM). Most studies of children's upper respiratory tract are culture-based and focus on culture of bacterial OM pathogens. However, in our bodies, bacterial cells outnumber human cells. Therefore, it is likely that other taxa not routinely studied influence OM susceptibility. Methods: In the first project (Chapter 2), clinical isolates of the same genetic background (sequence type (ST199)) but different serotypes (15B/C, 19A) were compared in vitro and for pathogenic potential in a chinchilla model of middle ear disease. A qPCR assay was developed to quantitatively assess each isolate, circumventing the need for selectable markers. The second and third studies (Chapters 3 and 4) used nasal swabs and clinical and demographic data collected in a cross sectional study of Philadelphia children during two winter respiratory virus seasons. Advanced pyrosequencing technology was used to characterize the microbial communities residing in children's anterior nares. URI is a major risk factor for OM. Chapter 3 characterized children experiencing URI with and without concurrent OM. Results were used to direct analyses in Chapter 4, which included healthy children as well as children experiencing URI with and without concurrent acute OM. Swabs were cultured for S. pneumoniae . Microbial communities were described using Shannon diversity and Evenness indices. Principal component analysis (PCA), which identifies a small number of variables that account for the majority of variance within a population, was used in a novel application to highlight influential microbial community taxa and group correlated taxa into factors. In Chapter 4, a qPCR assay was also included to identify the presence of OM pathogens: S. pneumoniae, H. influenzae, and M. catorrhalis. Results/Discussion: In Chapter 2 serotype 19A grew faster in vitro, however both 19A and 15B/C were equally capable of colonization and middle ear infection in the chinchilla model. Serotype 19A is included in the new conjugate vaccines while 15B/C is not. Serotype 15B/C should be considered for future capsule vaccine development. Interestingly, Chapters 3 and 4 demonstrated that, regardless of health status, S. pneumoniae is associated with decreased levels of diversity. In Chapter 3, PCA identified taxa that may be important in OM susceptibility and/or OM pathogen colonization. These results were applied to the Chapter 4 analysis with equivalent results. Potentially protective taxa included: Corynebacterium, Dolosigranulum, and Lactococcus. Taxa that may contribute to the causal pathway of OM include Actinomyces, Neisseria, Rothia, and Veillonella . An increased understanding of upper respiratory tract microbial communities may contribute to the development of prevention strategies, such as multispecies probiotics.