Genetic requirement for pneumococcal ear infection

Ear infection or otitis media (OM) accounts for most bacterial respiratory infections in children in both developed and developing nations. Streptococcus pneumoniae, nontypeable Haemophilus influenzae, and Moraxella catarrhalis are the major OM pathogens. However, little is known about the genetic basis of bacterial OM largely due to practical difficulties in conducting research in ear infection models and genetically manipulating clinical isolates. Here, we report the first genome-scale in vivo screen for bacterial genes required for ear infection in a chinchilla model by signature tagged mutagenesis (STM), a high throughput mutant screen technique.

STM strains were constructed with a multi-drug resistant OM isolate ST556 (serotype 19F) and screened in a chinchilla OM model. Out of 5,280 mutants tested, 248 mutants were substantially underrepresented in the mutant pools recovered from the middle ear fluids of the infected chinchillas, indicating the impaired ability to survive and replicate in the middle ears due to genetic disruptions in the chromosome of strain ST556. Further DNA sequencing analysis mapped the mutations to 169 pneumococcal genes. Surprisingly, only 52 of these genes were required for pneumococcal nasopharyngeal colonization in a murine model. This infection site-specific gene requirement was verified by targeted mutagenesis in the selected genes.

These findings suggest that there are a subset of pneumococcal genes required for ear infection and that these may be distinct from those required for nasal colonization. Our data thus provide comprehensive gene targets for mechanistic understanding of pneumococcal ear infection. Finally, this study has also developed a model for future genome-scale search for virulence determinants in other pathogens associated with ear infections.