Role of the bacterial cell wall in middle ear inflammation caused by Streptococcus pneumoniae

The pathogenesis of middle ear inflammation caused by Streptococcus pneumoniae was explored in the chinchilla model with different pneumococcal cell wall (CW) preparations, including isolated native CW, M1 muramidase CW (M1-CW) digest, amidase CW digest, and M1 peptidoglycan (M1-PG) digest. Inflammatory cell and lysozyme concentrations in middle ear fluid (MEF) were measured between 6 and 72 h after the middle ears were inoculated with one of the preparations or sterile saline. Middle ear histopathology was measured quantitatively at 72 h. Native CW, M1-CW digest, and amidase-CW digest caused significantly more inflammatory cell influx and lysozyme accumulation in MEF than saline did. M1-PG digest also caused more inflammatory cell influx and lysozyme accumulation in MEF than saline did but caused less inflammation than native CW or either CW digest. Epithelial metaplasia was significantly greater in ears inoculated with native CW than in ears inoculated with the CW or PG digest or with saline. Pneumococcal CW is, therefore, the principal factor that initiates middle ear inflammation in acute pneumococcal otitis media, and CW teichoication seems to be important in initiating this response.