Moraxella catarrhalis-induced purulent otitis media in the rat middle ear. Structure, protection, and serum antibodies.

To study the effects of viable and heat-killed Moraxella catarrhalis bacteria on the middle ear mucosa and to evaluate the protection after whole-cell immunizations, Sprague-Dawley rats were challenged and rechallenged with four different M. catarrhalis strains. The animals were monitored by clinical observations, bacterial and histological samples from middle ears, and serum IgG levels. Only viable bacteria at a high concentration induced purulent otitis media, which was culture positive in 58% of the cases on day 4. The infection was characterized by a mild acute reaction lasting otomicroscopically about 8 days, together with quantitative and qualitative changes of the goblet cells. Structurally the mucosal effects of the heat-killed bacteria were less pronounced in the early phase compared to the viable bacteria, but similar at the end of the experiment at 6 months. The intrabullar and subcutaneous immunizations evoked an IgG antibody response in all animals, and the protection rate after immunization was 50% or more. The induced protection was not strain-specific. The study showed the rat to be a possible alternative for the study of different aspects of M. catarrhalis otitis media, an infection that is clinically and structurally different from that elicited by Streptococcus pneumoniae and Haemophilus influenzae in the rat.