• To define the relative importance of the local and systemic immune mechanisms in otitis media, middle ear infections were induced in 47 chinchillas by unilateral intrabullar inoculation of Streptococcus pneumoniae type 6A. Bacterial challenges were repeated ipsilaterally or contralaterally several times during the five-month course of the study. The middle ear condition was assessed with the use of otomicroscopy, tympanometry, and direct inspection and culture through a bullar opening. Pneumococcal 6A antibodies were assayed periodically in the serum and middle ear effusions. The concentration levels of serum antibodies did not correlate with the course of otitis media, and antibiotics did not protect either the ipsilateral or contralateral ear from infection. There was a minimal number of bacteria that rendered the middle ear resistant to subsequent infection. This resistance was a local phenomenon and occurred only when the middle ear had previously been exposed to the same bacteria.
(Arch Otolaryngol 1981;107:532-539)