Experimental otitis media with Moraxella (Branhamella) catarrhalis

Two hundred fifty gerbils and 7 chinchillas were utilized in 11 experiments to determine the effect of inoculating viable and heat-killed suspensions of Moraxella (Branhamella) catarrhalis into the middle ear cavity. Development of otitis media was observed by otoscopy and histopathology. Gerbils were found to be susceptible to 2 x 10(4) viable M catarrhalis cells. Depending on the number of cells inoculated, the resulting untreated inflammation was a very mild to moderately severe, self-limiting disease with no permanent sequelae except in animals inoculated with high (10(6) to 10(7)) numbers of bacteria. Viable bacteria could not be isolated from the middle ears 24 hours after inflammation was induced. Heat-killed cells produced less severe acute inflammation with no permanent sequelae. We conclude 1) gerbils and chinchillas are susceptible to a self-limited inflammation caused by M catarrhalis, 2) no infection occurs, since viable bacteria cannot be recovered from middle ear aspirates, and 3) viable cells produce more severe inflammation than heat-killed cells.