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Despite the clinical importance of anaerobic bacteria in otitis media and the uncertainty regarding the proper treatment of the anaerobic focal infection, few experimental studies focused upon the role of these microorganisms in otitis media have been publis­hed. In the present investigation a guinea-pig model for the induc­tion of anaerobic monoinfections in the middle ear was described. Bacteroides fragilis and Propionibacterium acnes (4.0-10x10 colony forming units) injected via the tympanic membrane were capable of inducing clinical and histological otitis media with persistent seguele in the middle ear cavity. Bacteroides asacc-harolyticus, Peptostreptococcus micros and Peptost repto- coccus anaerobius failed to induce otitis media.

B. fragilis otitis was accompanied by increased serum IgG and IgM antibody titres against the challenge organism, whereas P. acnes and P. anaerobius did not induce a humoral immune response. The results suggested true virulence of B. fragilis in guinea-pig middle ear monoinfections.

Metronidazole was found to accelerate the elimination of B.fragilis from the middle ear. However, even high doses of metro­nidazole were nojt fully effective perhaps reflecting an incomplete anaerobiosis at the site of infection in some instances. At pre­sent, nitroimidazoles in chronic otitis media must be regarded as a possible alternative reguiring further study, particularly with regard to the dosage.

In order to gain further knowledge of the interaction between poly­morphonuclear leukocytes and bacteria under anaerobic conditions an in vitro model was established. It was shown that P. acnes was readily phagocytosed with the aid of C3 activated either via the classical or alternative pathway and that killing of P. acnes was inefficient during anaerobiosis. The results suggest that P.acnes is maintained in the pus in chronic otitis media because it sur­vives phagocytosis.

Finally, the interaction between the most common pathogen in acute purulent otitis media, Streptococcus pneumoniae, and human poly­morphonuclear leukocytes under anaerobic conditions was studied. Since purulent maxillary sinus effusion (and probably also purulent middle ear effusion), invariably has a pO^ approaching zero, such studies are highly relevant with regard to the host defence in sinuitis and perhaps also in otitis media.

S. pneumoniae was killed by the phagocytes under anaerobic condi­tions although at a slower rate than in air. Degradation of pneumo­coccal teichoic acid, DNA and RNA took place after phagocytosis under aerobic as well as anaerobic conditions, whereas degradation of unsaturated cell membrane lipids took place only under aerobic conditions. Furthermore, the pneumococcal autolytic system did not participate in the killing or the degradation of the bacteria.