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Acute otitis media (AOM) is a common disease in childhood and is one of the most common causes for outpatient antibiotic treatment. The major aetiological agents of AOM have varied over the decades. Now the three most common pathogens are Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis. The resistance patterns of these organisms have also varied from the beginning of the antibiotic era to the situation we have today with an increasing incidence of penicillin-resistant S. pneumoniae and a moderate to high frequency of beta-lactamase production in H. influenzae and M. catarrhalis. In Sweden we have continued to use the Scandinavian treatment policy of penicillins as the first-line antibiotic treatment of AOM, which has been implemented with good results in the past. The question is if this policy will continue to have acceptable treatment results.

In order to investigate aspects of treatment, protection and structural changes in AOM, an animal model was used.

Amoxicillin treatment of AOM caused by H. influenzae was studied. Amoxicillin treatment was shown to shorten the duration of the infection and to reduce the morphological changes normally observed after an untreated AOM. The influence of antibiotic treatment on recurrent AOM was evaluated. Amoxicillin treatment did not lead to less protection against reinfection. Abstaining from antibiotics did not improve the levels of serum IgG antibodies. The IgG levels were significantly higher in treated animals after rechallenge. AOM caused by H. influenzae with a non-beta-lactamase-mediated resistance to beta-lactams was investigated and it was observed that during amoxicillin treatment the chromosomal changes mediating resistance were possibly advantageous for the bacterium. In cultures from children with AOM, there is sometimes growth of several bacteria. The possibility of a sheltering effect of beta-lactamase-producing H. influenzae on a penicillin-sensitive S. pneumoniae in a mixed infection was investigated, and amoxicillin was shown to eradicate the pneumococci from the middle ear despite the presence of beta-lactamase. An increasingly cultured bacterium in nasopharynx and in AOM is M. catarrhalis. It is now beta-lactamase-producing in almost 100% of cases and is thus not eradicated by penicillins. An animal model of AOM caused by beta-lactamase-producing M. catarrhalis was established to study the course of this infection with the possibility of evaluating aspects of virulence between AOM pathogens. The AOM observed was a self-limiting disease.

The results obtained in this study in a rat model support the continuing use of penicillins as first-line drugs in the treatment of AOM. Penicillins are not sufficient to treat all causative agents, but the majority of pathogens including the most virulent bacteria are eradicated from the middle ear.