In vitro susceptibility to tetracycline and doxycycline in clinical isolates of Haemophilus influenzae.

The most common indication for the use of tetracyclines in Sweden is respiratory tract infections. Among the tetracyclines, doxycycline dominates with about 75% of the consumption. It is therefore used for routine susceptibility testing, while tetracycline is the test drug in most other countries. Six hundred strains of Haemophilus influenzae isolated from different parts of Sweden were tested for susceptibility to doxycycline and tetracycline. The results were compared with those from earlier reports on resistance rates in Sweden and other countries. The MIC50's of the strains were slightly lower for tetracycline than for doxycycline, but some strains with reduced susceptibility to tetracycline were fully susceptible to doxycycline. The level of resistance to doxycycline in H. influenzae was very low (less than 1%) and has not changed significantly over the past ten years, making doxycycline a suitable antibiotic for instance in the treatment of chronic bronchitis when H. influenzae is involved.