Tetracycline resistance in Haemophilus influenzae is plasmid-linked.

A strain of Haemophilus influenzae resistant to tetracycline (MIC 16-32 mug/ml) was "cured" of tetracycline resistance by ethidium bromide. DNA centrifugation in cesium chloride + ethidium bromide density gradients showed the existence of a band of non-chromosomal DNA molecules in the original resistant strain. This band was not found in lysates of a strain "cured" of tetracycline resistance by ethidium bromide. These facts support the hypothesis that in this strain of Haemohpilus influenzae, tetracycline resistance is plasmid mediated.