Restriction Enzyme Analysis of Plasmids from Haemophilus influenzae

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Abstract
Examination of Haemophilus influenzae strains isolated in New Orleans revealed ampicillin-resistant strains with plasmids of size classes not previously detected in North America. The molecular weight of plasmids in five ampicillin-resistant strains ranged from 0.8 × 106 daltons (0.8 Mdal) to 36 Mdal. The molecular weights of the plasmids were determined by sucrose gradient centrifugation, electron microscopy, and agarose gel electrophoresis. Plasmids of the previously detected 30-Mdal size class were found in three of the five ampicillin-resistant strains. Restriction enzyme analysis is consistent with a close relationship between these three 30-Mdal plasmids. Of the two remaining ampicillin-resistant strains, one contained a single plasmid of 36 Mdal and the other contained two plasmids of 0.8 and 2.3 Mdal.