Characterization of a plasmid isolated from Branhamella catarrhalis and detection of plasmid sequences within the genome of a B. catarrhalis strain.

We isolated a 12.2-kb plasmid from two clinical strains of Branhamella catarrhalis and evaluated its distribution among other B. catarrhalis strains by colony hybridization experiments using the whole plasmid as a probe. Homology was detected with the two plasmid-bearing strains and also with a third B. catarrhalis strain named E7, which is plasmidless. Southern transfer analysis of total digested E7 DNA using the purified plasmid as a probe revealed a single band of hybridization, different from those observed in plasmid-bearing strains, for each restriction enzyme used. The region of the plasmid hybridizing with DNA of strain E7 was located within a 4.5-kb SstI-EcoRV fragment. No homology was noted between our B. catarrhalis plasmid and about 20 other strains of different genera tested by colony hybridization.