Characterization of Plasmids

Antibiotic resistant strains of Haemophilus influenzae appeared in the early 1970s and studies on these strains led to the detection of large 40–50 kb conjugative plasmids (1 –3 ). Early work was hampered by the failure to easily detect extrachromosomal plasmid in clinical isolates. However, closed circular plasmid was readily detected in exconjugants following conjugal transfer of plasmid (4 ). Early, it was recognized that small, approx 5-kb, nonconjugative plasmid was associated with a proportion of strains exhibiting β-lactamasepositive ampicillin resistance (1 ,2 ). We concentrate here on characterization of large conjugative plasmids.