Molecular epidemiology of antibiotic resistant Haemophilus influenzae isolated in Denmark 1981‐87

The molecular basis of antibiotic resistance was studied in 32 epidemiologically unrelated Danish clinical isolates of Haemophilus influenzae. Both non-encapsulated and capsulated type b strains were represented as well as four different biotypes. Plasmid DNA was found in 11 strains, all of which were antibiotic resistant. Antibiotic resistance was transferred to an Rd Haemophilus influenzae recipient from 5 of 6 prospective donors. Ampicillin and chloramphenicol resistance were linked markers while tetracycline resistance--when unselected--was lost in 18% of the transconjugants. Loss of Tcr was associated with loss of a plasmid DNA segment. Restriction enzyme profiles of plasmid DNA lead to the conclusion that the drug resistance plasmids are derivatives of a common, not too distant, ancestor. There is evidence of both clonal spread and horizontal transmission of related drug resistance plasmids in H. influenzae in Denmark.