MOLECULAR CHARACTERIZATION OF RESISTANCE PLASMIDS IN EPIDEMIOLOGICALLY UNRELATED STRAINS OF MULTIRESISTANT HAEMOPHILUS-INFLUENZAE

Thirty-three epidemiologically unrelated strains of ampicillin-chloram phenicol-resistant isolates of Haemophilus influenzae (22 type b, 11 u nencapsulated), isolated over 10 years in Belgium, were compared with 53 ampicillin-resistant chloramphenicol-susceptible isolates (22 type b, 31 unencapsulated). All ampicillin-chloramphenicol-resistant and 76 % of ampicillin-resistant chloramphenicol-susceptible strains were res istant to tetracycline, kanamycin, or both. Resistance to these antibi otics was specified by a 37- to 44-MDa conjugative plasmid. The geneti c relatedness of these plasmids and of those in multiresistant strains from Spain was investigated. Plasmids specifying ampicillin-chloramph enicol-tetracycline-kanamycin resistance in Belgium or in Spain had hi ghly related restriction fragment patterns. By homoduplex analysis, th ey had similar molecular organization and contained a structure identi cal to Tn 10-TnCm, a transposon previously identified in chloramphenic ol-tetracycline-resistant H. influenzae. Plasmids coding for different resistance phenotypes had less resemblance by restriction endonucleas e analysis; however, study of heteroduplex molecules indicated they sh ared a high proportion of core sequences. These findings support the h ypothesis of independent transposition events resulting in resistance plasmids of close molecular organization.