Plasmid-mediated ampicillin resistance in Haemophilus ducreyi

Three of 19 strains of Haemophilus ducreyi, isolated during a recent outbreak of chancroid, were found to produce beta-lactamase and to harbor a 6.0 x 10(6)-dalton plasmid. Escherichia coli transformed with this plasmid acquired beta-lactamase-mediated resistance to ampicillin. The guanine-plus-cytosine content of the plasmid was found to be 41 mol%. Restriction endonuclease digestion studies suggest that a relatively large portion of the Tn1 translocon is carried by this plasmid. Whereas this plasmid could not be transferred to H. influenzae by mating on membrane filters, a strain of H. ducreyi was able to receive and donate a 30 x 10(6)-dalton ampicillin resistance plasmid from H. influenzae. The ability of H. ducreyi to receive and donate conjugative plasmids may result in the appearance of multiply resistant strains.