ABSTRACT
Chancroid caused by Haemophilus ducreyi has been described as a significantly predisposing factor of HIV heterosexual transmission in an endemic region of both diseases. The fastidious, H. ducreyi has been reported world wide with various antimicrobial susceptibility patterns. A high tendency of drug resistances has generally been found among isolates derived in Thailand. In this study, the plasmids of H. ducreyi were isolated and analysed from 63 clinically derived organisms. Twenty-nine out of 63 isolates (46%) revealed the same plasmid profiles. Plasmid DNA was further cloned into Escherichia coli and transformants were selected. A 3.6 kb plasmid (pCb) carrying ampicillin resistance was subsequently identified. The pCb conferred resistance to various beta-lactam antibiotics including penicillin G, carbenicillin, piperacillin, cefazolin, cefoperazone, ampicillin-sulbactam, and amoxicillin-clavulanate but not to cefoxitin. Co-resistance to streptomycin, chloramphenicol and tetracycline was not detected. Beta-lactamase gene was located on the major pCb fragment of EcoRI and AatII cutting.