Plasmid-mediated antimicrobial resistance in Haemophilus pleuropneumoniae.

Haemophilus pleuropneumoniae, isolated from the lungs of pig, was resistant to ampicillin, streptomycin, and sulfadiazine. This isolate, SD-1, possessed 1 plasmid (pVM105) coded for resistance to ampicillin and sulfadiazine, and the other (pVM104) for resistance to streptomycin and sulfadiazine. A 2nd isolate of H pleuropneumoniae, SD-2, recovered from a pig in a different outbreak of porcine pleuropneumonia, was resistant to streptomycin and sulfadiazine. These resistance determinants were on 1 plasmid (pVM106). All 3 patients were nontransmissible and small (pVM105 = 3.6 x 10(6) daltons, pVM104 and pVM106 = 2.3 x 10(6) daltons. The basis for resistance to ampicillin, encoded on plasmid pVM105, was due to a beta-lactamase. This beta-lactamase was highly active on penicillin G and ampicillin, moderately active on cephalothin, and inactive on oxacillin, indicating a TEM type of beta-lactamase.