Physical size of the donor locus and transmission of Haemophilus influenzae ampicillin resistance genes by deoxyribonucleic acid-mediated transformation.

The properties of donor deoxyribonucleic acid (DNA) from three clinical isolates and its ability to mediate the transformation of competent Rd strains to ampicillin resistance were examined. A quantitative technique for determining the resistance of individual Haemophilus influenzae cells to ampicillin was developed. When this technique was used, sensitive cells failed to tolerate levels of ampicillin greater than 0.1 to 0.2 mug/ml, whereas three resistant type b beta-lactamase-producing strains could form from the colonies in 1- to 3-mug/ml levels of the antibiotic. DNA extracted from the resistant strains elicited transformation of the auxotrophic genes in a multiply auxotrophic Rd strain. For two of the donors, transformation to ampicillin resistance occurred after the uptake of a single DNA molecule approximately 104-fold less frequently than transformation of auxotrophic loci and was not observed to occur at all with the third. The frequency of transformation to ampicillin resistance was two- to fivefold higher in strain BC200 (Okinaka and Barnhart, 1974), which was cured of a defective prophage. All three clinical ampicillin-resistant strains were poor recipients, but the presence of the ampicillin resistant genes in strain BC200 did not reduce its competence. Sucrose gradients of DNA from ampicillin-resistant transformants of BC200 and from the original ampicillin-resistant strains showed that: (i) all the DNA preparations had high molecular weights; (ii) donor activity for ampicillin resistance sedimented heterogeneously and in parallel with genome DNA up to the highest molecular weights observed (100 x 106 to 200 x 106); and (iii) genetic transformation of ampicillin resistance from strain BC200-amp90383 required the physical integrity of a linearly integrated segment of DNA having a molecular weight of about 25 x 106 to 30 x 106.