SUMMARY: We have evaluated 66 β-lactamase-producing (β-Lac) clinical isolates of for the presence of plasmid DNA, ability to conjugate with an type b recipient, and stability of the β-Lac phenotype. Of the 66 strains, 23 (35%) contained a plasmid demonstrable by either of two methods of cleared cell lysate preparation. The molecular weight of 17 of the plasmids was approximately 30 x 10. Four strains carried a plasmid of mol.wt 3 x 10 and single strains carried plasmids of mol. wt 36 x 10 or 50 x 10. Of 23 plasmids that were unstable , 10 (43%) had molecular weights of 30 x 10 and coded only for β-lactamase. Stability of the β-Lac phenotype was observed in all 43 strains in which plasmid DNA was not demonstrated by physical methods. When β-Lac,‘plasmid-free’ strains were conjugated with strain Eagan ( type b), ampicillin-resistant transconjugants were isolated in 21 out of 43 matings (the lower limit of detection was 10 per donor cell per 30 min mating). With few exceptions, homologous (type b x type b) crosses with β-Lac, plasmid-containing donors were more efficient (frequency 10—10) than heterologous (untypable x type b) matings (frequency 10-10). β-Lac,‘plasmid-free’ strains were inefficient donors (frequency 10-10) irrespective of serotype, but all transconjugants from these crosses were efficient donors (frequency 10-10) and contained a plasmid of mol. wt 30 x 10 that was unstable in 9 cases out of 26 (35%). The increase in transferability and phenotypic instability of plasmid-bearing β-Lac strains reinforces the concept of a chromosomal locus for β-lactamase genes in‘plasmid-free’ β-Lac strains. Carriage of β-lactamase genes on 30 x 10 mol. wt R plasmids aids transferability at the expense of genetic stability.


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