Interrelationships between physical state, phenotypic stability and transferability of beta-lactamase genes in Haemophilus influenzae.

SUMMARY: We have evaluated 66 β-lactamase-producing (β-Lac+) clinical isolates of Haemophilus influenzae for the presence of plasmid DNA, ability to conjugate with an H. influenzae 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 106. Four strains carried a plasmid of mol.wt 3 x 106 and single strains carried plasmids of mol. wt 36 x 106 or 50 x 106. Of 23 plasmids that were unstable in vitro, 10 (43%) had molecular weights of 30 x 106 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 (H. influenzae type b), ampicillin-resistant transconjugants were isolated in 21 out of 43 matings (the lower limit of detection was 10-7 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 10-2—10-3) than heterologous (untypable x type b) matings (frequency 10-6-10-7). β-Lac+,‘plasmid-free’ strains were inefficient donors (frequency 10-6-10-7) irrespective of serotype, but all transconjugants from these crosses were efficient donors (frequency 10-1-10-4) and contained a plasmid of mol. wt 30 x 106 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 106 mol. wt R plasmids aids transferability at the expense of genetic stability.