Interrelationships Between Physical State, Phenotypic Stability and Transferability of -Lactamase Genes in Haemophilus influenzae

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Abstract
.beta.-Lactamase-producing (.beta.-Lac+) clinical isolates (66) of H. influenzae were evalauted for the presence of plasmid DNA, ability to conjugate with an H. influenzae type b recipient and stability of the .beta.-Lac+ phenotype. Of the 66 strains, 23 (35%) contained a plasmid demonstrable by either of 2 methods of cleared cell lysate preparation. The MW of 17 of the plasmids was .apprx. 30 .times. 106. Four strains carried a plasmid of MW 3 .times. 106 and single strains carried plasmids of MW 36 .times. 106 or 50 .times. 106. Of 23 plasmids that were unstable in vitro, 10 (43%) had MW of 30 .times. 106 and coded only for .beta.-lactamase. Stability of the .beta.-Lac+ phenotype was observed in all 43 strains in which plasmid DNA was not demonstrated by physical methods. When .beta.-Lac+, plasmid-free strains were conjugated with strain Eagan (H. influenza type b), ampicillin-resistant transconjugants were isolated in 21 of 43 matings (the lower limit of detection was 10-7/donor cell per 30 min mating). With few exceptions, homologous (type b .times. type b) crosses with .beta.-Lac+, plasmid-containing donors were more efficient (frequency 10-2-10-3) than heterologous (untypable .times. type b) matings (frequency 10-6-10-7). .beta.-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 MW 30 .times. 106 that was unstable in 9 cases out of 26 (35%). The increase in transferability and phenotypic instability of plasmid-bearing .beta.-Lac+ strains reinforced the concept of a chromosomal locus for .beta.-lactamase genes in plasmid-free .beta.-Lac- strains. Carriage of .beta.-lactamase genes on 30 .times. 106 MW R plasmids aids transferability at the expense of genetic stability.

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