MOBILIZATION OF TYPE b CAPSULE GENE(S) IN HAEMOPHILUS INFLUENZAE b (Hib) BY CELL TO CELL CONTACT

Abstract

Little is known about the genetics of the type b capsule (cap), the major virulence factor of Hib. By contrast, transfer of antibiotic resistance is well known to be mediated primarily by R-plasmids, though chromosomal gene transfer also occurs under conditions facilitating conjugation. During experiments to characterize plasmid-mediated Amp^R we mated 29 Amp^R Sm^S Alaskan Hib with an unencapsulated Amp^S Sm^R recipient. We noted large smooth cfus among the Amp^R transconjugants (TCs) in 25 of 29 matings. Testing these smooth strains for b cap. (antiserum agar and agglutination with anti b antiserum) revealed that all were type b. The background mutation freq. of the 25 donor strains to Sm^R was ≤10^-9 and the mutation freq. of the unencapsulated recipient to produce type b cap. was undetectable (<10^-9). These data suggest that cap. expression was the result of gene transfer to the recipient and not a mutational event. To determine if this phenomenon occurs in other R-plasmid bearing HI, we mated an Amp, Cm and Te resistant Hib isolate with 2 different unencapsulated recipients. The type b cap. transferred with the antibiotic resistance determinants in a subset of TCs. Conjugal cap. transfer from this strain was not detected in the presence of DNAase, suggesting gene transfer occurred by transformation. We conclude that the type b cap. genes of these strains transfer concurrently with the plasmid as a common event in vitro. Whether the cap. b gene can be carried on an R plasmid or whether it is transferred by plasmid-facilitated chromosomal exchange remains to be determined.

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