Genes involved in Haemophilus influenzae type b capsule expression are frequently amplified

The genes involved in Haemophilus influenzae type b capsule expression are present as a duplication of an--18-kb DNA segment (the Cap b locus). It has been shown previously that recombination occurs between the two copies of the repeat, resulting in deletion of one copy and loss of capsule expression at frequencies of 0.1%-0.5%. The present study tested the hypothesis that the duplicated arrangement could serve as a template for further amplification of capsule gene sequences. Southern hybridization analysis of 66 type b invasive isolates showed that ampli-fications exist and are moderately common (23/66 were amplified). In addition to three copies of the 18-kb repeat, four copies were detected in some strains, and up to five copies in 1 isolate. By ELISA, a five-copy strain made about six times more capsular polysaccharide than did an iso-genic two-copy derivative. The evolutionary significance of the duplicated arrangement may be its ability to rapidly amplify under conditions where it is advantageous to produce more capsule. Haemophilus influenzae type b is a common cause of men-ingitis and other serious infections in young children. The type b capsular polysaccharide, a polymer of ribose ribitol phosphate (PRP), is considered to be an important virulenc