Alteration of the cell wall of Haemophilus influenzae type b by transformation with cloned DNA: association with attenuated virulence.

A virulent strain of Haemophilus influenzae type b was used to construct a lambda library of chromosomal DNA in Charon 4, amplified in Escherichia coli. From this library a recombinant (I-69) phage was isolated that contained a 10.2-kilobase-pair fragment of DNA eliciting H. influenzae transformants whose colonies had a distinctive opaque phenotype. Compared with their H. influenzae parent strains the opaque I-69 transformants had two defined cell wall alterations: one in the lipopolysaccharide (greater mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis) and one in the outer membrane proteins. The I-69 transformant of virulent type b strain Rd-/b+ had stable expression of type b capsule. In contrast to strain Rd-/b+, the Rd-/b+/I-69 transformant was serum sensitive in vitro and avirulent in vivo in rats. Thus the potential of H. influenzae type b organisms to cause invasive infection can be substantially attenuated by altering the expression of one or more genes that affect the cell wall composition.