Genetics of spontaneous, high-frequency loss of b capsule expression in Haemophilus influenzae.

We determined that the frequency of spontaneous capsule loss in Haemophilus influenzae type b is 0.1 to 0.3%. All of 10 independent capsule-deficient variants (derived from four different type b strains) were found to be missing an identical 9-kilobase EcoRI restriction fragment when probed with a cloned piece of DNA containing sequences known to be necessary for type b capsule expression. These results suggest the existence of a specific mechanism for shutting off type b capsule synthesis at a high frequency. Intranasal infection of infant rats showed that capsule loss occurred in vivo at frequencies comparable to those observed in vitro.