Induction of competence in nonencapsulated and encapsulated strains ofHaemophilus influenzae

A simple procedure for induction of competence in nonencapsulated and encapsulated strains ofHaemophilus influenzae is described, which consists of growing cells without shaking in brain-heart infusion broth under aerobic conditions. Competence emerged at the end of the exponential phase and reached a peak at the stationary phase. InH. influenzae Rd competence was maintained for at least 6 h at 37°C, whereas in two encapsulated clinical isolates ofH. influenzae type b a decrease in competence was observed after 4 h. Competence was maintained for 24 h at 22°C and 4°C as well as by freezing the cells in 15% glycerol and storing them at –70°C. Transformation frequencies of three chromosomal markers—streptomycin, nalidixic acid, and erythromycin resistance—were 0.5% to 1% inH. influenzae Rd and about tenfold lower in the two encapsulated clinical isolates ofH. influenzae type b. The advantage of this procedure is that it is simpler than the previously described procedures and yields stable, highly transformable cells. Unlike the standard M IV method, the static aerobic procedure does not interfere with the capsule synthesis and can be used for testing transforming activity of encapsulated virulent isolates ofH. influenzae.