Fimbriation of Haemophilus species isolated from the respiratory tract of adults.

Twenty-two clinical isolates of Haemophilus species were studied within two passages of their original isolation for the presence of fimbriae by negative-staining electron microscopy. Six isolates were identified as fimbriated, including three strains of nontypable Haemophilus influenzae, one strain of Haemophilus parainfluenzae, and two strains of Haemophilus haemolyticus. In fresh isolates of fimbriated strains of nontypable H. influenzae, approximately 40%-50% of cells had fimbriae; after five passages in vitro, less than 1% of the cells had fimbriae. Thus, a variety of Haemophilus species that colonize the human respiratory tract can be fimbriated, and this fimbriation is rapidly lost on passage in vitro.