Phenotypic changes in nonfimbriated smooth strains of Aggregatibacter actinomycetemcomitans grown in low-humidity solid medium.

Aggregatibacter actinomycetemcomitans is the primary etiologic agent of localized aggressive periodontitis. In vitro, it can undergo fimbriated rough to nonfimbriated smooth phenotypic transition, accompanied by an increase in invasive ability and a decrease in adhesive ability. No opposite direction phenotypic transition was reported. To better understand its pathogenicity, the authors studied the morphological changes of nonfimbriated smooth strains induced by growth environmental humidity. Transmission electron microscopy was used to identify fimbriae expression change. It was found that the lower medium humidity, the more fimbriae reexpressed. In conclusion, the smooth strain of A. actinomycetemcomitans can reexpress the fimbriae in lower humidity environment.