Phenotypic variation in Actinobacillus actinomycetemcomitans during laboratory growth: implications for virulence

This study examined alteration of specific virulence traits associated with phenotypic changes seen when a low-passage disease-associated and well mai ntained parent strain of Actinobacillus actinomycetemcomitans was compared to a laboratory-grown spontaneous variant/mutant. Clinical isolates of A. a ctinomycetemcomitans recovered from periodontitis patients typically grow a s rough, adherent colonies on primary culture but undergo transformation to smooth, non-adherent colonies following repeated passage in vitro. The rel ationship of these phenotypic changes to the virulence of the organism or t o the processes that underlie this transformation are not understood. A fre sh clinical isolate, designated strain CU1000, was obtained from the first molar site of a patient with classical signs of localized juvenile periodon titis and used as the parent strain to study virulence-related understood. A fresh clinical isolate, designated strain CU1000, was obtained phenotypes . Following several passages of CU1000 on selective agar, a spontaneous var iant that demonstrated smooth, opaque, non-adherent colonies was isolated a nd designated strain CU1060. This study compared the properties of these tw o strains with respect to colony morphology, autoaggregation, surface appen dages, adherence to saliva-coated hydroxyapatite (SHA), LPS chemotype and a ctivity, induction of fibroblast proteinase activity and antigenic properti es. CU1000 demonstrated rough, raised, star-positive colonies which upon el ectron microscopic examination revealed the presence of large, flexible, bu ndled fibrils. In addition, CU1000 showed adherence to SHA, several unique protein antigens and elevated endotoxin and fibroblast proteinase activity. CU1060, on the other hand, showed minimal adherence to SHA and fewer react ive proteins compared to the fresh clinical isolates. This strain formed sm ooth, opaque colonies on agar, showed minimal fibril formation and limited endotoxin and fibroblast-proteinase-inducing activity, These findings demon strate that clinical isolates of A. actinomycetemcomitans undergo significa nt virulence-reducing phenotypic alterations during in vitro passage and su pport the need to study this organism in its clinical form.