Lack of serotype antigen in A. actinomycetemcomitans

Aggregatibacter actinomycetemcomitans is divided into 6 serotypes. occurrence of non-serotypeable strains is known,but background reasons are unclear. we hypothesized that non-serotypeable strains represent new serotypes or have altered expression of serotype-specific polysaccharide antigen (s-pa). we first characterized 311 strains from 189 individuals using both immunoassay- and pcr-based serotyping. next,using natural human infection and rabbit immunization approaches,we clarified whether the phenotypically non-serotypeable strains expressed s-pa. immunoassay identified serotypes a-f among 216 strains from 159 individuals. the remaining 95 strains from 30 individuals were phenotypically non-serotypeable. yet,all these strains were identified by pcr-typing as serotype a-,b-,c-,or f. non-serotypeability was confirmed by western immunoblot with respective rabbit antisera. patient sera remained non-reactive with autologous non-serotypeable strains at the serotype-specific region. rabbit immunization with a phenotypically non-serotypeable strain induced no antibody production against s-pa. thus,phenotypically non-serotypeable strains did not include novel serotypes,but lacked s-pa expression.