Structural and genetic analyses of O polysaccharide from Actinobacillus actinomycetemcomitans serotype f

The oral bacterium Actinobacillus actinomycetemcomitans is implicated as a causative agent of localized juvenile periodontitis (LJP). A. actinomycetem comitans is classified into five serotypes (a to e) corresponding to five s tructurally and antigenically distinct O polysaccharide (O-PS) components o f their respective lipopolysaccharide molecules. Serotype b has been report ed to be the dominant serotype isolated from LJP patients. We determined th e lipopolysaccharide O-PS structure from A. actinomycetemcomitans CU1000, a strain isolated from a 13-year-old African-American female with LJP which had previously been classified as serotype b. The O-PS of strain CU1000 con sisted of a trisaccharide repeating unit composed Of L-rhamnose and 2-aceta mido-2-deoxy-D-galactose (molar ratio, 2:1) with the structure -->2)-alpha -L-Rhap-(1-3)-2-O-(beta -D-GalpNAc)-alpha -L-Rhap-(1-->. O-PS from strain C U1000 was structurally and antigenically distinct from the O-PS molecules o f the five known A. actinomycetemcomitans serotypes. Strain CU1000 was muta genized with transposon IS903 phi kan, and three mutants that were deficien t in O-PS synthesis were isolated. All three transposon insertions mapped t o a single 1-kb region on the chromosome. The DNA sequence of a 13.1-kb reg ion surrounding these transposon insertions contained a cluster of 14 open reading frames that was homologous to gene clusters responsible for the syn thesis of A. actinomycetemcomitans serotype b, c, and e O-PS antigens. The CU1000 gene cluster contained two genes that were not present in serotype-s pecific O-PS antigen clusters of the other five known A. actinomycetemcomit ans serotypes. These data indicate that strain CU1000 should be assigned to a new A. actinomycetemcomitans serotype, designated serotype f. A PCR assa y using serotype-specific PCR primers showed that 3 out of 20 LJP patients surveyed (15%) harbored A. actinomycetemcomitans strains carrying the serot ype f gene cluster. The finding of an A. actinomycetemcomitans serotype sho wing serological cross-reactivity with anti-serotype b-specific antiserum s uggests that a reevaluation of strains previously classified as serotype b may be warranted.