Heterogeneity of Actinobacillus actinomycetemcomitans strains in various human infections and relationships between serotype, genotype, and antimicrobial susceptibility.

Actinobacillus actinomycetemcomitans, an oral pathogen, only occasionally causes nonoral infections. In this study 52 A. actinomycetemcomitans strains from 51 subjects with nonoral infections were serotyped and genotyped by arbitrarily primed PCR (AP-PCR) to determine whether a certain clone(s) is specifically associated with nonoral infections or particular in vitro antimicrobial susceptibility patterns. The promoter structure of leukotoxin genes was additionally investigated to find the deletion characteristic of highly leukotoxic A. actinomycetemcomitans strains. The nonoral A. actinomycetemcomitans strains included all five known serotypes and nonserotypeable strains, the most common serotypes being b (40%) and c (31%). AP-PCR distinguished 10 different genotypes. A. actinomycetemcomitans serotype b strains were more frequently found in blood samples of patients with bacteremia or endocarditis than in patients with focal infections. One AP-PCR genotype was significantly more frequently found among strains originating in focal infections than in blood samples. Resistance to benzylpenicillin was significantly more frequent among A. actinomycetemcomitans serotype b strains than among strains of other serotypes. No differences in the leukotoxin gene promoter region or benzylpenicillin resistance between nonoral and oral A. actinomycetemcomitans strains were observed. Nonoral A. actinomycetemcomitans strains showed great similarity to the oral strains, confirming that the oral cavity is the likely source of nonoral A. actinomycetemcomitans infections. The predominance of serotype b strains in endocarditis and bacteremia supports the hypothesis of a relationship between certain A. actinomycetemcomitans clones and some nonoral infections. The mechanisms behind the exceptionally high rate of occurrence of benzylpenicillin resistance among A. actinomycetemcomitans serotype b strains are to be elucidated in further studies.