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

Actinobacillus actinomycetemcomitans, an oral pathogen, only occasionally c auses nonoral infections. In this study 52 A. actinomycetemcomitans strains from 51 subjects with nonoral infections were serotyped and genotyped by a rbitrarily primed PCR (AP-PCR) to determine whether a certain clone(s) is s pecifically associated with nonoral infections or particular in vitro antim icrobial susceptibility patterns. The promoter structure of leukotoxin gene s was additionally investigated to find the deletion characteristic of high ly leukotoxic A. actinomycetemcomitans strains. The nonoral A. actinomycete mcomitans strains included all five known serotypes and nonserotypeable str ains, the most common serotypes being b (40%) and c (31%). AP-PCR distingui shed 10 different genotypes. A. actinomycetemcomitans serotype b strains we re more frequently found in blood samples of patients with bacteremia or en docarditis than in patients with focal infections. One AP-PCR genotype,vas significantly more frequently found among strains originating in focal infe ctions than in blood samples. Resistance to benzylpenicillin was significan tly more frequent among A. actinomycetemcomitans serotype b strains than am ong strains of other serotypes. No differences in the leukotoxin gene promo ter region or benzylpenicillin resistance between nonoral and oral A. actin omycetemcomitans strains were observed. Nonoral A. actinomycetemcomitans st rains showed great similarity to the oral strains, confirming that the oral cavity is the likely source of nonoral. A. actinomycetemcomitans infection s. The predominance of serotype b strains in endocarditis and bacteremia su pports the hypothesis of a relationship between certain A. actinomycetemcom itans clones and some nonoral infections. The mechanisms behind the excepti onally high rate of occurrence of benzylpenicillin resistance among A. acti nomycetemcomitans serotype b strains are to be elucidated in further studie s.