Genetic structure of population of Bacillus cereus and B-thuringiensis isolates associated with periodontitis and other human infections

The genetic diversity and relationships among 35 Bacillus cereus and Bacill us thuringiensis isolates recovered from marginal and apical periodontitis in humans and from various other human infections were investigated using m ultilocus enzyme electrophoresis. The strains were isolated in Norway, exce pt for three strains isolated from periodontitis patients in Brazil, The ge netic diversity of these strains was compared to that of 30 isolates from d airies in Norway and Finland. Allelic variation in 13 structural gene loci encoding metabolic enzymes was analyzed. Twelve of the 13 loci were polymor phic, and 48 unique electrophoretic types (ETs) were identified, representi ng multilocus genotypes, The mean genetic diversity among the 48 genotypes was 0.508. The genetic diversity of each source group of isolates varied fr om 0.241 (periodontal infection) to 0.534 (dairy), Cluster analysis reveale d two major groups separated at a genetic distance of greater than 0.6. One cluster, ETs 1 to 13, included solely isolates from dairies, while the oth er cluster, ETs 14 to 49, included all of the human isolates as well as iso lates from dairies in Norway and Finland, The isolates were serotyped using antiflagellar antiserum. A total of 14 distinct serotypes were observed, H owever, little association between serotyping and geontyping was seen. Most of the strains were also analyzed with pulsed-field gel electrophoresis, s howing the presence of extrachromosomal DNA in the size range of 15 to 600 kb. Our results indicate a high degree of heterogeneity among daily strains . In contrast, strains isolated from humans had their genotypes in one clus ter. Most strains from patients with periodontitis belonged to a single lin eage, suggesting that specific clones of B. cereus and B. thuringiensis are associated with oral infections.