Colonization and persistence of rough and smooth colony variants of Actinobacillus actinomycetemcomitans in the mouths of rats

Fresh isolates of Actinobacillus actinomycetemcomitans (Aa) bind avidly to surfaces in vitro, but existing in vivo studies of the adherence of Aa are limited. This study had two goals: (1) to compare the oral colonization of two isogenic strains of Aa-CU1010, a clinical isolate that expresses the ad herent phenotype, and CU1012, a minimally adherent laboratory variant-and ( 2) to check for phenotypic reversion of these strains in a clinical setting . Rifampicin-resistant strains, developed for tracking in Sprague-Dawley ra ts, were tested in vitro to determine their stability and binding. In study I, after antibiotic suppression, six rats (group I) received CU1010 in the ir feed. The eight rats in group II received CU1012 in their feed and four were supplemented by oral swabbing and four by gastric gavage. Group III co nsisted of three sham-inoculated controls. All rats were inoculated for 4 d ays. Microbiological data were collected at 1, 4 and 8 weeks after inoculat ion. Supporting data were supplied by antibody titres and clinical measures of alveolar bone loss. Study 2 consisted of six rats in each of three grou ps as above, but tagged strains of Aa were delivered by food alone. At all time-points in both studies, Aa was absent before inoculation and controls had no Aa or antibody to Aa. In study 1, all six rats in group I yielded po sitive cultures for Aa at 8 weeks. In group II, five of eight had positive cultures for Aa at 1 week, two of eight at 4 weeks and none had Aa at 8 wee ks (P less than or equal to 0.001). All six rats in group I had serum anti- Aa titres compared to group II, where titres were seen in four of eight rat s (P less than or equal to 0.015). In vitro data paralleled those found in vivo. No phenotypic reversion of either strain was seen in vivo. In study 2 , four of six rats in group I showed Aa and had titres to Aa, while no othe r animals showed Aa at any time. The model provides convincing evidence tha t, unlike laboratory variants, clinical isolates colonize, persist and inte grate into an already established, albeit reduced, econiche. (C) 2001 Publi shed by Elsevier Science Ltd.