Investigation of the Actinobacillus actinomycetemcomitans genome by pulsed field gel electrophoresis

Transmission of periodontal bacteria and models of infection

BACKGROUND

Bacteria play an essential role in the aetiology of periodontitis. Most bacterial species isolated from subgingival plaque are indigenous to the oral cavity. Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis are detected infrequently in periodontal health, which makes these species prime candidates to study person-to-person transmission. The aim of the present study was to review the literature on transmission of these periodontal bacterial species.

METHOD

We review the literature on bacterial typing techniques and summarize the information on clonal distribution of A. actinomycetemcomitans and P. gingivalis in family units based on different typing techniques in order to establish the likelihood for person-to-person transmission of these periodontal pathogens.

RESULTS

Vertical transmission of A. actinomycetemcomitans is estimated to be between 30% and 60%, whereas vertical transmission of P. gingivalis has rarely been observed. Horizontal transmission between spouses ranges between 14% and 60% for A. actinomycetemcomitans and between 30% and 75% for P. gingivalis. There is some evidence to show that cohabitation with a periodontitis patient influences the periodontal status of the spouse; however, substantially more information is needed to prove this hypothesis.

CONCLUSIONS

Transmission of putative periodontal pathogens between family members has been shown. The clinical consequences of these events have been poorly documented. Based on the current knowledge, screening for and prevention of transmission of specific virulent clones of A. actinomycetemcomitans may be feasible and effective in preventing some forms of periodontal disease. P. gingivalis is usually recovered from diseased adult subjects, and transmission of this pathogens seems largely restricted to adult individuals. Horizontal transmission of P. gingivalis may therefore be controlled by periodontal treatment involving elimination or significant suppression of the pathogen in diseased individuals and by a high standard of oral hygiene.

Analysis of serum antibody responses to periodontopathogens in early-onset periodontitis patients from different geographical locations

Serum antibody specificity to oral micro-organisms was used to delineate the pathogens associated with early-onset periodontal diseases in a Turkish population. Additionally, comparison of the findings to those derived from a clinically similar US patient population described differences in bacterial specific antibody between these 2 geographic regions. Serum from 89 (LJP), 86 (RPP) and 94 (normal) subjects was analyzed (ELISA) to determine IgG antibody to 14 oral micro-organisms. All LJP patients from Turkey exhibited elevated antibody levels to A. actinomycetemcomitans (serotypes c and a significantly increased), while antibody levels to A. actinomycetemcomitans Y4 and JP2 (serotype b) were significantly higher in US LJP patients. 50% of the Turkish RPP patients also showed elevated anti-A. actinomycetemcomitans antibody, although the US RPP patients exhibited significantly higher antibody levels and frequency of elevated antibody to the A. actinomycetemcomitans serotypes. Healthy subjects and LJP and RPP patients from the US exhibited higher antibody levels to all 3 P. gingivalis serogroups compared to those from Turkey, although, the frequency of elevated antibody to the P. gingivalis serogroups was significantly higher in LJP and RPP patients from Turkey than from the US. Interestingly, 87% and 77% of the LJP patients in the Turkish population had elevated antibody responses to P. gingivalis and E. corrodens, respectively, which was not observed in the US LJP patients. These data suggested that considerable variation exists in the systemic antibody levels to periodontopathogens between these 2 countries. This supports potential differences in subgingival colonization or antigenic composition of these pathogens between patient populations from different geographical regions.

Functional analysis of pVT745, a plasmid from Actinobacillus actinomycetemcomitans

Plasmid pVT745 is a 25.1-kb replicon isolated from Actinobacillus actinomycetemcomitans strain VT745. A previous report described the hybridization of pVT745 in 5 strain-specific patterns to chromosomal DNA from 15 other A. actinomycetemcomitans strains. However, pVT745 does not share homology with the chromosome of the strain from which it was isolated, VT745. It was hypothesized that the shared areas of homology might represent insertion sequence elements and/or transposons possibly encoding resistance to one or more antibiotics. An antibiogram of strain VT745 demonstrated that this strain was uniformly susceptible to all antibiotics examined. Because insertion sequence elements and transposons are mobile genetic elements, a series of cell passaging experiments, followed by Southern hybridization was conducted in a attempt to detect transposition of pVT745 homologous DNA within the chromosomes of several A. actinomycetemcomitans strains. The results of these experiments suggested stability of the homologous DNA both within the chromosome and on the plasmid. It was also possible that pVT745 represented a lysogenic bacteriophage. Phage induction experiments were conducted under conditions that induced a previously described A. actinomycetemcomitans lysogenic phage, but no phage could be induced from strain VT745. Attempts to obtain isolates of VT745 cured of pVT745 were also unsuccessful.

Chromosome sizes and phylogenetic relationships between serotypes of Actinobacillus pleuropneumoniae

The genome size of Actinobacillus pleuropneumoniae was determined by pulsed field gel electrophoresis of AscI and ApaI digested chromosomal DNA. The genome size of the type strain 4074T (serotype 1) was determined to be 2404 +/- 40 kb. The chromosome sizes for the reference strains of the other serotypes range between 2.3 and 2.4 Mb. The restriction pattern profiles of AscI, ApaI and NheI digested chromosomes showed a high degree of polymorphism among the different serotype reference strains and allowed their discrimination. The analysis of the macrorestriction pattern polymorphism revealed phylogenetic relationships between the different serotype reference strains which reflect to some extent groups of serotypes known to cross-react serologically. In addition, different pulsed fields gel electrophoresis patterns also revealed heterogeneity in the chromosomal structure among different field strains of serotypes 1, 5a, and 5b, while strains of serotype 9 originating from most distant geographical places showed homogeneous ApaI patterns in pulsed field gel electrophoresis.