Introduction

This study investigated the bacterial communities residing in the apical portion of human teeth with apical periodontitis in primary and secondary infections using a culture-independent molecular biology approach.

Methods

Root canal samples from the apical root segments of extracted teeth were collected from 18 teeth with necrotic pulp and 8 teeth with previous endodontic treatment. Samples were processed for amplification via polymerase chain reaction (PCR) and separated with denaturing gradient gel electrophoresis (DGGE). Selected bands were excised from the gel and sequenced for identification.

Results

Comparable to previous studies of entire root canals, the apical bacterial communities in primary infections were significantly more diverse than in secondary infections (p=0.0003). Inter- and intra-patient comparisons exhibited similar variations in profiles. Different roots of the same teeth with secondary infections displayed low similarity in bacterial composition, while an equivalent sample collected from primary infection contained almost identical populations. Sequencing revealed a high prevalence of fusobacteria, Actinomyces sp. and oral Anaeroglobus geminatus in both types of infection. Many secondary infections contained Burkholderiales or Pseudomonas sp. both of which represent opportunistic environmental pathogens.

Conclusion

Certain microorganisms exhibit similar prevalence in primary and secondary infection indicating that they are likely not eradicated during endodontic treatment. The presence of Burkholderiales and Pseudomonas sp. underscores the problem of environmental contamination. Treatment appears to affect the various root canals of multi-rooted teeth differently, resulting in local changes of the microbiota.

It is a well-recognized fact that the composition of human salivary microbial community is greatly affected by its nutritional environment. However, most studies are currently focused on major carbon or nitrogen sources with limited attention to trace elements like essential mineral ions. In this study, we examined the effect of iron availability on the bacterial profiles of an in vitro human salivary microbial community as iron is an essential trace element for the survival and proliferation of virtually all microorganisms. Analysis via a combination of PCR with denaturing gradient gel electrophoresis (DGGE) demonstrated a drastic change in species composition of an in vitro human salivary microbiota when iron was scavenged from the culture medium by addition of the iron chelator 2,2’- bipyridyl (Bipy). This shift in community profile was prevented by the presence of excessive ferrous iron (Fe2+). Most interestingly, under iron deficiency, the in vitro grown salivary microbial community became dominated by several hemolytic bacterial species, including Streptococcus spp., Gemella spp. and Granulicatella spp.all of which have been implicated in infective endocarditis. These data provide evidence that iron availability can modulate host-associated oral microbial communities, resulting in a microbiota with potential clinical impact.

Since the initial observations of oral bacteria within dental plaque by van Leeuwenhoek using his primitive microscopes in 1680, an event that is generally recognized as the advent of oral microbiological investigation, oral microbiology has gone through phases of “reductionism” and “holism”. From the small beginnings of the Miller and Black period, in which microbiologists followed Koch’s postulates, took the reductionist approach to try to study the complex oral microbial community by analyzing individual species; to the modern era when oral researchers embrace “holism” or “system thinking”, adopt new concepts such as interspecies interaction, microbial community, biofilms, poly-microbial diseases, oral microbiological knowledge has burgeoned and our ability to identify the resident organisms in dental plaque and decipher the interactions between key components has rapidly increased, such knowledge has greatly changed our view of the oral microbial flora, provided invaluable insight into the etiology of dental and periodontal diseases, opened the door to new approaches and techniques for developing new therapeutic and preventive tools for combating oral poly-microbial diseases.