Methanogenic Archaea and oral infections - ways to unravel the black box

Abstract

Archaea, organisms that make up the third domain of cellular life are members of the human oral microflora. They are strikingly less diverse than oral bacteria and appear to be relatively rare with respect to their numerical abundance. Since they have been exclusively found in association with oral infections such as periodontitis and apical periodontitis and given their unique physiology and energy metabolism, it is highly plausible that they are more than just secondary colonizers of infected areas, but instead are actively involved in the overall poly-microbial infection process. Conversely, it is a highly challenging task to clearly demonstrate their possible active participation - mostly due to the difficulty to grow them in routine microbiology laboratories. This current review points out the importance for understanding the medical impact of methanogens and aims at devising strategies for elucidating the true function of archaea in the oral ecosystem.

Keywords: human microbial ecosystems; interspecies hydrogen transfer; methanogenic archaea; oral infections.

Fig. 1

Dendrogramm depicting major taxonomic groups of Archaea. The dendrogramm is based on representative 16S rRNA gene sequences (clustered in boxes) calculated with the ARB software package (www.arb-home.de). The size and shape of the boxes roughly reflect the proportions of known phylotypes and phylogenetic depth. Note, that the tree is not a rigorous phylogenetic analysis, but an attempt to convey sequence relationship among archaeal organisms. The names in frames indicate four major orders of methanogens. Another fifth order of methanogens that is represented by only one species, namely, Methanopyrus kandleri is not included in the tree. The boxes that contain recognized human methanogens (i.e. Methanobrevibacter oralis as primary colonizer of the oral cavity, M. smithii and Methanosphaera stadtmanae as primary colonizer of the gut system) are highlighted in black. The stars within light boxes indicate sporadic detection of 16S rRNA genes (single star: detection in the human gut system, double star: detection in the gut system and oral cavity). For further information see main text.

Fig. 2

Schematic representation of a SIP experiment. Clinical samples (e.g. from periodontal plaque) are incubated anaerobically in a liquid medium with addition of ¹³C-labelled butyrate as substrate. If syntrophic growth occurs between a butyrate-degrading bacterium and methanogens (via interspecies hydrogen transfer), the labeled ¹³C will ultimately be assimilated and incorporated into their DNA. After some time of incubation, total DNA is extracted and the ‘heavy’ and ‘light’ fractions are physically separated. Analysis of ‘heavy’ DNA by PCR, cloning, and comparative sequence analysis will then allow identification of the active microbial populations in the microcosm. For further information see main text.