The proportional lack of archaeal pathogens: Do viruses/phages hold the key?

Abstract

Although Archaea inhabit the human body and possess some characteristics of pathogens, there is a notable lack of pathogenic archaeal species identified to date. We hypothesize that the scarcity of disease-causing Archaea is due, in part, to mutually-exclusive phage and virus populations infecting Bacteria and Archaea, coupled with an association of bacterial virulence factors with phages or mobile elements. The ability of bacterial phages to infect Bacteria and then use them as a vehicle to infect eukaryotes may be difficult for archaeal viruses to evolve independently. Differences in extracellular structures between Bacteria and Archaea would make adsorption of bacterial phage particles onto Archaea (i.e. horizontal transfer of virulence) exceedingly hard. If phage and virus populations are indeed exclusive to their respective host Domains, this has important implications for both the evolution of pathogens and approaches to infectious disease control.

Figure 1

Phage/virus receptor locations in Gram-positive Bacteria, Gram-negative Bacteria, and two types of Archaea. Cell wall and membrane structures are depicted for select classic groups of Bacteria and Archaea (i.e. mesophilic Archaea noted for colonizing humans). Known phage/virus receptor sites are indicated by stars. Note that most of the components to which bacterial phages attach (peptidoglycans, lipopolysaccharides, lipoteichoic acid, and fatty acid D-glycerol ester phospholipids) are absent in Archaea. Although both Bacteria and Archaea possess membrane proteins and pili, they are not highly conserved between these Domains of life.