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It was recently shown that haloarchaeal strains of different genera are able to adhere to surfaces and form surface-attached biofilms. However, the surface structures mediating the adhesion were still unknown. We have identified a novel surface structure with  Halobacterium salinarum  strain R1, crucial for surface adhesion. Electron microscopic studies of surface-attached cells frequently showed pili-like surface structures of two different diameters that were irregularly distributed on the surface. The thinner filaments, 7–8 nm in diameter, represented a so far unobserved novel pili-like structure. Examination of the  Hbt. salinarum  R1 genome identified two putative gene loci ( pil-1  and  pil-2 ) encoding type IV pilus biogenesis complexes besides the archaellum encoding  fla  gene locus. Both  pil-1  and  pil-2  were expressed as transcriptional units, and the transcriptional start of  pil-1  was identified.  In silico  analyses revealed that the  pil-1  locus is present with other euryarchaeal genomes whereas the  pil-2  is restricted to haloarchaea. Comparative  real time  qRT-PCR studies indicated that the general transcriptional activity was reduced in adherent vs. planktonic cells. In contrast, the transcription of  pilB1  and  pilB2 , encoding putative type IV pilus assembly ATPases, was induced in comparison to the archaella assembly/motor ATPase ( flaI ) and the ferredoxin gene. Mutant strains were constructed that incurred a  flaI  deletion or  flaI / pilB1  gene deletions. The absence of  flaI  caused the loss of the archaella while the additional absence of  pilB1  led to loss of the novel pili-like surface structures. The Δ flaI /Δ pilB1  double mutants showed a 10-fold reduction in surface adhesion compared to the parental strain. Since surface adhesion was not reduced with the non-archaellated Δ flaI  mutants, the  pil-1  filaments have a distinct function in the adhesion process.

Novel pili-like surface structures of Halobacterium salinarum strain R1 are crucial for surface adhesion