AUTHOR=Losensky Gerald , Vidakovic Lucia , Klingl Andreas , Pfeifer Felicitas , Fröls Sabrina 

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

JOURNAL=Frontiers in Microbiology

VOLUME=5

YEAR=2015

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2014.00755

DOI=10.3389/fmicb.2014.00755

ISSN=1664-302X

ABSTRACT=<p>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 <italic>Halobacterium salinarum</italic> 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 <italic>Hbt. salinarum</italic> R1 genome identified two putative gene loci (<italic>pil-1</italic> and <italic>pil-2</italic>) encoding type IV pilus biogenesis complexes besides the archaellum encoding <italic>fla</italic> gene locus. Both <italic>pil-1</italic> and <italic>pil-2</italic> were expressed as transcriptional units, and the transcriptional start of <italic>pil-1</italic> was identified. <italic>In silico</italic> analyses revealed that the <italic>pil-1</italic> locus is present with other euryarchaeal genomes whereas the <italic>pil-2</italic> is restricted to haloarchaea. Comparative <italic>real time</italic> qRT-PCR studies indicated that the general transcriptional activity was reduced in adherent vs. planktonic cells. In contrast, the transcription of <italic>pilB1</italic> and <italic>pilB2</italic>, encoding putative type IV pilus assembly ATPases, was induced in comparison to the archaella assembly/motor ATPase (<italic>flaI</italic>) and the ferredoxin gene. Mutant strains were constructed that incurred a <italic>flaI</italic> deletion or <italic>flaI</italic>/<italic>pilB1</italic> gene deletions. The absence of <italic>flaI</italic> caused the loss of the archaella while the additional absence of <italic>pilB1</italic> led to loss of the novel pili-like surface structures. The Δ<italic>flaI</italic>/Δ<italic>pilB1</italic> 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<italic>flaI</italic> mutants, the <italic>pil-1</italic> filaments have a distinct function in the adhesion process.</p>