Gliding motility and Por secretion system genes are widespread among members of the phylum bacteroidetes

Abstract

The phylum Bacteroidetes is large and diverse, with rapid gliding motility and the ability to digest macromolecules associated with many genera and species. Recently, a novel protein secretion system, the Por secretion system (PorSS), was identified in two members of the phylum, the gliding bacterium Flavobacterium johnsoniae and the nonmotile oral pathogen Porphyromonas gingivalis. The components of the PorSS are not similar in sequence to those of other well-studied bacterial secretion systems. The F. johnsoniae PorSS genes are a subset of the gliding motility genes, suggesting a role for the secretion system in motility. The F. johnsoniae PorSS is needed for assembly of the gliding motility apparatus and for secretion of a chitinase, and the P. gingivalis PorSS is involved in secretion of gingipain protease virulence factors. Comparative analysis of 37 genomes of members of the phylum Bacteroidetes revealed the widespread occurrence of gliding motility genes and PorSS genes. Genes associated with other bacterial protein secretion systems were less common. The results suggest that gliding motility is more common than previously reported. Microscopic observations confirmed that organisms previously described as nonmotile, including Croceibacter atlanticus, "Gramella forsetii," Paludibacter propionicigenes, Riemerella anatipestifer, and Robiginitalea biformata, exhibit gliding motility. Three genes (gldA, gldF, and gldG) that encode an apparent ATP-binding cassette transporter required for F. johnsoniae gliding were absent from two related gliding bacteria, suggesting that the transporter may not be central to gliding motility.

Fig 1

Proteins involved in F. johnsoniae gliding motility and protein secretion. SprB and RemA (orange) are thought to function as adhesins that are propelled along the cell surface by the some of the other proteins shown. GldA, GldF, and GldG (red) comprise an ATP-binding cassette transporter whose exact role in gliding is not known. GldI (yellow) is a peptidylprolyl isomerase involved in protein folding. Proteins in blue (GldK, GldL, GldM, GldN, SprA, SprE, SprT) constitute the PorSS and are required for secretion of SprB and RemA and for motility. They also secrete the chitinase ChiA (white), which is not involved in motility. Proteins in green (GldB, GldD, GldH, and GldJ) are also required for gliding. Black lines indicate lipid tails on lipoproteins. Proteins are not drawn to scale, stoichiometry of components is not known, and it is not yet known whether any of the lipoproteins localize to the cytoplasmic membrane (CM) instead of the outer membrane (OM) as shown.

Fig 2

Distribution of gld and spr orthologs in completed Bacteroidetes genomes. Orthologs were identified by reciprocal BLASTP analysis using the F. johnsoniae motility protein sequences. Maximum E values were set at 1e−50 for GldA and 1e−5 for the other proteins. In order to confirm weak hits for bacteria distantly related to F. johnsoniae (class Flavobacteriia), genomes were also searched with protein sequences from C. hutchinsonii (class Cytophagia), P. heparinus (class Sphingobacteriia), and P. gingivalis (class Bacteroidia). A colored square indicates the presence of an ortholog, and a white square indicates the absence of an ortholog. Colors of squares correspond to those assigned to individual proteins in Fig. 1 based on predicted or verified functional groups: red, ABC transporter components; yellow, peptidylprolyl isomerase; blue, PorSS components; green, additional proteins required for gliding. The core gliding motility genes as defined here correspond to the green and blue columns. Species are listed alphabetically within each class. Species previously known to exhibit gliding motility are in bold.

Fig 3

Colony morphologies of the gliding bacteria C. algicola and Maribacter sp. HTCC2170, which lack homologs of gldA, gldF, and gldG. (A) Spreading colony of C. algicola on Cytophaga agar after incubation for 5 days at 15°C; (B) nonspreading colony of Maribacter sp. HTCC2170 on marine agar after incubation for 9 days at 25°C. Photomicrographs were taken with a Photometrics CoolSNAP_cf² camera mounted on an Olympus IMT-2 phase-contrast microscope. The bar indicates 1 mm and applies to both panels.

Fig 4

C. atlanticus, R. biformata, “G. forsetii,” R. anatipestifer, and P. propionicigenes exhibit gliding motility. Cells gliding on agar or glass were recorded using a Photometrics Cool-SNAP_cf² camera mounted on an Olympus BH-2 phase-contrast microscope. The images were taken from Movies S2 to S6 in the supplemental material. (A) Cells of C. atlanticus in marine broth gliding on glass in a tunnel slide. The images come from Movie S2 between 4 and 8 s and correspond to the middle portion of the field. (B) Cells of R. biformata on marine agar. The images come from the second part of Movie S3 and correspond to the top middle portion of the field. (C) Cells of “G. forsetii” on modified Cytophaga agar (yeast extract and tryptone omitted). The images come from Movie S4 and correspond to the top middle portion of the field. (D) Cells of R. anatipestifer on TS agar. The images come from the second part of Movie S5 and correspond to the lower right portion of the field. (E) Cells of P. propionicigenes on PYG agar. The images come from the second part of Movie S6 and correspond to the lower left portion of the field. All cells were incubated at 25°C during analysis. The numbers indicate time (minutes and seconds). Arrows indicate moving cells, and arrowheads indicate stationary reference points. The bar at the lower right indicates 10 μm and applies to all panels. The movies in the supplemental material contain additional footage demonstrating gliding.