Deinococcus geothermalis E50051 forms tenuous biofilms on paper machine surfaces. Field emission electron microscopy analysis revealed peritrichous appendages which mediated cell-to-surface and cell-to-cell interactions but were absent in planktonically grown cells. The major protein component of the extracellular extract of D. geothermalis had an N-terminal sequence similar to the fimbrial protein pilin annotated in the D. geothermalis DSM 11300 draft sequence. It also showed similarity to the type IV pilin sequence of D. radiodurans and several gram-negative pathogenic bacteria. Other proteins in the extract had N-terminal sequences identical to D. geothermalis proteins with conservative motifs for serine proteases, metallophosphoesterases, and proteins whose function is unknown. Periodic acid-Schiff staining for carbohydrates indicated that these extracellular proteins may be glycosylated. A further confirmation for the presence of glycoconjugates on the cell surface was obtained by confocal laser scanning imaging of living D. geothermalis cells stained with Amaranthus caudatus lectin, which specifically binds to galactose residues. The results indicate that the thread-like appendages of D. geothermalis E50051 are glycosylated type IV pili, bacterial attachment organelles which have thus far not been described for the genus Deinococcus.Deinococcus geothermalis is an important primary biofilm former found in paper machine water (17,35). It forms tenuous biofilms on abiotic surfaces and is difficult or impossible to remove from the surfaces using industrial washing procedures (18). The attachment of D. geothermalis involves thread-like structures that connect the cells to the abiotic surface while allowing sliding movement, to escape mechanical stress (18).Capsules and slimes are involved in the adhesion of bacteria onto living and nonliving substrates (6, 15). However, adhesion of D. geothermalis seems to occur in the absence of either of these (18,29). Another attachment mode by means of pili has been described for several gram-negative pathogenic bacteria, including, for example, Neisseria meningitidis, N. gonorrhoeae (22,36), and Pseudomonas aeruginosa (38), and for some grampositive bacteria, such as Enterococcus faecalis (11), Actinomyces spp. (19), and Ruminococcus albus (28). So far there is no experimental evidence that members of the genus Deinococcus produce flagella or pili, even though several genes encoding pilus-associated functions have been found in the genome of D. radiodurans (20). Within the phylum Deinococcus-Thermus, only the Thermus thermophilus strain HB27 has been reported to express pili during natural transformation (8).This study describes the ultrastructure of the thread-like appendages expressed by the industrially relevant strain D. geothermalis E50051. It is proposed that they represent pili and glycoconjugates necessary for adhesion and biofilm formation of D. geothermalis. MATERIALS AND METHODSBacterial strain and growth conditions. Deinococcus geothermalis E50051 (HAMBI 2411) ...