FIG. 2.

Absence of F plasmid or pilus formation delays the development of biofilms. The images show the biofilm growth patterns and morphologies of different bacterial strains, including F-plasmid-containing strain MG1655(F), nonconjugative F-plasmid-containing strain MG1655(pNCF), F-plasmid-containing strain MG1655(F) treated with Zn²⁺, and plasmid-free strain MG1655. (A) CSLM images of developing biofilms. MG1655 did not form dense mature biofilms at 48 h, in contrast to F-plasmid-containing strain MG1655(F), which formed mushroom-type biofilms. At 24 h, a clear delay in biofilm formation was observed when pilus formation was impaired by expression of a nonconjugative F plasmid [MG1655(pNCF)] or by Zn²⁺ treatment. However, both strains formed flat and uniform biofilms at 48 h. (B) SEM images of the different biofilms at 48 h. The MG1655 strain lacked adhesive appendages, while pili (P), colanic acid (CA), and curli meshwork (C) were abundant in MG1655(F) biofilms. Both the MG1655(pNCF) and MG1655(F) strains treated with Zn²⁺ lacked pili but contained colanic acid particles. Only MG1655(F) treated with Zn²⁺ could synthesize curli. All experiments were performed in duplicate, and representative images are shown. (C) Quantification of the substratum coverage, the average thickness, and the roughness coefficient of the biofilms. The values were calculated with the programs PHLIP and COMSTAT. The results are the means for 20 image stacks representing 10 image stacks from two independent experiments. The error bars indicate the standard deviations.