Hampered motility promotes the evolution of wrinkly phenotype in Bacillus subtilis

Abstract

Selection for a certain trait in microbes depends on the genetic background of the strain and the selection pressure of the environmental conditions acting on the cells. In contrast to the sessile state in the biofilm, various bacterial cells employ flagellum-dependent motility under planktonic conditions suggesting that the two phenotypes are mutually exclusive. However, flagellum dependent motility facilitates the prompt establishment of floating biofilms on the air-medium interface, called pellicles. Previously, pellicles of B. subtilis were shown to be preferably established by motile cells, causing a reduced fitness of non-motile derivatives in the presence of the wild type strain. Here, we show that lack of fully assembled flagella promotes the evolution of matrix overproducers that can be distinguished by the characteristic wrinkled colony morphotype. The wrinkly phenotype is associated with amino acid substitutions in the master repressor of biofilm-related genes, SinR. By analyzing one of the mutations, we show that it alters the tetramerization and DNA binding properties of SinR, allowing an increased expression of the operon responsible for exopolysaccharide production. Finally, we demonstrate that the wrinkly phenotype is advantageous when cells lack flagella, but not in the wild type background.\n\nGraphical Abstract\n\nO_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=194 SRC=\"FIGDIR/small/288951v1_ufig1.gif\" ALT=\"Figure 1\">\nView larger version (47K):\norg.highwire.dtl.DTLVardef@1a70c4org.highwire.dtl.DTLVardef@1e16453org.highwire.dtl.DTLVardef@819999org.highwire.dtl.DTLVardef@8b2bde_HPS_FORMAT_FIGEXP M_FIG C_FIG Abbreviated SummaryDuring biofilm establishment at the air-liquid interface, Bacillus subtilis evolves matrix overproducers with a wrinkly colony phenotype (WS). This is caused by mutations in the regulator SinR which alter its dimerization and DNA interaction properties. The matrix overproducers appear mostly in a non-motile mutant where they possess a competitive advantage for biofilm formation, which is not present in the wild type background.