Inverse regulatory coordination of motility and curli-mediated adhesion in Escherichia coli

The BLUF-EAL protein YcgF acts as a direct anti-repressor in a blue-light response of Escherichia coli

The blue light using FAD (BLUF)-EAL protein YcgF is a known blue-light sensor of Escherichia coli, but its direct regulatory output and physiological function have remained unknown. Here, we demonstrate that unlike other EAL domain proteins, YcgF does not degrade the signaling molecule c-di-GMP, but directly binds to and releases the MerR-like repressor YcgE from its operator DNA upon blue-light irradiation. As a consequence, a distinct regulon of eight small proteins (of 71-126 amino acids) is strongly induced. These include YmgA and YmgB, which, via the RcsC/RcsD/RcsB two-component phosphorelay system, activate production of the biofilm matrix substance colanic acid as well as acid resistance genes and the biofilm-associated bdm gene and down-regulate adhesive curli fimbriae. Thus, small proteins under YcgF/YcgE control seem to act as "connectors" that provide additional signal input into a two-component signaling pathway. Moreover, we found ycgF and ycgE expression to be strongly activated at low temperature, and we elucidate how blue light, cold, and starvation signals are integrated in the expression and activity of the YcgF/YcgE/small protein signaling pathway. In conclusion, this pathway may modulate biofilm formation via the two-component network when E. coli has to survive in an extrahost aquatic environment.

Nickel promotes biofilm formation by Escherichia coli K-12 strains that produce curli

The survival of bacteria exposed to toxic compounds is a multifactorial phenomenon, involving well-known molecular mechanisms of resistance but also less-well-understood mechanisms of tolerance that need to be clarified. In particular, the contribution of biofilm formation to survival in the presence of toxic compounds, such as nickel, was investigated in this study. We found that a subinhibitory concentration of nickel leads Escherichia coli bacteria to change their lifestyle, developing biofilm structures rather than growing as free-floating cells. Interestingly, whereas nickel and magnesium both alter the global cell surface charge, only nickel promotes biofilm formation in our system. Genetic evidence indicates that biofilm formation induced by nickel is mediated by the transcriptional induction of the adhesive curli-encoding genes. Biofilm formation induced by nickel does not rely on efflux mechanisms using the RcnA pump, as these require a higher concentration of nickel to be activated. Our results demonstrate that the nickel-induced biofilm formation in E. coli is an adaptational process, occurring through a transcriptional effect on genes coding for adherence structures. The biofilm lifestyle is obviously a selective advantage in the presence of nickel, but the means by which it improves bacterial survival needs to be investigated.

Prokaryotic phototaxis

Microorganisms have various mechanisms at their disposal to react to (changes in) their ambient light climate (i.e., intensity, color, direction, and degree of polarization). Of these, one of the best studied mechanisms is the process of phototaxis. This process can be described as a behavioral migration-response of an organism toward a change in illumination regime. In this chapter we discuss three of these migration responses, based on swimming, swarming, and twitching motility, respectively. Swimming motility has been studied using a wide range of techniques, usually microscopy based. We present a detailed description of the assays used to study phototaxis in liquid cultures of the phototrophic organisms Halobacterium salinarum, Halorhodospira halophila, and Rhodobacter sphaeroides and briefly describe the molecular basis of these responses. Swarming and twitching motility are processes taking place at the interface between a solid phase and a liquid or gas phase. Although assays to study these processes are relatively straightforward, they are accompanied by technical complications, which we describe. Furthermore, we discuss the molecular processes underlying these forms of motility in Rhodocista centenaria and Synechocystis PCC6803. Recently, it has become clear that also chemotrophic organisms contain photoreceptor proteins that allow them to respond to their ambient light climate. Surprisingly, light-modulated motility responses can also be observed in the chemotrophic organisms Escherichia coli and Acinetobacter calcoaceticus. In the light-modulated surface migration not only "che-like" signal transduction reactions may play a role, but in addition processes as modulation of gene expression and even intermediary metabolism.