Biofilms are consortia of bacteria that are held together by an extracellular matrix. Cyanobacterial biofilms, which are highly ubiquitous and inhabit diverse niches, are often associated with biological fouling and cause severe economic loss. Information on the molecular mechanisms underlying biofilm formation in cyanobacteria is scarce. We identified a mutant of the cyanobacterium Synechococcus elongatus, which unlike the wild type, developed biofilms. This biofilm-forming phenotype is caused by inactivation of homologues of type II secretion /type IV pilus assembly systems and is associated with impairment of protein secretion. The conditioned medium from a wild-type culture represses biofilm formation by the secretion-mutants. This suggested that the planktonic nature of the wild-type strain is a result of a self-suppression mechanism, which depends on the deposition of a factor to the extracellular milieu. The particular niche conditions will determine whether the inhibitor will accumulate to effective levels and thus the described mechanism allows switching to a sessile mode of existence.
Left panels: The T2SE-mutant (T2SEΩ) adheres to the growth tube, in contrast to the wild-type strain, which is characterized by planktonic growth. Right panels: Fluorescence microscopy reveals biofilms of T2SEΩ.
Environ Microbiol. 2013 Jun;15(6):1786-94.
Self-suppression of biofilm formation in the cyanobacterium Synechococcus elongatus.
Schatz D, Nagar E, Sendersky E, Parnasa R, Zilberman S, Carmeli S, Mastai Y, Shimoni E, Klein E, Yeger O, Reich Z, Schwarz R
We identified genes that are essential for biofilm formation. These genes include small proteins characterized by a double-glycine secretion motif as well as a component of transport system responsible for secretion and maturation of these small proteins.
Sequence of the double-glycine motif of the product of pcc7942_1134 and selected secreted peptides. Cerein7B of Bacillus cereus, EnterocinA and EnterocinB of Enterococcus faceum, MicrocinE492 of Klebsiella pneumonia, and Microcin24, MicrocinH47, and ColicinV of Escherichia coli. Black shading indicates the double-glycine or glycine-alanine present just prior to the peptide cleavage site (arrow).
Scientific Reports –Nature 2016 Aug 25;6:32209
Small secreted proteins enable biofilm development in the cyanobacterium Synechococcus elongatus.
Parnasa R, Nagar E, Sendersky E, Reich Z, Simkovsky R, Golden S, Schwarz R
The transition between planktonic growth and biofilm formation represents a tightly regulated developmental shift that has substantial impact on cell fate. Here, we highlight different mechanisms through which bacteria limit their own biofilm development. The mechanisms involved in these self-inhibition processes include: (i) regulation by secreted small molecules, which govern intricate signalling cascades that eventually decrease biofilm development, (ii) extracellular polysaccharides capable of modifying the physicochemical properties of the substratum and (iii) extracellular DNA that masks an adhesive structure. These mechanisms, which rely on substances produced by the bacterium and released into the extracellular milieu, suggest regulation at the communal level. In addition, we provide specific examples of environmental cues (e.g. blue light or glucose level) that trigger a cellular response reducing biofilm development. All together, we describe a diverse array of mechanisms underlying self-inhibition of biofilm development in different bacteria and discuss possible advantages of these processes.
Biofilm inhibition in S. elongatus by an extracellular inhibitor.
Wild-type cells produce and secrete an inhibitor (red ‘no-entrance’ sign) that suppresses transcription of the genes Synpcc7942_1133 and Synpcc7942_1134. Inactivation of t2sE, encoding a homologue of subunit E of type two secretion systems (T2S), enables biofilm development. The biofilm-forming mutant, T2SEΩ, is most likely impaired in secretion of the inhibitory factor, and therefore expresses ‘biofilm-genes’ at a higher level, and develops biofilms.
Environmental Microbiology 2015 May;17(5):1477-86.
To be or not to be planktonic? Self-inhibition of biofilm development.
Nagar E, Schwarz R.