Polyfunctional Nanofibril Appendages Mediate Attachment, Filamentation, and Filament Adaptability in Leptothrix cholodnii

Alcanivorax borkumensis biofilms enhance oil degradation by interfacial tubulation

During the consumption of alkanes, Alcanivorax borkumensis will form a biofilm around an oil droplet, but the role this plays during degradation remains unclear. We identified a shift in biofilm morphology that depends on adaptation to oil consumption: Longer exposure leads to the appearance of dendritic biofilms optimized for oil consumption effected through tubulation of the interface. In situ microfluidic tracking enabled us to correlate tubulation to localized defects in the interfacial cell ordering. We demonstrate control over droplet deformation by using confinement to position defects, inducing dimpling in the droplets. We developed a model that elucidates biofilm morphology, linking tubulation to decreased interfacial tension and increased cell hydrophobicity.

Identification of lthB, a Gene Encoding a Putative Glycosyltransferase Family 8 Protein Required for Leptothrix Sheath Formation

The bacterium Leptothrix cholodnii generates cell chains encased in sheaths that are composed of woven nanofibrils. The nanofibrils are mainly composed of glycoconjugate repeats, and several glycosyltransferases (GTs) are required for its biosynthesis. However, only one GT (LthA) has been identified to date. In this study, we screened spontaneous variants of L. cholodnii SP6 to find those that form smooth colonies, which is one of the characteristics of sheathless variants. Genomic DNA sequencing of an isolated variant revealed an insertion in the locus Lcho_0972, which encodes a putative GT family 8 protein. We thus designated this protein LthB and characterized it using deletion mutants and antibodies. LthB localized adjacent to the cell envelope. ΔlthB cell chains were nanofibril free and thus sheathless, indicating that LthB is involved in nanofibril biosynthesis. Unlike the ΔlthA mutant and the wild-type strain, which often generate planktonic cells, most ΔlthB organisms presented as long cell chains under static conditions, resulting in deficient pellicle formation, which requires motile planktonic cells. These results imply that sheaths are not required for elongation of cell chains. Finally, calcium depletion, which induces cell chain breakage due to sheath loss, abrogated the expression of LthA, but not LthB, suggesting that these GTs cooperatively participate in glycoconjugate biosynthesis under different signaling controls. IMPORTANCE In recent years, the regulation of cell chain elongation of filamentous bacteria via extracellular signals has attracted attention as a potential strategy to prevent clogging of water distribution systems and filamentous bulking of activated sludge in industrial settings. However, a fundamental understanding of the ecology of filamentous bacteria remains elusive. Since sheath formation is associated with cell chain elongation in most of these bacteria, the molecular mechanisms underlying nanofibril sheath formation, including the intracellular signaling cascade in response to extracellular stimuli, must be elucidated. Here, we isolated a sheathless variant of L. cholodnii SP6 and thus identified a novel glycosyltransferase, LthB. Although mutants with deletions of lthA, encoding another GT, and lthB were both defective for nanofibril formation, they exhibited different phenotypes of cell chain elongation and pellicle formation. Moreover, LthA expression, but not LthB expression, was influenced by extracellular calcium, which is known to affect nanofibril formation, indicating the functional diversities of LthA and LthB. Such molecular insights are critical for a better understanding of ecology of filamentous bacteria, which, in turn, can be used to improve strategies to control filamentous bacteria in industrial facilities.

Porous Pellicle Formation of a Filamentous Bacterium, Leptothrix

The bacterium Leptothrix cholodnii generates filaments encased in a sheath comprised of woven nanofibrils. In static liquid culture, L. cholodnii moves toward the air-liquid interface, where it forms porous pellicles. Observations of aggregation at the interface reveal that clusters consisting of only a few bacteria primarily grow by netting free cells. These growing clusters hierarchically enlarge through the random docking of other small clusters. We find that the bacteria swim using their polar flagellum toward the interface, where their sheath assists them in intertwining with others and thereby promotes the formation of small clusters. In contrast, sheathless hydrophobic mutant cells get stuck to the interface. We find that the nanofibril sheath is vital for robust pellicle formation as it lowers cell surface hydrophobicity by 60%, thereby reducing their adsorption and enabling cells to move toward and stick together at the air-liquid interface. IMPORTANCE Efficient and sustainable management of water resources is becoming a fundamental issue for supporting growing populations and for developing economic activity. Fundamental to this management is the treatment of wastewater. Microorganisms are the active component of activated sludge that is employed in the biodegradation process of many wastewater treatment facilities. However, uncontrolled growth of filamentous bacteria such as Sphaerotilus often results in filamentous bulking, lowering the efficiency of water treatment systems. To prevent this undesirable condition, strategies based on a fundamental understanding of the ecology of filamentous bacteria are required. Although the filamentous bacterium Leptothrix cholodnii, which is closely related to Sphaerotilus, is a minor inhabitant of activated sludge, its complete genome sequence is known, making gene manipulation relatively easy. Moreover, L. cholodnii generates porous pellicles under static conditions, which may be a characteristic of filamentous bulking. We show that both swimming motility and nanofibril-mediated air-liquid interface attachment are required for porous pellicle formation. These insights are critical for a better understanding of the characteristics of filamentous bulking and might improve strategies to control activated sludge.

Conditional filamentation as an adaptive trait of bacteria and its ecological significance in soils

Bacteria can regulate cell morphology in response to environmental conditions, altering their physiological and metabolic characteristics to improve survival. Conditional filamentation, in which cells suspend division while continuing lateral growth, is a strategy with a range of adaptive benefits. Here, we review the causes and consequences of conditional filamentation with respect to bacterial physiology, ecology and evolution. We describe four major benefits from conditional filamentation: stress tolerance, surface colonization, gradient spanning and the facilitation of biotic interactions. Adopting a filamentous growth habit involves fitness trade-offs which are also examined. We focus on the role of conditional filamentation in soil habitats, where filamentous morphotypes are highly prevalent and where environmental heterogeneity can benefit a conditional response. To illustrate the use of information presented in our review, we tested the conditions regulating filamentation by the forest soil isolate Paraburkholderia elongata 5N^T . Filamentation by P. elongata was induced at elevated phosphate concentrations, and was associated with the accumulation of intracellular polyphosphate, highlighting the role of filamentation in a phosphate-solubilizing bacterium. Conditional filamentation enables bacteria to optimize their growth and metabolism in environments that are highly variable, a trait that can impact succession, symbioses, and biogeochemistry in soil environments.

Leptothrix cholodnii Response to Nutrient Limitation

Microorganisms are widely utilized for the treatment of wastewater in activated sludge systems. However, the uncontrolled growth of filamentous bacteria leads to bulking and adversely affects wastewater treatment efficiency. To clarify the nutrient requirements for filament formation, we track the growth of a filamentous bacterium, Leptothrix cholodnii SP-6 in different nutrient-limited conditions using a high aspect-ratio microfluidic chamber to follow cell-chain elongation and sheath formation. We find that limitations in Na⁺, K⁺, and Fe²⁺ yield no observable changes in the elongation of cell chains and sheath formation, whereas limitations of C, N, P, or vitamins lead to more pronounced changes in filament morphology; here we observe the appearance of partially empty filaments with wide intercellular gaps. We observe more dramatic differences when SP-6 cells are transferred to media lacking Mg²⁺ and Ca²⁺. Loss of Mg²⁺ results in cell autolysis, while removal of Ca²⁺ results in the catastrophic disintegration of the filaments. By simultaneously limiting both carbon and Ca²⁺ sources, we are able to stimulate planktonic cell generation. These findings paint a detailed picture of the ecophysiology of Leptothrix, which may lead to improved control over the unchecked growth of deleterious filamentous bacteria in water purification systems.

Structural determination of the sheath-forming polysaccharide of Sphaerotilus montanus using thiopeptidoglycan lyase which recognizes the 1,4 linkage between α-d-GalN and β-d-GlcA

Sphaerotilus natans is a filamentous sheath-forming bacterium commonly found in activated sludge. Its sheath is assembled from a thiolic glycoconjugate called thiopeptidoglycan. S. montanus ATCC-BAA-2725 is a sheath-forming member of stream biofilms, and its sheath is morphologically similar to that of S. natans. However, it exhibits heat susceptibility, which distinguishes it from the S. natans sheath. In this study, chemical composition and solid-state NMR analyses suggest that the S. montanus sheath is free of cysteine, indicating that disulfide linkage is not mandatory for sheath formation. The S. montanus sheath was successfully solubilized by N-acetylation, allowing solution-state NMR analysis to determine the sugar sequence. The sheath was susceptible to thiopeptidoglycan lyase prepared from the thiopeptidoglycan-assimilating bacterium, Paenibacillus koleovorans. The reducing ends of the enzymatic digests were labeled with 4-aminobenzoic acid ethyl ester, followed by HPLC. Two derivatives were detected, and their structures were determined. We found that the sheath has no peptides and is assembled as follows: [→4)-β-d-GlcA-(1→4)-β-d-Glc-(1→3)-β-d-GalNAc-(1→4)-α-d-GalNAc-(1→4)-α-d-GalN-(1→]_n (β-d-Glc and α-d-GalNAc are stoichiometrically and substoichiometrically 3-O-acetylated, respectively). Thiopeptidoglycan lyase was thus confirmed to cleave the 1,4 linkage between α-d-GalN and β-d-GlcA, regardless of the peptide moiety. Furthermore, vital fluorescent staining of the sheath demonstrated that elongation takes place at the tips, as with the S. natans sheath.

Competence-Stimulating-Peptide-Dependent Localized Cell Death and Extracellular DNA Production in Streptococcus mutans Biofilms

Extracellular DNA (eDNA) is a biofilm component that contributes to the formation and structural stability of biofilms. Streptococcus mutans, a major cariogenic bacterium, induces eDNA-dependent biofilm formation under specific conditions. Since cell death can result in the release and accumulation of DNA, the dead cells in biofilms are a source of eDNA. However, it remains unknown how eDNA is released from dead cells and is localized within S. mutans biofilms. We focused on cell death induced by the extracellular signaling peptide called competence-stimulating peptide (CSP). We demonstrate that nucleic acid release into the extracellular environment occurs in a subpopulation of dead cells. eDNA production induced by CSP was highly dependent on the lytF gene, which encodes an autolysin. Although lytF expression was induced bimodally by CSP, lytF-expressing cells further divided into surviving cells and eDNA-producing dead cells. Moreover, we found that lytF-expressing cells were abundant near the bottom of the biofilm, even when all cells in the biofilm received the CSP signal. Dead cells and eDNA were also abundantly present near the bottom of the biofilm. The number of lytF-expressing cells in biofilms was significantly higher than that in planktonic cultures, which suggests that adhesion to the substratum surface is important for the induction of lytF expression. The deletion of lytF resulted in reduced adherence to a polystyrene surface. These results suggest that lytF expression and eDNA production induced near the bottom of the biofilm contribute to a firmly attached and structurally stable biofilm.IMPORTANCE Bacterial communities encased by self-produced extracellular polymeric substances (EPSs), known as biofilms, have a wide influence on human health and environmental problems. The importance of biofilm research has increased, as biofilms are the preferred bacterial lifestyle in nature. Furthermore, in recent years it has been noted that the contribution of phenotypic heterogeneity within biofilms requires analysis at the single-cell or subpopulation level to understand bacterial life strategies. In Streptococcus mutans, a cariogenic bacterium, extracellular DNA (eDNA) contributes to biofilm formation. However, it remains unclear how and where the cells produce eDNA within the biofilm. We focused on LytF, an autolysin that is induced by extracellular peptide signals. We used single-cell level imaging techniques to analyze lytF expression in the biofilm population. Here, we show that S. mutans generates eDNA by inducing lytF expression near the bottom of the biofilm, thereby enhancing biofilm adhesion and structural stability.