Systems view of Bacillus subtilis pellicle development

Maximizing the benefits of biofilms in fermentation processes

Biofilm-based fermentation has great potential, as it possesses inherent characteristics such as self-immobilization, high resistance to reactants, and long-term activity. This forum focuses on research targets for promoting biofilm engineering to maximize the beneficial features of biofilms and to effectively utilize them in biofilm-mediated fermentation.

Stress and alignment response to curved obstacles in growing bacterial monolayers

Monolayers of growing bacteria, confined within channel geometries, exhibit self-organization into a highly aligned laminar state along the axis of the channel. Although this phenomenon has been observed in experiments and simulations under various boundary conditions, the underlying physical mechanism driving this alignment remains unclear. In this study, we conduct simulations of growing bacteria in two-dimensional channel geometries perturbed by fixed obstacles, either circular or arc shaped, placed at the channel's center. Our findings reveal that even sizable obstacles cause only short-ranged disruptions to the baseline laminar state. These disruptions arise from a competition between local planar anchoring and bulk laminar alignment. At smaller obstacle sizes, bulk alignment fully dominates, while at larger sizes planar anchoring induces increasing local disruptions. Furthermore, our analysis indicates that the resulting configurations of the bacterial system display a striking resemblance to the arrangement of hard-rod smectic liquid crystals around circular obstacles. This suggests that modeling hard-rod bacterial monolayers as smectic, rather than nematic, liquid crystals may yield successful outcomes. The insights gained from our study contribute to the expanding body of research on bacterial growth in channels. Our work provides perspectives on the stability of the laminar state and extends our understanding to encompass more intricate confinement schemes.

Exudate-Induced Gelatinizable Nanofiber Membrane with High Exudate Absorption and Super Bactericidal Capacity for Bacteria-Infected Wound Management

Invasion of bacteria and continuous oozing of exudate are significant causes of interference with the healing of infected wounds. Therefore, an exudate-induced gelatinizable and near-infrared (NIR)-responsive nanofiber membrane composed of polyvinyl alcohol (PVA), carboxymethyl chitosan (CMC), and Fe-doped phosphomolybdic acid (Fe-PMA) with exceptional exudate absorption capacity and potent bactericidal efficacy is developed and denoted as the PVA-FP-CMC membrane. After absorbing exudate, the fiber membrane can transform into a hydrogel membrane, forming coordination bonds between the Fe-PMA and CMC. The unique exudate-induced gelation process imparts the membrane with high exudate absorption and retention capability, and the formed hydrogel also traps the bacteria that thrive in the exudate. Moreover, it is discovered for the first time that the Fe-PMA exhibits an enhanced photothermal conversion capability and photocatalytic activity compared to the PMA. Therefore, the presence of Fe-PMA provides the membrane with a photothermal and photodynamic therapeutic effect for killing bacteria. The PVA-FP-CMC membrane is proven with a liquid absorption ratio of 520.7%, a light-heat conversion efficiency of 41.9%, high-level generation of hydroxyl radical (•OH) and singlet oxygen (1O2), and a bacterial killing ratio of 100% for S. aureus and 99.6% for E. coli. The treatment of infected wounds on the backs of rats further confirms the promotion of wound healing by the PVA-FP-CMC membrane with NIR irradiation. Overall, this novel functional dressing for the synergistic management of bacteria-infected wounds presents a promising therapeutic strategy for tissue repair and regeneration.

Multiscale spatial segregation analysis in digital images of biofilms

Quantifying the degree of spatial segregation of two bacterial strains in mixed biofilms is an important topic in microbiology. Spatial segregation is dependent on spatial scale as two strains may appear to be well mixed if observed from a distance, but a closer look can reveal strong separation. Typically, this information is encoded in a digital image that represents the binary system, e.g., a microscopy image of a two species biofilm. To decode spatial segregation information, we have developed quantitative measures for evaluating the degree of the spatial scale-dependent segregation of two bacterial strains in a digital image. The constructed algorithm is based on the new segregation measures and overcomes drawbacks of existing approaches for biofilm segregation analysis. The new approach is implemented in a freely available software and was successfully applied to biofilms of two strains and bacterial suspensions for detection of the different spatial scale-dependent segregation levels.

Bacillus subtilis NDmed, a model strain for biofilm genetic studies

The Bacillus subtilis strain NDmed was isolated from an endoscope washer-disinfector in a medical environment. NDmed can form complex macrocolonies with highly wrinkled architectural structures on solid medium. In static liquid culture, it produces thick pellicles at the interface with air as well as remarkable highly protruding ''beanstalk-like'' submerged biofilm structures at the solid surface. Since these mucoid submerged structures are hyper-resistant to biocides, NDmed has the ability to protect pathogens embedded in mixed-species biofilms by sheltering them from the action of these agents. Additionally, this non-domesticated and highly biofilm forming strain has the propensity of being genetically manipulated. Due to all these properties, the NDmed strain becomes a valuable model for the study of B. subtilis biofilms. This review focuses on several studies performed with NDmed that have highlighted the sophisticated genetic dynamics at play during B. subtilis biofilm formation. Further studies in project using modern molecular tools of advanced technologies with this strain, will allow to deepen our knowledge on the emerging properties of multicellular bacterial life.

Inactivation of Bacillus subtilis spores by a combination of high-pressure thermal treatment and potassium sorbate

Combining High-pressure Thermal Treatment (HPTT) and Potassium Sorbate (PS) may have a stronger spore inactivation effect. Spores of Bacillus subtilis were subjected to HPTT at 600 MPa-65 °C/75 °C and a combination of HPTT and PS of 0.1% and 0.2% concentrations. After these treatments, different procedures and techniques were employed to investigate the spore's inactivation. The results revealed that 4.92 ± 0.05 log spores were inactivated after treatment at 600 MPa-75 °C, while 5.97 ± 0.09 log spores were inactivated when the HPTT treatment was combined with 0.2% PS. Changes in permeability of the spore's inner membrane were characterized by OD600 value and release rates of nucleic acids, protein, and dipicolinic acid (DPA). Compared with HPTT treatment at 600 MPa-75 °C, the OD600 value of spores decreased further by about 50% after treatment with a combination of HPTT and 0.2% PS. Additionally, the combined treatments resulted in a significant increase in the OD260 and OD280 values, as well as the DPA release. The spore size analysis indicated a significant decrease in the size of spores treated with a combination of HPTT at 600 MPa-75 °C and PS of 0.2% concentration. Furthermore, the flow cytometry analysis and confocal laser scanning microscopy (CLSM) analysis indicated that the inner membrane damage of spores was higher after combined treatments than that after HPTT treatment alone. A significant reduction was also found in the Na+/K+-ATPase activity after the combined treatments. Also, the FTIR analysis revealed that the combined treatments resulted in significant adverse changes in the spores' inner membrane, cell wall, cortex, and nucleic acid. Therefore, the combination of HPTT and PS has a stronger inactivation effect and can be suggested as a promising strategy for the inactivation of Bacillus subtilis spores.

Living microecological hydrogels for wound healing

Chronic hard-to-heal wounds draw great attention worldwide, as their treatments are limited by infections and hypoxia. Inspired by the natural oxygen production capacity of algae and the competitive advantage of beneficial bacteria over other microbes, we presented a living microecological hydrogel (LMH) with functionalized Chlorella and Bacillus subtilis encapsulation to realize continuous oxygen delivery and anti-infections for promoting chronic wound healing. As the hydrogel consisted of thermosensitive Pluronic F-127 and wet-adhesive polydopamine, the LMH could keep liquid at a low temperature while quickly solidifying and tightly adhering to the wound bed. It was demonstrated that by optimizing the proportion of the encapsulated microorganism, the Chlorella could continuously produce oxygen to relieve hypoxia and support the proliferation of B. subtilis, while B. subtilis could eliminate the colonized pathogenic bacteria. Thus, the LMH substantially promoted the healing of infected diabetic wounds. These features make the LMH valuable for practical clinical applications.

Microdomains and stress distributions in bacterial monolayers on curved interfaces

Monolayers of growing non-motile rod-shaped bacteria act as active nematic materials composed of hard particles rather than the flexible components of other commonly studied active nematics. The organization of these granular monolayers has been studied on flat surfaces but not on curved surfaces, which are known to change the behavior of other active nematics. We use molecular dynamics simulations to track alignment and stress in growing monolayers fixed to curved surfaces, and investigate how these vary with changing surface curvature and cell aspect ratio. We find that the length scale of alignment (measured by average microdomain size) increases with cell aspect ratio and decreases with curvature. Additionally, we find that alignment controls the distribution of extensile stresses in the monolayer by concentrating stress in negative-order regions. These results connect active nematic physics to bacterial monolayers and can be applied to model bacteria growing on droplets, such as oil-degrading marine bacteria.

eDNA Provides a Scaffold for Autoaggregation of B. subtilis in Bacterioplankton Suspension

The self-binding of bacterial cells, or autoaggregation, is, together with surface colonization, one of the first steps in the formation of a mature biofilm. In this work, the autoaggregation of B. subtilis in dilute bacterial suspensions was studied. The dynamics of cell lysis, eDNA release, and bacterial autoaggregate assembly were determined and related to the spatial autocorrelation of bacterial cells in dilute planktonic bacterial suspensions. The non-random distribution of cells was associated with an eDNA network, which stabilized the initial bacterial cell-cell aggregates. Upon the addition of DNase I, the aggregates were dispersed. The release of eDNA during cell lysis allows for the entrapment of bacterial drifters at a radius several times the size of the dying bacteria. The size of bacterial aggregates increased from 2 to about 100 μm in diameter in dilute bacterial suspensions. The results suggest that B. subtilis cells form previously unnoticed continuum of autoaggregate structures during planktonic growth.

Nutrient Availability and Biofilm Polysaccharide Shape the Bacillaene-Dependent Antagonism of Bacillus subtilis against Salmonella Typhimurium

Salmonella enterica is one of the most common foodborne pathogens and, due to the spread of antibiotic resistance, new antimicrobial strategies are urgently needed to control it. In this study, we explored the probiotic potential of Bacillus subtilis PS-216 and elucidated the mechanisms that underlie the interactions between this soil isolate and the model pathogenic strain S. Typhimurium SL1344. The results reveal that B. subtilis PS-216 inhibits the growth and biofilm formation of S. Typhimurium through the production of the pks cluster-dependent polyketide bacillaene. The presence of S. Typhimurium enhanced the activity of the PpksC promoter that controls bacillaene production, suggesting that B. subtilis senses and responds to Salmonella. The level of Salmonella inhibition, overall PpksC activity, and PpksC induction by Salmonella were all higher in nutrient-rich conditions than in nutrient-depleted conditions. Although eliminating the extracellular polysaccharide production of B. subtilis via deletion of the epsA-O operon had no significant effect on inhibitory activity against Salmonella in nutrient-rich conditions, this deletion mutant showed an enhanced antagonism against Salmonella in nutrient-depleted conditions, revealing an intricate relationship between exopolysaccharide production, nutrient availability, and bacillaene synthesis. Overall, this work provides evidence on the regulatory role of nutrient availability, sensing of the competitor, and EpsA-O polysaccharide in the social outcome of bacillaene-dependent competition between B. subtilis and S. Typhimurium. IMPORTANCE Probiotic bacteria represent an alternative for controlling foodborne disease caused by Salmonella enterica, which constitutes a serious concern during food production due to its antibiotic resistance and resilience to environmental stress. Bacillus subtilis is gaining popularity as a probiotic, but its behavior in biofilms with pathogens such as Salmonella remains to be elucidated. Here, we show that the antagonism of B. subtilis is mediated by the polyketide bacillaene and that the production of bacillaene is a highly dynamic trait which depends on environmental factors such as nutrient availability and the presence of competitors. Moreover, the production of extracellular polysaccharides by B. subtilis further alters the influence of these factors. Hence, this work highlights the inhibitory effect of B. subtilis, which is condition-dependent, and the importance of evaluating probiotic strains under conditions relevant to the intended use.

Experimental challenges in determining the rheological properties of bacterial biofilms

Bacterial biofilms are communities living in a matrix consisting of self-produced, hydrated extracellular polymeric substances. Most microorganisms adopt the biofilm lifestyle since it protects by conferring resistance to antibiotics and physico-chemical stress factors. Consequently, mechanical removal is often necessary but rendered difficult by the biofilm's complex, viscoelastic response, and adhesive properties. Overall, the mechanical behaviour of biofilms also plays a role in the spreading, dispersal and subsequent colonization of new surfaces. Therefore, the characterization of the mechanical properties of biofilms plays a crucial role in controlling and combating biofilms in industrial and medical environments. We performed in situ shear rheological measurements of Bacillus subtilis biofilms grown between the plates of a rotational rheometer under well-controlled conditions relevant to many biofilm habitats. We investigated how the mechanical history preceding rheological measurements influenced biofilm mechanics and compared these results to the techniques commonly used in the literature. We also compare our results to measurements using interfacial rheology on bacterial pellicles formed at the air-water interface. This work aims to help understand how different growth and measurement conditions contribute to the large variability of mechanical properties reported in the literature and provide a new tool for the rigorous characterization of matrix components and biofilms.

Kin Discrimination Modifies Strain Distribution, Spatial Segregation, and Incorporation of Extracellular Matrix Polysaccharide Mutants of Bacillus subtilis Strains into Mixed Floating Biofilms

Microorganisms in nature form multicellular groups called biofilms. In biofilms, bacteria embedded in the extracellular matrix (ECM) interact intensely due to their proximity. Most studies have investigated genetically homogeneous biofilms, leaving a gap in knowledge on genetically heterogeneous biofilms. Recent insights show that a Gram-positive model bacterium, Bacillus subtilis, discriminates between strains of high (kin) and low (nonkin) genetic similarity, reflected in merging (kin) and boundaries (nonkin) between swarms. However, it is unclear how kinship between interacting strains affects their fitness, the genotype assortment, and incorporation of the mutant lacking the main structural ECM polysaccharide (EpsA-O) into floating biofilms (pellicles). We cultivated Bacillus subtilis strains as mixtures of isogenic, kin, and nonkin strain combinations in the biofilm-promoting minimal medium under static conditions, allowing them to form pellicles. We show that in nonkin pellicles, the dominant strain strongly reduced the frequency of the other strain. Segregation of nonkin mixtures in pellicles increased and invasion of nonkin EpsA-O-deficient mutants into pellicles decreased compared to kin and isogenic floating biofilms. Kin and isogenic strains had comparable relative frequencies in pellicles and showed more homogenous cell mixing. Overall, our results emphasize kin discrimination as a social behavior that shapes strain distribution, spatial segregation, and ECM mutant ability to incorporate into genetically heterogenous biofilms of B. subtilis. IMPORTANCE Biofilm communities have beneficial and harmful effects on human societies in natural, medical, and industrial environments. Bacillus subtilis is a biotechnologically important bacterium that serves as a model for studying biofilms. Recent studies have shown that this species engages in kin discriminatory behavior during swarming, which may have implications for community assembly, thus being of fundamental importance. Effects of kin discrimination on fitness, genotype segregation, and success of extracellular matrix (ECM) polysaccharide (EpsA-O) mutant invasion into biofilms are not well understood. We provide evidence that kin discrimination depends on the antagonism of the dominant strain against nonkin by using environmental strains with determined kin types and integrated fluorescent reporters. Moreover, this antagonism has important implications for genotype segregation and for when the bacteria are mixed with ECM producers. The work advances the understanding of kin-discrimination-dependent bacterial sociality in biofilms and its role in the assembly of multicellular groups.

Systematic microscopical analysis reveals obligate synergy between extracellular matrix components during Bacillus subtilis colony biofilm development

Single-species bacterial colony biofilms often present recurring morphologies that are thought to be of benefit to the population of cells within and are known to be dependent on the self-produced extracellular matrix. However, much remains unknown in terms of the developmental process at the single cell level. Here, we design and implement systematic time-lapse imaging and quantitative analyses of the growth of Bacillus subtilis colony biofilms. We follow the development from the initial deposition of founding cells through to the formation of large-scale complex structures. Using the model biofilm strain NCIB 3610, we examine the movement dynamics of the growing biomass and compare them with those displayed by a suite of otherwise isogenic matrix-mutant strains. Correspondingly, we assess the impact of an incomplete matrix on biofilm morphologies and sessile growth rate. Our results indicate that radial expansion of colony biofilms results from the division of bacteria at the biofilm periphery rather than being driven by swelling due to fluid intake. Moreover, we show that lack of exopolysaccharide production has a negative impact on cell division rate, and the extracellular matrix components act synergistically to give the biomass the structural strength to produce aerial protrusions and agar substrate-deforming ability.