Topography quantifications allow for identifying the contribution of parental strains to physical properties of co-cultured biofilms

Vertical growth dynamics of biofilms

During the biofilm life cycle, bacteria attach to a surface and then reproduce, forming crowded, growing communities. Many theoretical models of biofilm growth dynamics have been proposed; however, difficulties in accurately measuring biofilm height across relevant time and length scales have prevented testing these models, or their biophysical underpinnings, empirically. Using white light interferometry, we measure the heights of microbial colonies with nanometer precision from inoculation to their final equilibrium height, producing a detailed empirical characterization of vertical growth dynamics. We propose a heuristic model for vertical growth dynamics based on basic biophysical processes inside a biofilm: diffusion and consumption of nutrients and growth and decay of the colony. This model captures the vertical growth dynamics from short to long time scales (10 min to 14 d) of diverse microorganisms, including bacteria and fungi.

Tilt-induced polar order and topological defects in growing bacterial populations

Rod-shaped bacteria, such as Escherichia coli, commonly live forming mounded colonies. They initially grow two-dimensionally on a surface and finally achieve three-dimensional growth. While it was recently reported that three-dimensional growth is promoted by topological defects of winding number +1/2 in populations of motile bacteria, how cellular alignment plays a role in nonmotile cases is largely unknown. Here, we investigate the relevance of topological defects in colony formation processes of nonmotile E. coli populations, and found that both ±1/2 topological defects contribute to the three-dimensional growth. Analyzing the cell flow in the bottom layer of the colony, we observe that +1/2 defects attract cells and -1/2 defects repel cells, in agreement with previous studies on motile cells, in the initial stage of the colony growth. However, later, cells gradually flow toward -1/2 defects as well, exhibiting a sharp contrast to the existing knowledge. By investigating three-dimensional cell orientations by confocal microscopy, we find that vertical tilting of cells is promoted near the defects. Crucially, this leads to the emergence of a polar order in the otherwise nematic two-dimensional cell orientation. We extend the theory of active nematics by incorporating this polar order and the vertical tilting, which successfully explains the influx toward -1/2 defects in terms of a polarity-induced force. Our work reveals that three-dimensional cell orientations may result in qualitative changes in properties of active nematics, especially those of topological defects, which may be generically relevant in active matter systems driven by cellular growth instead of self-propulsion.

Wetting and Imbibition Characteristics of Pseudomonas fluorescens Biofilms Grown on Stainless Steel

This study aims to provide insights into biofilm resistance associated with their structural properties acquired during formation and development. On this account, the wetting and imbibition behavior of dehydrated Pseudomonas fluorescens biofilms grown on stainless steel electropolished substrates is thoroughly examined at different biofilm ages. A polar liquid (water) and a non-polar liquid (diiodomethane) are employed as wetting agents in the form of sessile droplets. A mathematical model is applied to appraise the wetting and imbibition performance of biofilms incorporating the evaporation of sessile droplets. The present results show that the examined biofilms are hydrophilic. The progressive growth of biofilms leads to a gradual increase of substrate surface coverage─up to full coverage─accompanied by a gradual decrease of biofilm surface roughness. It is noteworthy that just after 24 h of biofilm growth, the surface roughness increases about 6.7 times the roughness of the clean stainless steel surface. It is further found that the imbibition of liquid in the biofilm matrix is restricted only to the biofilm region under the sessile droplet. The lack of further capillary imbibition into the biofilm structure, beyond the droplet deposition region, implies that the biofilm matrix is not in the form of an extended network of interconnected micro/nanopores. All in all, the present results indicate a resilient biofilm structure to biocide penetration despite its hydrophilic nature.

In vitro effect of farnesol on planktonic cells and dual biofilm formed by Candida albicans and Escherichia coli

Many biofilm studies have focused on axial biofilms, however biofilms in nature and in vivo environment are multi-species. Farnesol is a sesquiterpene alcohol found in many essential oils. This study investigated the in vitro effects of farnesol on planktonic cells and biofilms of Candida albicans and Escherichia coli. The ultrastructural morphology of farnesol treated cells was evaluated by TEM. According to the XTT results, farnesol caused a significant decrease in metabolic activity and scanning electron microscope images confirmed a reduction in the preformed biofilm as a result of farnesol treatment for single species C. albicans and E. coli biofilms. Although farnesol has less effect on dual species biofilm compared to the single species biofilms, its effect on the dual biofilm was found to be stronger than amphotericin B or ampicillin. Further studies are needed to clarify the role of farnesol on fungal-bacterial biofilms.

Biofilm hydrophobicity in environmental isolates of Bacillus subtilis

Biofilms are communities of bacteria that are attached to a surface and surrounded by an extracellular matrix. The extracellular matrix protects the community from stressors in the environment, making biofilms robust. The Gram-positive soil bacterium Bacillus subtilis, particularly the isolate NCIB 3610, is widely used as a model for studying biofilm formation. B. subtilis NCIB 3610 forms colony biofilms that are architecturally complex and highly hydrophobic. The hydrophobicity is linked, in part, to the localisation of the protein BslA at the surface of the biofilm, which provides the community with increased resistance to biocides. As most of our knowledge about B. subtilis biofilm formation comes from one isolate, it is unclear if biofilm hydrophobicity is a widely distributed feature of the species. To address this knowledge gap, we collated a library of B. subtilis soil isolates and acquired their whole genome sequences. We used our novel isolates to examine biofilm hydrophobicity and found that, although BslA is encoded and produced by all isolates in our collection, hydrophobicity is not a universal feature of B. subtilis colony biofilms. To test whether the matrix exopolymer poly γ-glutamic acid could be masking hydrophobicity in our hydrophilic isolates, we constructed deletion mutants and found, contrary to our hypothesis, that the presence of poly γ-glutamic acid was not the reason for the observed hydrophilicity. This study highlights the natural variation in the properties of biofilms formed by different isolates and the importance of using a more diverse range of isolates as representatives of a species.