Presence of Calcium Lowers the Expansion of Bacillus subtilis Colony Biofilms.

Robust colony formation by Bacillus subtilis is recognized as one of the sessile, multicellular lifestyles of this bacterium. Numerous pathways and genes are responsible for the architecturally complex colony structure development. Cells in the biofilm colony secrete extracellular polysaccharides (EPS) and protein components (TasA and the hydrophobin BslA) that hold them together and provide a protective hydrophobic shield. Cells also secrete surfactin with antimicrobial as well as surface tension reducing properties that aid cells to colonize the solid surface. Depending on the environmental conditions, these secreted components of the colony biofilm can also promote the flagellum-independent surface spreading of B. subtilis, called sliding. In this study, we emphasize the influence of Ca2+ in the medium on colony expansion of B. subtilis. Interestingly, the availability of Ca2+ has no major impact on the induction of complex colony morphology. However, in the absence of this divalent ion, peripheral cells of the colony expand radially at later stages of development, causing colony size to increase. We demonstrate that the secreted extracellular compounds, EPS, BslA, and surfactin facilitate colony expansion after biofilm maturation. We propose that Ca2+ hinders biofilm colony expansion by modifying the amphiphilic properties of surfactin.

Effects of oxygen plasma treatment on interfacial shear strength and post-peak residual strength of a PLGA fiber-reinforced brushite cement.

Biodegradable calcium phosphate cements (CPCs) are promising materials for minimally invasive treatment of bone defects. However, CPCs have low mechanical strength and fracture toughness. One approach to overcome these limitations is the modification of the CPC with reinforcing fibers. The matrix-fiber interfacial shear strength (ISS) is pivotal for the biomechanical properties of fiber-reinforced CPCs. The aim of the current study was to control the ISS between a brushite-forming CPC and degradable PLGA fibers by oxygen plasma treatment and to analyze the impact of the ISS alterations on its bulk mechanical properties. The ISS between CPC matrix and PLGA fibers, tested in a single-fiber pull-out test, increased up to 2.3-fold to max. 3.22±0.92MPa after fiber oxygen plasma treatment (100-300W, 1-10min), likely due to altered surface chemistry and morphology of the fibers. This ISS increase led to more efficient crack bridging and a subsequent increase of the post-peak residual strength at biomechanically relevant, moderate strains (up to 1%). At the same time, the work of fracture significantly decreased, possibly due to an increased proportion of fractured fibers unable to further absorb energy by frictional sliding. Flexural strength and flexural modulus were not affected by the oxygen plasma treatment. This study shows for the first time that the matrix-fiber ISS and some of the resulting mechanical properties of fiber-reinforced CPCs can be improved by chemical modifications such as oxygen plasma treatment, generating the possibility of avoiding catastrophic failures at the implant site and thus enhancing the applicability of biodegradable CPCs for the treatment of (load-bearing) bone defects.

Enhanced mechanical properties of a novel, injectable, fiber-reinforced brushite cement.

Injectable, brushite-forming calcium phosphate cements (CPCs) have great potential as bone replacement materials due to enhanced degradability and long-term inclusion in bone remodeling. However, the use of such brushite-forming CPCs in load-bearing areas is limited by their low mechanical strength. One approach to overcome this limitation is the use of reinforcing fibers. Thus, an injectable, biodegradable, brushite-forming CPC based on beta-tricalcium phosphate/phosphoric acid with fiber reinforcement was developed for minimally invasive surgery. The fibers (diameter 25 µm; length 0.25, 1 or 2mm) were extruded from poly(l-lactide-co-glycolide) acid (PLGA) and added to the CPC (2.5, 5 or 7.5% (w/w)). Independent of the fiber content, injectability of the CPC was retained up to a fiber length of 1mm. The addition of all PLGA fiber types increased diametral tensile strength, biaxial flexural strength, and flexural strength by up to 25% (p ≤ 0.05 for the diametral tensile strength for the CPC with 5% (w/w) 1mm fibers and the biaxial flexural strength of the CPC with 5% (w/w) 0.25 mm fibers). In contrast, the work of fracture strongly and significantly increased (p<0.01) by up to 12.5-fold. At constant fiber content, the mechanical properties of the fiber-reinforced CPC were mostly augmented with increasing fiber length. Also, the addition of PLGA fibers to the brushite-forming CPC (up to 7.5% (w/w)) only transiently delayed cell growth and did not decrease cell viability. Fiber reinforcement of CPCs thus augments their mechanical strength while preserving the injectability and biocompatibility required for their application in modern surgery.