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    Enhancing the high-temperature resistance of self-healing bio-cementitious composites using tea waste as a bacterial carrier
    (Elsevier, 2025) Yildirim, Musa; Ozhan, Hacer Bilir; Oz, Hilal Girgin; Ogut, Hamdi
    Bacterial composites exhibit advanced self-healing capabilities; however, their effectiveness is often constrained by production processes and environmental conditions. To enhance bacterial viability, protective carriers are required, with natural fibers recently utilized for this purpose. Fiber reinforcement has been shown to improve self-healing efficiency by limiting crack propagation. This study investigates the potential of tea waste as a bacterial carrier in cementitious composites. Bacillus megaterium spores were absorbed into tea waste and incorporated into mortar specimens at varying concentrations. The durability of bacterial composites under hightemperature exposure, a critical yet underexplored aspect, was also evaluated. Mortar specimens containing bacterial tea waste were subjected to different high-temperature conditions in both undamaged and pre-cracked states, followed by compressive strength assessments. Post-exposure microstructural changes were analysed via scanning electron microscopy (SEM). The findings demonstrated that tea waste effectively functioned as a bacterial carrier, exhibiting behaviour comparable to natural fibers. Additionally, it contributed to enhanced residual strength by mitigating thermal stress and promoting calcite precipitation, facilitating damage repair. These results highlight the potential of tea waste as a sustainable and effective medium for improving the durability of bacterial composites against high-temperature effects.
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    Optimization of Bacterial Cellulose Production by Komagataeibacter rhaeticus K23
    (Mdpi, 2024) Ugurel, Ceyda; Ogut, Hamdi
    The use of bacterial cellulose (BC), having high purity, a high degree of crystallinity, water-holding capacity, tensile strength and adaptability on a broad scale is limited because of the low yield. In this study, the optimal conditions for bio-cellulose production by Komagataeibacter rhaeticus K23 were investigated. Optimal values for temperature, pH, inoculum concentration and incubation time were determined via Taguchi design. The maximum BC production, 9.1 +/- 0.66 gL-1 (dry weight), was obtained from 32 degrees C, pH 5.5, 8 log CFUmL(-1) and 14 days of incubation. The inoculum concentration was the most significant factor affecting BC yield. A value of 8 log CFUmL(-1) and 14 days of incubation led to significantly higher levels of BC yield than other concentrations (8.5, 9, 9.5, 10 and 10.5 log CFUmL(-1)) (p < 0.002) and days (15, 16, 17, 21 and 28) (p < 0.001). The studied features, namely absorption peaks (Fourier transform infrared spectroscopy), pattern and the crystallinity index (X-ray diffraction analysis) of the BC obtained in this study were all in parallel with the characteristics of cellulose I. The study demonstrates that optimized parameters were effective in producing BC with high water-holding capacity, tensile strength, elongation and Young's modulus (mechanical tests) by K. rhaeticus K23.
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    Optimizing hyaluronic acid production by Streptococcus zooepidemicus using taguchi method: effects of temperature and pH
    (Springer, 2025) Oz, Hilal Girgin; Ogut, Hamdi
    The cost-effective and high-yield production of hyaluronic acid (HA) by microbial means remains challenging, necessitating the optimization of existing processes through the implementation of novel approaches. The present study investigates the production of HA under varying temperature and pH conditions utilizing independent, fully controlled fermenters. The synthesis of HA was evaluated at four temperatures (32 degrees C, 35 degrees C, 37 degrees C, 40 degrees C) and four pH levels (6.5, 7.0, 7.5, and 8.0), with optimal parameters identified through the Taguchi design methodology. The Taguchi optimization method effectively identified 37 degrees C and pH 6.5 as the optimal conditions for HA production corresponding to the highest signal-to-noise (S/N) ratios of 58.18 and 57.55, respectively. These conditions resulted in a maximum yield of 1.08 g.L-1, demonstrating the efficacy of this parameter combination in maximizing production efficiency. The carbazole method was utilized to quantify the production of HA following a five-hour fermentation period, with the culture conditions subjected to a statistical comparison. A temperature of 37 degrees C yielded significantly higher HA levels than 32 degrees C and 40 degrees C (Dunn test, respectively p = 0.019, p = 0.001). The lowest HA production was observed at 40 degrees C, while the 32 degrees C group exhibited relatively low variation in HA production. Furthermore, the hourly bacterial counts demonstrated a direct correlation between bacterial proliferation and HA synthesis, with the highest bacterial growth observed at 37 degrees C and pH 7.0. This study highlights the trends in HA concentration under different temperature and pH conditions during the pre-fermentation phase, offering critical insights for optimizing HA production.
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    Repair of Cracks in Concrete with the Microbial-Induced Calcite Precipitation (MICP) Method
    (Sciendo, 2023) Ozhan, Hacer Bilir; Yildirim, Musa; Ogut, Hamdi; Oz, Hilal Girgin
    In this study, the microbiologically-induced calcium carbonate precipitation (MICP) method was employed to examine its potential for repairing cracks in concrete. In addition, specific gravity and porosity values were measured to examine the effect of calcite formations on concrete surfaces and microstructures. Bacteria-supplemented concrete repaired cracks up to 0.4 mm wide by filling them with CaCO3. Furthermore, this study not only examined the healing of the width but also the length of cracks. However, as the width of the treated cracks decreased, their length increased. This indicated that the MICP treatment is more effective in a limited crack range. Specific gravity values increased, and porosity values decreased in concrete supplemented with calcifying bacteria. SEM analyses showed that calcite is a bacterial product that forms a very tight bond with a cement gel and that calcite fills visible cracks and voids and creates more of a void-free and undamaged concrete structure.
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    Utilization of tea waste as a bacterial carrier in self-healing mortars
    (Springernature, 2025) Yildirim, Musa; Ozhan, Hacer Bilir; Oz, Hilal Girgin; Ogut, Hamdi
    The protection of bacteria in self-healing composites is crucial for an effective healing process. However, materials used for protection often require advanced techniques and incur additional costs. This study investigates the use of tea waste as a bacterial carrier and natural fibre reinforcement in mortar. Tea waste, a fibrous by-product with high water absorption capacity, was used as a bacterial carrier for Bacillus megaterium spores. Mortar samples with different amounts of tea waste containing bacteria were evaluated through crack healing analysis, compressive strength, water absorption, ultrasonic pulse velocity (UPV), capillarity, and microstructural analyses. Additionally, the healing efficiency of the mortars was investigated using pre-cracked specimens. The results showed that tea waste effectively absorbed and protected the bacterial spores. Due to its fibrous nature, tea waste restricted cracks and created a more favourable condition for self-healing. Bacterial mortars with tea waste repaired cracks up to 0.68 mm wide and exhibited a 26.49% increase in compressive strength compared to control samples after 90 days. Moisture from the tea waste improved the bacterial environment and promoted internal curing, leading to denser structures with higher UPV, reduced water absorption, and lower capillarity. Scanning electron microscopy analysis confirmed that tea waste supported bacterial calcite formation. FT-IR analysis demonstrated that tea waste was highly compatible with the bacterial mortar matrix. Consequently, the incorporation of tea waste led to a notable presence of bacterial calcite products within the mortar structure. The findings indicate that tea waste can be effectively utilized to enhance the self-healing capabilities of mortars.