Harnessing Chromogenic Bacteria for Eco-Friendly Textile Dyeing: Pigment Production and Process Optimization

Abstract

Microbial pigments are emerging as sustainable alternatives to synthetic dyes due to their environmental compatibility and potential for circular bio-economy applications. This study focused on the identification of potent pigment-producing bacterial isolates, the use of agro waste substrates as low-cost substrates, and the optimization of culture parameters to enhance pigment yield. The study systematically explored pigment-producing bacterial isolates from environmental samples, employing morphological, biochemical, and MALDI-TOF analyses for identification. Eco-physiological tolerance was assessed using the one-variable-at-a-time approach, while culture optimization was conducted through Plackett–Burman Design followed by response surface methodology. Agro-waste extracts as low-cost substrates were evaluated via optical density measurements, and pigments were extracted using organic solvents. Pigments were characterized using UV-Vis spectroscopy, LC-MS, and ATR-FTIR. The pigments' stability under varying stressor conditions was assessed by monitoring absorbance at their characteristic wavelengths. Fabrics dyeing were conducted using immersion technique. Dye exhaustion and fixation to fabrics were evaluated via absorbance method after washing and exposure to sunlight. From purified colonies on streak plates, six isolates; namely: Acinetobacter sp., Exiguobacterium aurantiacum, two Kocuria spp. (from different samples), Micrococcus luteus, and one unidentified strain, exhibited chromogenic potential. Among the chromogenic isolates screened, Micrococcus luteus exhibited the highest pigment yield (1.47 g/L) under orange-waste extract cultivation, likely due to the isolate's inherent carotenoid biosynthetic capacity, stress tolerance, and efficient substrate utilization. Optimization of pigment production by Exiguobacterium aurantiacum using tomato-waste extract revealed that agitation rate, pH, and yeast extract concentrations were the most significant factors influencing culture growth (p<0.001), leading to 1.6-fold yield increment compared to un-optimized conditions. The characteristic UV-Visible absorbance peaks (400 550 nm) indicated the pigments are likely carotenoid compounds with functional group signatures (O–H, C–H, C=C, C=O) from infrared spectra analyses. The LC-MS analyses further resolved pigment mixtures, provided molecular weights and fragmentation patterns, indicating pigment heterogeneity. Stability assays of Micrococcus luteus pigment indicated susceptibility to oxidative and thermal stress, while dyeing trials demonstrated vibrant and uniform coloration with moderate fixation on fabrics. While microbial pigment production has been widely reported, existing studies predominantly rely on synthetic media. The present study is fundamentally novel, integrating systematic screening, agro-waste valorization, and multivariate statistical optimization, marking the first such effort in Ethiopia. The findings highlight Micrococcus luteus as a robust candidate for scalable, eco-friendly pigment production under low-cost sustainable culture conditions. The findings highlight the feasibility of low-cost, eco-friendly microbial pigments for industrial applications as promising alternatives to synthetic dyes.