The motivation behind the work is stark. The textile sector is one of the world’s largest polluters, accounting for roughly 20 percent of global wastewater, with dyeing processes alone consuming between 30 and 50 liters of water per kilogram of fabric. Synthetic dyes, particularly azo and nitro compounds, are notoriously resistant to natural degradation, and more than 280,000 tons of them are released into water bodies every year. Their persistence causes reduced sunlight penetration, impaired oxygen exchange, ecological disruption and potential carcinogenic risks. Microbial pigments, by contrast, are biodegradable, non-toxic, water soluble and can be produced by fermenting cheap agro-waste substrates in minimal space, making them an attractive candidate for greener coloration.

The research team had previously identified six pigment-producing bacterial isolates from environmental samples using MALDI-TOF mass spectrometry. Three of these proved to be non-pathogenic strains: Micrococcus luteus, Exiguobacterium aurantiacum and Kocuria rosea. When cultivated on optimized agro-waste extracts, all three produced yellowish pigments, but with markedly different yields of 1.47, 0.96 and 0.56 grams per liter respectively. The authors suggest that the yield differences reflect inherent genetic and biochemical variation between the isolates, and note that bacteria from harsher environments may produce more pigment as a stress response. The similar coloration across three distinct species may represent functional convergence, perhaps tied to shared ecological roles such as ultraviolet protection.

Because Micrococcus luteus delivered the highest yield and the strongest growth in orange waste extract, its pigment was selected for detailed analysis. Among the organic solvents tested, methanol proved the most efficient extraction medium, outperforming previous reports of bacterial pigment yields from agro-industrial substrates. Ultraviolet-visible spectroscopy revealed a characteristic absorption peak at 476 nanometers, squarely within the 400 to 550 nanometer range typical of carotenoids, the family of conjugated molecules responsible for many yellow, orange and red hues in nature. Liquid chromatography-mass spectrometry confirmed molecular diversity among the three pigments, with distinct mass-to-charge ratios indicating structural variation despite their similar appearance.

Stability testing revealed a pigment with clear vulnerabilities. Exposure to temperatures ranging from 20 to 80 degrees Celsius caused a progressive decline in absorbance and visible fading of the yellow color, a pattern consistent with thermal degradation of the conjugated double-bond system that gives carotenoids their color. Regression analysis identified temperature as a statistically significant predictor of pigment stability, with a coefficient of determination of 0.68, and pinpointed 40 degrees Celsius as the optimal temperature for absorbance. Similarly, increasing concentrations of hydrogen peroxide produced steady oxidative degradation, with a strikingly strong regression fit of 0.89 and a p-value of 0.0013. By contrast, the pigment showed no observable color change when exposed to glucose as a reducing agent, demonstrating high tolerance to reducing conditions.

The pH story was more nuanced. Absorbance increased as conditions approached neutrality, rising from 0.127 in acidic environments to 0.196, then falling to 0.118 under alkaline conditions. Although analysis of variance confirmed that pH significantly affected absorbance, with nearly half of pairwise comparisons showing significant differences, regression analysis revealed that pH alone was a poor linear predictor of absorbance, with a weak coefficient of determination of just 0.087. The authors interpret this non-linear relationship as evidence that buffering effects or molecular stability mechanisms may dominate pigment behavior, and note that this isolate displays a narrower pH tolerance than some previously studied carotenoid producers.

With the stability profile established, the team turned to dyeing. Five-gram samples of 100 percent cotton and Tetron 6000, a blend of 65 percent polyester and 35 percent cotton, were pre-treated with aluminum sulfate and potassium sulfate mordants at 60 degrees Celsius for 45 minutes to enhance dye fixation. The pigment was dissolved in methanol at 3 percent concentration and applied in dye baths at a liquor ratio of 1:20, with fabrics soaked at 25 degrees Celsius, gradually heated to 60 degrees Celsius and held there for 30 minutes with continuous stirring. The process produced vibrant, uniform shades on both fabrics, demonstrating that the pigment can be applied through standard immersion dyeing equipment.

Quantitative analysis showed that cotton absorbed significantly more pigment than the polyester blend, with dye exhaustion of 75 percent versus 65 percent and fixation of 54.6 percent versus 37 percent, differences that were statistically significant. The authors attribute cotton’s superior performance to its hydrophilic cellulose structure, which bonds more readily with the microbial pigment, while the synthetic surface of Tetron 6000 resists fixation. Compared with synthetic dye benchmarks of 84 percent exhaustion and 79 percent fixation reported in the literature, the natural pigment’s performance was lower, a trade-off the researchers acknowledge between environmental safety and durability. Washing with detergent and prolonged sunlight exposure caused noticeable fading on both fabrics, with the effect more pronounced on the polyester blend.

The study’s conclusions are measured. The Micrococcus luteus pigment demonstrated moderate stability and dyeing performance, making it a plausible candidate for further scale-up investigation rather than an immediate industrial replacement. The authors caution that their laboratory-scale experiments, and the visual color fastness assessments that could introduce observer bias, are not sufficient to conclude industrial applicability without successful scale-up to industrial equipment. They also note that future work should screen more potent chromogenic bacteria for higher yields, conduct detailed quantitative dye uptake studies, and investigate stabilization techniques to improve color fastness and durability.

Even so, the research adds to a growing body of evidence that bacteria cultivated on food waste could help decolorize one of the world’s dirtiest industries. Advances in fermentation technology, nanoparticle-assisted pigment binding and low-liquor-ratio dyeing are steadily improving the economics of microbial coloration, and pigments like this one may eventually deliver added value beyond color, since some bacterial pigments confer antimicrobial properties to finished textiles. For now, the humble yellow pigment of Micrococcus luteus stands as a vivid demonstration that the future of sustainable fashion may be growing, quite literally, in the soil.

Subject of Research: Stability and textile dyeing performance of a carotenoid pigment produced by the bacterium Micrococcus luteus

Article Title: Stability and dyeing performance of Micrococcus luteus pigment on cotton and polyester fabrics

Article References: Stability and dyeing performance of Micrococcus luteus pigment on cotton and polyester fabrics. (n.d.). https://doi.org/10.1007/s44340-025-00038-7

Image Credits: AI Generated

DOI: 10.1007/s44340-025-00038-7

Keywords: Micrococcus luteus, bacterial pigment, carotenoid, natural dye, textile dyeing, cotton, polyester, pigment stability, eco-friendly dye, color fastness, dye exhaustion, sustainable textiles