Bacterium Degrades Kraft Lignin and Cuts Its Toxicity in Lab Study

A bacterial strain has shown promise for breaking down kraft lignin, a recalcitrant byproduct of the pulp and paper industry that colors wastewater brown and contributes to the toxicity of effluents discharged into aquatic environments. In a study published in Annals of Microbiology, researchers report that Serratia liquefaciens, a ligninolytic bacterium, degraded kraft lignin by 53 percent over 120 hours in laboratory conditions, and that bacterial treatment reduced the compound’s phytotoxicity.

Kraft lignin is a complex aromatic polymer generated during pulping operations. According to the study’s background, its structure resists conventional wastewater treatment, which the authors state is unable to reduce its toxicity because the molecule is resistant to chemicals. Fungal systems have historically been used to degrade lignin, but the authors note that fungi become unstable in harsh environments, motivating the search for bacterial alternatives that are more environmentally adaptable and cost-effective while leaving low residues.

The research team characterized kraft lignin before and after bacterial treatment using instrumental analysis to confirm that biodegradation had occurred. During the degradation process, S. liquefaciens exhibited lignin peroxidase enzyme activity of 22 IU per milliliter, an enzymatic capability associated with the breakdown of lignin’s intricate chemical structure.

To maximize degradation efficiency, the researchers turned to process optimization. Previous studies had demonstrated that S. liquefaciens could be used for lignin biodegradation, but the complex structure of the polymer posed challenges. In the present study, the team optimized biodegradation conditions using Response Surface Methodology with a Box-Behnken design, a statistical approach for evaluating the effects of multiple process variables and identifying conditions that enhance degradation.

Toxicity was assessed through phytotoxicity testing using Vigna radiata L., the mung bean, a common plant bioassay organism. When seeds were exposed to undiluted kraft lignin solution at a concentration of 1000 milligrams per liter, sprout length was reduced by 85 percent compared to the control. After bacterial treatment, sprout length inhibition dropped to 60 percent, a measurable reduction in toxicity.

The authors attribute this detoxification to structural changes in the lignin molecule. Enzyme-treated kraft lignin exhibited weaker toxicity than untreated lignin because the enzymes degraded lignin’s complex structure, reducing the amount of toxic phenolic compounds present. Depolymerization and the loss of chemical bonds were associated with the observed toxicity reduction.

The findings suggest that S. liquefaciens has significant potential for treating kraft lignin and could be employed as a bioremediation application in the paper industry, according to the study’s conclusions. By modifying the lignin structure through biodegradation, the approach may also lead to more sustainable usage of a waste stream that currently poses environmental risks when discharged without proper treatment.

The study, published as an open access original article on 2 September 2026, opens up avenues to increase lignin utilization, the authors write. As with any laboratory-scale investigation, further work would be needed to translate these results into full-scale industrial wastewater treatment, but the combination of measurable degradation, statistical optimization and demonstrated toxicity reduction offers a bacterial route to managing one of the pulp and paper sector’s most persistent pollutants.

Subject of Research: Bacterial biodegradation of kraft lignin and assessment of its toxicity reduction

Article Title: Bacterial degradation of kraft lignin, its metabolites analysis, and toxicity assessment

Article References: Haq, I. (2026). Bacterial degradation of kraft lignin, its metabolites analysis, and toxicity assessment. Annals of Microbiology. https://doi.org/10.1186/s13213-026-01869-3

Image Credits: AI Generated

DOI: 10.1186/s13213-026-01869-3

Keywords: kraft lignin, Serratia liquefaciens, biodegradation, pulp and paper industry, lignin peroxidase, phytotoxicity, Vigna radiata, bioremediation, Response Surface Methodology, wastewater treatment, phenolic compounds, Annals of Microbiology