Abdul Wahab Khan
Panaji
Researchers at the Birla Institute of Technology and Science (BITS) Pilani, K K Birla Goa campus, have found that a naturally occurring bacterium can partially break down highly vulcanised waste tyre rubber, offering a potential approach to managing one of the
most persistent forms of solid waste.
The study, published in RSC Advances on July 27 identified chemical changes in rubber and provided evidence that bacterial enzymes can break the polymer chains within discarded tyres.
The research, titled ‘Biodegradation of vulcanised waste tire rubber by Gordonia polyisoprenivorans Kd2: mechanistic insights and ecological context from indigenous rubber-degrading communities,’ was conducted by Pritish Raj Shukla, Utpal Roy, Sunil Bhand and Vivek Rangarajan. The researchers are affiliated with the Departments of Biological Sciences, Chemistry and Chemical Engineering at BITS Pilani’s Goa campus.
The study examined whether Gordonia polyisoprenivorans Kd2 could degrade real-world, highly vulcanised ground tyre rubber, how particle size affected bacterial activity and what chemical changes occurred within the rubber network. It also investigated the microbial communities present at waste tyre dumping sites in Goa.
The team tested two ground tyre rubber fractions — 3-4 mm and 400-600 micrometres — obtained from a recycling facility in Hyderabad. The bacterial strain, Kd2, was sourced from the DSMZ microbial culture collection in Germany. The rubber was treated with ethanol, dried and added to a mineral salts medium containing the bacterial strain. The cultures were incubated at 30°C for 28 days.
Bacterial growth was monitored through optical density and colony-forming unit counts. The team also measured rubber weight loss and conducted Fourier-transform infrared spectroscopy, scanning electron microscopy, elemental analysis, solvent-swelling tests and thermogravimetric analysis. Horikx analysis was used to investigate the mechanism of polymer breakdown.
To examine indigenous microbial communities, soil was collected
from two waste tyre dumping sites in Goa.
The samples underwent 90 days of enrichment in a rubber-based medium before 16S rRNA sequencing. Shotgun metagenomic analysis and KEGG pathway mapping were also used to investigate rubber-degradation genes and metabolic pathways. The authors proposed
that combining rubber-degrading bacteria with complementary microbial partners could improve degradation efficiency.
The smaller rubber particles supported substantially greater bacterial growth, with viable cell counts increasing by 2.49 × 10³-fold. The bacterial population reached 2.06 × 10¹¹ CFU per millilitre by day 16, compared with 8.29 × 10⁷ CFU per millilitre on the larger particles at day 24.
After 28 days, the smaller particles recorded a maximum weight loss of 5.80%, compared with 1.80% for the larger fraction and 1.40% in the abiotic control. The researchers cautioned that mass loss alone does not capture all chemical changes within the rubber.
Chemical analysis revealed surface oxidation and bacterial attachment, along with the formation of micropits. Sulphur and zinc concentrations on the treated rubber surface declined by 30.23% and 34.02%, respectively.
The crosslink density of the rubber decreased by 10.54%, from 9.58 × 10⁻³ to 8.57 × 10⁻³ mol cm⁻³. The soluble fraction increased from 2.73% to 5.51%.
The microbial survey found that Actinomycetota dominated one of the site communities, accounting for 42.9-44.6% of the reads. Nocardia was the most abundant genus across the samples, while Gordonia was also present.
The study identified the Lcp rubber-cleaving enzyme gene, K23117, in the enriched community.
The researchers concluded that pretreatment and mixed microbial cultures could improve degradation rates.
