Biodegradation of kraft-lignin by Bacillus sp. isolated from sludge of pulp and paper mill

Eight bacterial strains were isolated on kraft lignin (KL) containing mineral salt medium (L-MSM) agar with glucose and peptone from the sludge of pulp and paper mill. Out of these, ITRC-S8 was selected for KL degradation, because of its fast growth at highest tested KL concentration and use of various lignin-related low molecular weight aromatic compounds (LMWACs) as sole source of carbon and energy. The bacterium was identified by biochemical tests as Gram-positive, rod-shaped and non-motile. Subsequent 16S rRNA gene sequencing showed 95% base sequence homology and it was identified as Bacillus sp. In batch experiments, a decrease in pH was observed initially followed by an increase till it reached an alkaline pH, which did not alter the culture growth significantly. The bacterium reduced the colour and KL content of 500 mg l-¹ KL in MSM, in the presence of glucose and peptone, at pH 7.6, temperature 30°C, agitation of 120 rpm and 6 days of incubation by 65 and 37% respectively. Significant reduction in colour and KL content in subsequent incubations is indicative of a co-metabolism mechanism, possibly due to initial utilization of added co-substrates for energy followed by utilization of KL as a co-metabolic. The degradation of KL by bacterium was confirmed by GC-MS analysis indicating formation of several LMWACs such as t-cinnamic acid, 3, 4, 5-trimethoxy benzaldehyde and ferulic acid as degradation products, which were not present in the control (uninoculated) sample. This favours the idea of biochemical modification of the KL polymer to a single monomer unit.