Screening of Endophytic Bacteria from Azadirachta indica and Tridax procumbens for Waste-Engine-Oil-Associated Redox Activity: A DCPIP and Qualitative GC–MS Study

Background: Plant-associated endophytic bacteria are a potential source of microorganisms exhibiting oxidative responses to petroleum-derived substrates, but screening assays alone do not establish the mass of oil removed. Objective: To isolate bacterial endophytes from Azadirachta indica and Tridax procumbens, compare waste-engine-oil-associated redox responses, examine single-factor culture conditions, and describe qualitative gas chromatography–mass spectrometry (GC–MS) profiles. Methods: Twelve bacterial isolates were recovered, of which two were selected for morphological and biochemical characterisation and screening with the redox dye 2,6-dichlorophenolindophenol (DCPIP). The selected isolates were examined on mineral-salt agar containing waste engine oil. Isolate I underwent 16S rRNA gene analysis and carbon-source, temperature, and pH comparisons; treated and untreated oil samples were examined by GC–MS. Results: Within 72 h, the initial and final DCPIP absorbances were 0.332 and 0.047 for isolate I and 0.531 and 0.107 for isolate II, corresponding to unadjusted decreases in dye absorbance of 85.84% and 79.85%, respectively. These values are not percentages of oil degraded. Isolate I showed 99.86% sequence similarity to Kytococcus sedentarius. The single-factor tables ranked glucose, ambient incubation and pH 7 highest on the reported assay-response index. Comparative GC–MS peak lists showed differing relative profiles and tentative oxygenated-compound annotations, without demonstrating absolute compound loss or metabolic provenance. The final uninoculated-control value and assay-level replication were not documented, preventing attribution of these optical changes exclusively to oil-associated microbial metabolism. Conclusion: The observations justify further testing of a neem-associated isolate closely related to K. sedentarius as a candidate for hydrocarbon-related microbial activity. The extent of waste-engine-oil biodegradation, mineralisation, and environmental effectiveness remains undetermined. The experiments are exploratory and do not establish statistically supported differences between isolates.