Date Published: February 8, 2016
Publisher: Springer Berlin Heidelberg
Author(s): G. V. Subba Reddy, M. Md. Rafi, S. Rubesh Kumar, N. Khayalethu, D. Muralidhara Rao, B. Manjunatha, G. H. Philip, B. R. Reddy.
A novel aerobic gram-negative bacterial strain capable of utilizing 2-hydroxyquinoxaline (2-HQ) as sole source of carbon and energy was isolated from Indian agricultural soil and named as HQ1. Strain HQ1 was identified as Ochrobactrum sp. on the basis of morphology, physico-biochemical characteristics and 16S rRNA sequence analysis. The generation time of Ochrobactrum sp. HQ1 on 2-HQ at log phase is 0.71 h or 42.6 min. The degradation of 2-HQ by HQ1 under various physico-chemical parameters was analysed by HPLC and observed to be optimum with a high inoculum density (1.0 OD) at pH 7–8, temperatures 37–40°C and a high concentration of 2-HQ (500 ppm). Degradation of 2-HQ was also improved when additional nitrogen sources were used and this was attributed to the enhanced growth of the bacterium on the readily available nitrogen sources. Analysis of 2-HQ degradation by GC–MS resulted in elucidation of the degradation pathway for HQ1, a novel observation for aerobic Gram-negative bacteria. These findings are a possible indication of the application of HQ1 in the bioremediation of pesticide/metabolite contamination.
Quinalphos is one of the major and most widely used organophosphorus insecticides in agriculture and undergoes microbial metabolism/chemical hydrolysis to form major metabolites—2-hydroxyquinoxaline (2-HQ) and diethyl thiophosphate/diethyl phosphate (Babu et al. 1998; Menon and Gopal 2003; Goncalves et al. 2006; Gupta et al. 2012; Kaur and Sud 2012; Talwar et al. 2014) in soil and water. Diethyl phosphate/diethyl thiophosphate, formed from number of organophosphates including quinalphos, parathion and chlorpyrifos, undergoes rapid mineralization because of it easy utilization by microorganisms as carbon and phosphorus source (Cook et al. 1978). 2-HQ is getting accumulated in environment because of its longer persistence and extensive use of quinalphos (Babu et al. 1998). Accumulation of 2-HQ in the environment poses a health hazard to animals and human because of 2-HQ is capable of destroying various biogenic amines such as serotonin, melatonin, N1-acetyl-5-methoxy kynuramine, dopamine, epinephrine and norepinephrine and inducing oxidative stress as reflected by generation of H2O2 (Hardeland et al. 2000). This metabolite has been observed to display toxicity to even aquatic organisms such as ciliates, dinoflagellates and rotifer (Behrends et al. 2004) and has also been reported to be genotoxic to Salmonella typhimurium in Ames test (Riediger et al. 2007). Toxicity is not totally eliminated with degradation of the parent compound quinalphos to 2-HQ in the environment. 2-HQ is least understood in terms of biodegradation among metabolites formed from organophosphates (Singh and Walker 2006; Caceres et al. 2010). As microbial agents involved in the biodegradation of this metabolite in the environment are highly useful in decontamination of polluted environment. The aim of the present study was to isolate bacteria that play a role in the degradation of 2-HQ and evaluate the degradation potential under various environmental factors.
The bacterial isolate was identified to be Ochrobactrum sp. HQ1 through the selective culture enrichment method, morphological, biochemical and 16S rRNA sequence analyses. The optimum environmental conditions for growth and degradation of 2-HQ were analyzed in shaking conditions and recorded as the inoculum density of (1.OD), pH (7–8), 37–40°C temperature and high concentration of 2-HQ (500 ppm). This is the first report on degradation of 2-HQ by aerobic Gram negative bacterium and elucidation of pathway. Based on this pathway Ochrobactrum sp. HQ1 is the best remedial source in treatment of contaminated environment with pesticides and their metabolites.