Abstract
ε-Caprolactam-a toxic xenobiotic compound present in industrial polyamide waste was found to be degraded by caprolactam-degrading bacteria. Arthrobacter citreus was able to utilize up to 20 g ε-caprolactam/l as the sole source of carbon more efficiently as compared to the other Gram positive caprolactam-degrading bacteria Rhodococcus rhodochrous and Bacillus sphaericus. The cells of A. citreus remained viable in medium up to 40 g caprolactam/l. The degradation of 10 g caprolactam/l by A. citreus, when supplied as the sole source of carbon and nitrogen lead to the formation of 6-aminocaproic acid which was detected in broth and there was also an increase in the ammonium content. One of the other metabolites found to consistently accumulate in extracellular medium during the utilization of caprolactam by A. citreus was glutamic acid, though not reported in case of other caprolactam-degrading bacteria. A. citreus could metabolise caprolactam to form non toxic products such as 6-aminocaproic acid and glutamic acid which are amino acids of physiological and commercial importance. In the presence of 6-aminocaproic acid, the rate of caprolactam utilization by A. citreus was decreased but not inhibited and the viable count of cells was found to increase using both the substrates simultaneously. A. citreus was also suitable for degradation of caprolactam in presence of low phosphate as prevalent in soil, and in sterile soil without the supplementation of any other carbon or nitrogen, as well as in native non sterile soil where other microorganisms are present.
Highlights
Introduction εCaprolactam is a man-made compound used as the monomer to produce the polymer nylon-6
The separation and identification of such incompletely synthesised oligomer components of the solid waste by Thin layer chromatography (TLC) showed a total of eight components (Fig. 1a), depending on polarity and pH of According to literature, most of the caprolactamdegrading bacteria reported are Gram negative for example Pseudomonas spp., Proteus sp., A. faecalis and others (Boronin et al 1986; Otzen et al 2018; Sanuth et al 2013; Baxi and Shah 2002) and few Gram positive caprolactamdegrading bacteria have been reported as compared to Gram negative bacteria
As compared to Gram negative bacteria, the more predominant members found in soil are the Gram positive bacteria (Maier and Pepper 2000) and such bacteria are more inherently resistant to stress in environment (Schimel et al 2007; Mongodin et al 2006)
Summary
Introduction εCaprolactam is a man-made compound used as the monomer to produce the polymer nylon-6. The main product obtained when bacteria degrade caprolactam is the corresponding linear compound: 6-aminocaproic acid which is converted by further metabolism to adipic acid and succinic acid. Caprolactam degradation has been studied by various workers using different bacteria either in media only or for effluent treatment, the degradation of caprolactam in oligomer waste and the in situ bioremediation of soil polluted with caprolactam has not been reported. In the present study we report a strain: Arthrobacter citreus suitable for biodegradation of caprolactam in media as the sole source of carbon and nitrogen in absence of additional nutrients and growth factors and even in soil where the level of nutrients is low. The present study shows the capability of A. citreus cells to degrade caprolactam in situ and its conversion to amino acids which can be assimilated further or accumulated under appropriate conditions
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