Plasmid Encoded Toluene and Xylene Degradation by Phyllosphere Bacteria

Abstract

Air pollution by monoaromatic hydrocarbons (MAH) is a highly concerned great threat in modern world due to the high carcinogenicity and genotoxicity to all living beings. Most of these MAH releasing processes are oil refining processes and vehicular emission. These air trapped pollutants deposit on ground level and phyllosphere takes special place as a ground level exposure surface for these pollutants. Continuous deposition lead to make a MAH degrading microbial consortium in the phyllosphere and these microorganisms can be used as an efficient bioremediators in remediating MAH contaminants which is an environmental friendly solution compared to chemical remediations. The phyllosphere of plant species Ixora chinensis, Ervatamia divaricata, Hibiscus rosa-sinensis and Amaranthus cruentus which are highly abundant along the roadsides of polluted areas in Sri Lanka are rich with several species of bacteria belong to many genera and they were able to degrade toluene and xylene efficiently. The species of Alcaligenes feacalis, Alcaligenes DN25, Bacillus cereus and Bacillus methylotrophicus were able to degrade toluene and xylene efficiently. All these strains harbor plasmids conferring them resistance to ampicillin. Curing of the plasmids of A. feacalis and Alcaligenes sp. DN25 drastically reduced the ability in degrading these toluene and xylene. Upon transformation of plasmids of these two Alcaligenes sp. into E. coli JM109 enabled it to degrade the two hydrocarbons efficiently. But transformation and curing process of two Bacillus sp. into the E. coli JM109 was unsuccessful. Plasmid encoded toluene and xylene degradation of two Alcaligenes sp. suggested the presence of required catabolic genes in these plasmids. PCR amplification with degenerate primers and comparison of their nucleotide sequences with Genbank sequences indicated that plasmids of A. feacalis, Alcaligenes sp. DN25 and B. cereus harbor the genes xylQ involved in toluene and xylene degradation. RFLP and nucleotide sequence comparisons of xylQ amplicons revealed that both of these genes in two bacterial strains (A. feacalis and Alcaligenes sp. DN25) are homologous. But that was heterologous to the xylQ gene of B. cereus. Hence the results clearly showed the potential of A. feacalis, Alcaligenes sp. DN25 and B. cereus in degrading toluene and xylene and also the potential of using them in remediating aromatic hydrocarbonic contaminants.