Almost daily, there are novel bacteria being reported in the literature which have been shown to have the capacity to degrade certain xenobiots. Presumably the mutation which occurred during the evolution of the organism conferred an advantage, and selective pressure maintained that mutation in the DNA, thus pro-ducing a novel strain with an altered phenotype. Some example of such isolates are described here. Reference has already been made to some PAHs mimicking oestrogen which earns those chemicals the title of ‘endocrine disrupters’. This is in addition to some being toxic for other reasons and some being carcino-genic or teratogenic. The PAHs are derived primarily from the petrochemicals industry and are polycyclic hydrocarbons of three or more rings which include as members, naphthalene and phenanthrene and historically have been associated with offshore drilling, along with alkylphenols. Several genera of bacteria are now known to be able to degrade PAHs and recently, a novel strain of Vibriocyclotrophicus able to digest naphthalene and phenanthrene, was isolated fromcreosote-contaminated marine sediments from Eagle Harbour, Washington, USA. It would appear that bacteria isolated from the same marine or estuarine envi-ronments may vary quite considerably in their abilities to degrade certain PAHs. This observation is viewed as indicative of diverse catabolic pathways demon-strated by these organisms and awaiting our full understanding (Hedlund and Staley 2001).
Polycyclic hydrocarbons (PCBs) are xenobiotics which, due to their high level of halogenation, are substrates for very few pathways normally occurring in nature. However, a strain of Pseudomonas putida able to degrade PCBs, was isolated recently from wastewater outflow from a refinery. This was achieved by the bacterium employing two pathways encoded by two separate operons; the tod pathway employed in toluene degradation, and the cmt pathway which normally is responsible for the catabolism of p-cumate which is a substituted toluene. The mutation which allowed this strain to utilise the cmt pathway was found to be a single base change to the promoter-operator sequence. This allowed all the enzymes in this pathway to be expressed under conditions where their synthesis would normally be repressed. Thus, the two pathways could work in conjunction with each other to metabolise PCBs, a relationship described as mosaic (Ohta et al 2001).
The pthalates are substituted single-ring phenols and include terephthalic acid and its isomers, the major chemicals used in manufacture of polyester fibres, films, adhesives, coatings and plastic bottles. In a recent analysis of anaerobic sewage sludge, a methanogenic consortium of over 100 bacterial clones were found to have the capability to digest terephthalate. Characterisation of these by analysis of their ribosomal DNA sequences, revealed that almost 70% were archaeans most of which had not been previously identified, and that nearly 90% of the total bacteria comprised two of the novel archaean species. These two species are believed to be responsi-ble for the degradation of terephthalic acid (Wu et al 2001). During wastewater treatment, terepthalic acid is usually treated by aerobic processes. However, this consortium, or others like it provide an anaerobic alternative which, being methanogenic, may be structured to offset processing costs by the utilisation of the methane.
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