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IIT Bombay’s bacteria preferentially degrade aromatic compounds (The Hindu 06/04/2019)

 A unique bacterial strain isolated from soil contaminants was used in the study


          Using a unique strain of bacterium isolated from soil contaminated with petroleum products, IIT Bombay researchers can selectively remove from the environment toxic, aromatic pollutants such as benzoate (sodium benzoate is used as a food preservative), benzyl alcohol and naphthalene, to name a few. What makes the bacterial strain (Pseudomonas putida CSV86) unique is its preference for aromatic compounds and organic acid as a food source even when glucose is available. The strain can degrade aromatics and organic acids simultaneously.


Peculiar food choice


            Since breaking down aromatic compounds is difficult, bacteria generally prefer simple carbon sources such as glucose for obtaining energy. Even the bacteria that are known to degrade aromatic compounds tend to first prefer glucose and other simple carbon sources for energy and feed on aromatic compounds only when glucose gets exhausted. However, this bacteria strain displays a completely different order of food choices — it first feeds on aromatic compounds and organic acids and only when this gets exhausted does it start feeding on glucose.


          “This is the first time a bacteria strain that preferentially utilises aromatic compounds even in the presence of glucose has been ever reported,” says Prashant Phale from the Department of Biosciences and Bioengineering at IIT Bombay. “We isolated the bacteria from petroleum- contaminated soil in Bengaluru way back in 1986.” In studies carried out in the lab, the research team led by Prof. Phale found that even when both benzoate and glucose were available, the bacteria first utilised benzoate, and only when it was exhausted did it start feeding on glucose. “This gives an advantage to remove the pollutants with priority even in the presence of simple carbon source from the contaminated site,” says Prof. Phale. The results of the study were published in the journal Applied and Environmental Microbiology.


Molecular level


              The bacterial strain’s peculiar order of food preference comes from the suppression of glucose utilisation at a molecular level leading to a reduction in the level of proteins and enzymes necessary for transport breakdown of glucose. At the same time, proteins and enzymes needed for transport and breaking down of aromatic compounds are enhanced resulting in the preference for these pollutants as a source of food. “This is [the] complete opposite of what we see in aromatic compound-degrading bacteria. In those bacteria, glucose will inhibit metabolism of aromatic compounds by reducing the enzymes necessary for breaking down aromatics,” says Prof. Phale.


        When benzoate and succinate (organic acid) are available together, the bacteria simultaneously consume both pollutants. “The enzymes necessary for breaking down benzoate and succinate were produced in large amounts leading to equal uptake of both,” he says. However, when only glucose or benzoate is available the respective enzymes are produced and the carbon source is taken up by the bacteria. When the researchers introduced benzoate to the bacteria that were already growing on glucose, the level of enzymes responsible for glucose metabolism was seen to reduce while the level of enzymes for benzoate degradation increased. “We started seeing this effect in 15-30 minutes after benzoate was added. This [synthesising enzymes for benzoate degradation] is a very fast process occurring at the cellular level,” says Prof. Phale.


           “The bacterial strain is a very good candidate for bioremediation or waste-water treatment. We can increase the metabolic diversity and capacity by genetically engineering the strain,” he says. “We would first like to test the viability and efficiency of the strain in breaking down different aromatic compounds.” The team hopes to engineer the strain so it can be directly applied to the soil to preferentially degrade aromatic pesticides. The team is now trying to understand the molecular mechanisms and regulatory components involved in preferential degradation of aromatics over glucose using various molecular biology tools.