Toluene-Degrading Bacteria Are Chemotactic towards the Environmental Pollutants Benzene, Toluene, and Trichloroethylene

Abstract

The bioremediation of polluted groundwater and toxic waste sites requires that bacteria come into close physical contact with pollutants. This can be accomplished by chemotaxis. Five motile strains of bacteria that use five different pathways to degrade toluene were tested for their ability to detect and swim towards this pollutant. Three of the five strains (Pseudomonas putida F1, Ralstonia pickettii PKO1, and Burkholderia cepacia G4) were attracted to toluene. In each case, the response was dependent on induction by growth with toluene. Pseudomonas mendocina KR1 and P. putida PaW15 did not show a convincing response. The chemotactic responses of P. putida F1 to a variety of toxic aromatic hydrocarbons and chlorinated aliphatic compounds were examined. Compounds that are growth substrates for P. putida F1, including benzene and ethylbenzene, were chemoattractants. P. putida F1 was also attracted to trichloroethylene (TCE), which is not a growth substrate but is dechlorinated and detoxified by P. putida F1. Mutant strains of P. putida F1 that do not oxidize toluene were attracted to toluene, indicating that toluene itself and not a metabolite was the compound detected. The two-component response regulator pair TodS and TodT, which control expression of the toluene degradation genes in P. putida F1, were required for the response. This demonstration that soil bacteria can sense and swim towards the toxic compounds toluene, benzene, TCE, and related chemicals suggests that the introduction of chemotactic bacteria into selected polluted sites may accelerate bioremediation processes.Bacterial chemotaxis has been studied in detail for Escherichia coli and Salmonella enterica serovar Typhimurium (35). Simple sugars, amino acids, and organic acids are chemoattractants for these enteric bacteria. Aromatic acids such as benzoate, 4-hydroxybenzoate, and salicylate are attractants for Pseudomonas putida PRS2000 (15). Recently, the soil bacterium P. putida G7 was reported to be attracted to the pollutant naphthalene (12,24,31). This expanded the range of organic compounds that are known to serve as bacterial chemoattractants to include aromatic hydrocarbons. However, nothing is known about chemotaxis towards other common aromatic hydrocarbons such as toluene and benzene. Five distinct pathways have been described for the aerobic degradation of toluene. All pathways are initiated with the oxidation of toluene, but five different oxidation products are formed (Fig. 1). P. putida F1 contains toluene 2,3-dioxygenase, an enzyme that oxidizes the aromatic ring of toluene, incorporating both atoms of molecular oxygen. After a dehydrogenation step, 3-methylcatechol is formed. This compound is further degraded via meta ring fission (8, 10, 11). P. putida PaW15 (a leucine auxotroph of strain mt-2) initiates degradation at the methyl group of toluene, eventually forming benzoate. Benzoate is converted to catechol, which is also degraded by a meta cleavage route (41). Strains that monooxygenate the aromatic ring of toluene have also be...

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