Microbial metabolism of pesticides

MacRae, I.C. Microbial metabolism of pesticides and stmcturally related compounds. Rev. Environ. Contam. Toxicol, 109 2-87, 1989. 

Okey, R.W., Bogan, R.H. (1965) Apparent involvement of electronic mechanisms in limiting microbial metabolism of pesticides. J. Water Pollut. Control Eed. 37, 692-712. 

Bollag J.-M. (1982) Microbial metabolism of pesticides. In Microbial Transformations of Bioactive Compounds (ed. J. P. Rosazza). CRC Press, Boca Raton, FL, vol. 2,... 

The microbial metabolism of pesticides has often been subdivided into 2 distinct classes. The first of these is termed simply "catabolism". This process often results in the mineralization of some portion of an organic compound via enzymatic pathways to simple products of universal currency (C02, NH3). In some cases, one portion of the molecule may be mineralized and another portion may accumulate in soil. This is true for the soil microbial degradation of carbofuran (42,43). Therefore, catabolism should not be equated with mineralization or complete destruction of a pesticide. It should be pointed out, however, that mineralization of a pesticide in soil Is nearly always a consequence of microbial activity (44). The key to understanding catabolism is that it is primarily a process driven by the microbial quest for energy. Therefore, catabolism has come to be equated with utilization of a pesticide as an energy source and thus a growth substrate (40,41). Catabolism is most commonly linked to the conversion of pesticides into carbon skeleton... 

Another important variable that determines the microbial metabolism of soil-applied pesticides is the availability of the chemical to the microbial systems degrading it. The hydrolysis product and parent pesticide should be available to microbes so as to exert their toxicity or provide nutrient value. The lack of availability of some chemicals may result in resistance to microbial adaptation. 

Microbial reduction of toxic organics is carried out by reduction enzymes. Major reduction reactions of selected toxic organics are shown in Table 13.5. The reduction of the nitro group to amine involves the formation of a nitro and a hydroxyamino group. This type of reduction reaction occurs during the microbial metabolism of various pesticides. Organophosphorous pesticides such as para-thion, paraoxon, or fenitrothion are often reduced to nontoxic amino compounds (Miyamoto et al., 1966 Matsumura and Benezet, 1978). 

Munnecke DM, LM Johnson, HW Talbot, S Barik (1982) Microbial metabolism and enzymology of selected pesticides. In Biodegradation and Detoxification of Environmental Pollutants (Ed AM Chrakrabarty), pp. 1-32. CRC Press, Boca Raton, FL. 

The microbial metabolic process is the major mechanism for the transformation of toxic organic chemicals in the subsurface environment. The transformation process may be the result of a primary metabolic reaction, when the organic molecule is degraded by a direct microbial metabolism. Alternatively, the transformation process may be an indirect, secondary effect of the microbial population on the chemical and physical properties of the subsurface constituents. Bollag and Liu (1990), considering behavior of pesticides, defined five basic processes involved in microbially mediated transformation of toxic organic molecules in the soil upper layer environment. These processes are described next. 

Pesticides are susceptible to a variety of transformations in the environment, including both chemical degradation and microbial metabolism. Microbial transformations are cataly2ed exclusively by enzymes, whereas chemical transformations are mediated by a variety of organic and inorganic compounds. Many pesticide transformations can occur either chemically or biologically. Consequendy, most pesticide dissipation studies include sterile treatments to... 

For those pesticides which are utilized as microbial growth substrates, sigmoidal rates of biodegradation are frequendy observed (see Fig. 2). Sigmoidal data are more difficult to summarize than exponential (first-order) data because of their inherent nonlinearity. Sigmoidal rates of pesticide metabolism can be described using microbial growth kinetics (Monod) however, four kinetics constants are required. Consequently, it is more difficult to predict the persistence of these pesticides in the environment. 

Implications of Mobility on the Availability and Degradation of Pesticides in Soil. Repeated application of 2,4-dichlorophenol, p-nitrophenol, and salicylic acid (as observed in current studies) and carbofuran phenol (20) has induced enhanced microbial degradation of their parent compounds. Rf values of these hydrolysis products indicate intermediate to high mobility in soils. The p-nitrophenol, 2,4-dichlorophenol, and salicylic acid were utilized as energy sources by microbes, and their availability in soil may contribute to the induction of rapid microbial metabolism. Carbofuran phenol did not serve as a microbial substrate but also enhanced the degradation of its parent compound, carbofuran (20). Carbofuran phenol is freely available in anaerobic soils, but the significance of its availability is yet to be understood. 

Several interrelated factors complicate the relationship between growth and degradation multispecies interactions, cometabolic degradation, and substrate utilization. If multispecies consortia are required for degradation, then the amount of pesticide incorporated into microbial biomass of a particular species may be even lower. Multispecies interactions have been investigated Lappin et al. (21) isolated a consortium of 5 bacteria that metabolized the herbicide mecoprop in culture after a lag period of 20 hr. The growth of the consortium was in contrast to the failure of individual species to grow on mecoprop. The extent of these kinds of interactions in soils is essentially unknown. 

The bioaugmentation strategy involves the selection of adapted microbial strains that metabolize a pesticide as a carbon or nutrient source. Under these circumstances, the rate of degradation is enhanced compared to the rate normally observed in soil or water. The development of an adapted microbial strain usually begins with the enrichment and isolation of pesticide-degraders from the contaminated environment. Enrichment is relatively easy with compounds that are used as carbon or nutrient sources. Compounds that are not readily utilizable, however, require novel approaches to enrich and isolate potential degraders, which may be manipulated to enhance their degradative capabilities. 

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