Microorganisms toxic metabolite production

Paraoxon is the toxic metabolite produced in animals, whereas diethyl-phosphorothionate is a detoxication product. Microorganisms may, under anaerobic conditions, produce amino-parathion, which has a much lower animal toxicity than parathion. Therefore, parathion by oral administration is less toxic to ruminants than to other mammals. The highly active rumen flora detoxicates parathion by reducing it to amino-parathion. However, parathion deposited on leaves or dust particles can absorb light energy and be isomerized to iso-parathion, which has a high animal toxicity. 

Metabolites may be produced by biochemical transformation of the substrate rather than by degradation, or may result from partial abiotic reactions. These products may be (a) terminal and persistent or (b) toxic to other components of an ecosystem—including the microorganisms that produce them. Both of these represent important considerations that are illustrated by examples in this book. 

Molinate has a low toxicity to rats, oral LDso=720 mg/kg, and is rapidly metabolized by plants to CO2 (1) (5) and naturally occurring plant constituents (1). Molinate is also readily metabolized by soil microorganisms (6). After incubation of molinate with Bacillus sp. 24, Nocardia sp. 119, and Micrococcus sp. 22r which were isolated from Russian garden soils and rice field drains (7,8), it was found that molinate was completely degraded into various hydroxy and oxidized products in the medium. Molinate can be metabolized to its corresponding sulfoxide in the mouse in vivo and by the microsome-NADPH system of mouse liver (9, 10). Hubbell et al. (11) and DeBaun et al. (12) also found molinate sulfoxide along with other polar and nonpolar metabolites in rat urine. 

Interestingly, some metabolites undergoing biliary excretion are reabsorbed, usually after undergoing further metabolic change brought about by enzymes associated with microorganisms normally found in the intestines. There are notable examples of this phenomenon, and it can be important as a factor in toxicity production, but its discussion is beyond the scope of this book. 

Some volatile products accumulate at significant levels only at specific stages in the life cycles of microorganisms. This may be due in part to toxicity of metabolites which, when they reach threshold levels in the culture media, inhibit the organism. For example, Schindler and Bruns 51) found that Ceratocvstis variospora is inhibited by higher concentrations of its own terpene metabolites. They were able to significantly improve yields of these products by trapping the end products on resins. 

Several factors that influence enhanced microbial degradation include nutrient value of the metabolite molecule, toxicity of the metabolite to soil microorganisms, and the availability of the metabolite to soil microbes. Comparisons of several pesticides, and their respective degradation products provide insight into the question of why soil microbial populations can develop rapid degradation capabilities for some pesticides but not others. 

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