Mitigation and Termination Strategies
Risk Management


Mitigation and Termination Strategies
Response Infrastructure

Need for Mitigation and Termination Strategies

     Mitigation and termination are topics of relevance to introduction of organisms to the environment. There has been much discussion of the potential adverse effect of such introductions in general. The Risk Assessment will have identified any specific concerns, and in most cases will have noted that all possible events cannot be identified. Hence, especially for field trials, permission to conduct the release includes requirements for developing protocols for identifying the onset of an adverse effect and for terminating the trial and mitigating the effect. It must be pointed out that field trials involving microbes and most plants cannot generally be said to be contained. The proper term is confined.

Environmental Issues

     While there has been much public concern and speculation about the possibility of adverse environmental effects from the environmental application of runaway engineered organisms, there is little evidence of adverse effect. The possibility of effects on nontarget organisms, on biological cycles, and on human health has been discussed. Yet, to date, after almost 5000 field tests of engineered microbes or plants, there is no record of such problems arising. In one instance (Short et al. 1991) researchers evaluating the efficacy of Pseudomonas strains that had been engineered to degrade 2-4 Dichlorophenoxyacetate found that 2-4 Dichlorophenol (a toxic intermediate metabolite) accumulated in the soil. The accumulation of 2-4 Dichlorophenol resulted in a loss of 90% of the fungal population in the soil, soil decontaminated resulted in replenishment of normal flora. Nevertheless, the possibility of an adverse effect requires that environmental applications be reviewed for safety considerations.

     Cavalieri (1991) has proposed that microcosms be used to predict the environmental consequences of the application of engineered microorganisms. Microcosms can provide information about persistence, survival, and specific effects of the modified microbe in question relative to the unmodified host. While the information from microcosms may not be entirely representative of results under field conditions, it will provide a basis for deciding whether or how field testing should proceed. Similarly, based on microcosm data, the process could be modified, safety precautions instituted or devised, provisions for confinement or mitigation devised, and effective monitoring protocols designed.

     Total eradication of unwanted microorganisms is rare, but reduction to acceptable levels (i.e., below the level of unacceptable economic or health impact) is possible. Absolute containment of microorganisms is not possible and, based on experience with both beneficial and detrimental microorganisms, not essential (Vidaver and Stotzky 1992). Vidaver and Stotzky propose the use of the more realistic term "confinement" in lieu of containment. Confinement does not imply that the microbe will not spread beyond the point of application, but rather that it can be effectively managed and adverse effects minimized. Most microorganisms are confined biologically by their individual requirements for nutrients, moisture, and sensitivity to environmental conditions (i.e., their niche).

     Additional strategies involve the use of debilitated organisms or the construction and use of safe cloning vectors with limited ability to transfer or survive outside the original host and the use of replicons sensitive to temperature or other environmental factors. The use of debilitated microbes is not practical for environmental applications. However, several conditionally lethal systems for the control of released bacteria have been designed and tested. These include a temperature sensitive system ( e.g. where DNA repair does not occur at cold temperatures), a conditionally lethal construct wherein the organism has an inducible metabolic pathway that can be activated only by the presence of an innocuous chemical not normally present in the environment of the microbe and a "suicide" gene which will destroy a key feature without which the cell cannot survive. The gene is controlled by the presence (induced) or absence (derepressed) of the waste in question. If the waste concentration falls below a critical level, the gene is activated level; Alternatively the gene is always active and a second gene provides protection. Activity of the second gene is controlled by the concentration of the waste being treated.

     Decontamination (or mitigation) of the environment of microbes has been studied and is discussed by Vidaver and Stotzky (1992). It is important to keep in mind that each situation is different and that procedures for decontamination will differ. A case by case approach is essential. The type of organism, the physical environment, the nature of the modification and the season must all be considered. Knowledge about the organism, whether it is a wild type or has been modified, is critical to designing a decontamination protocol. Vidaver and Stotzky list specific methods of decontaminating soils, plants, and animals, if they are contaminated with a hazardous microbe, and provide an indication of the time required to achieve effectiveness. Details of soil sterilization to decrease the bacterial levels at the site are also specified. Specific soil fumigants in common use in the USA are identified.

     Plants and animals must be considered because of the possibility of escape during a field application. Thus, in the event of contamination of animals (straying on the site) incineration, quarantine or slaughter could be employed immediately to minimize spread of the microorganism. Birds, rodents and runoff water must be considered as alternate sources of microbial dispersal. Plants which are growing on the site may be contaminated with the microbe. If the microbe is considered a hazard the plants should be immediately destroyed (i.e. burning, tillage) or quarantined if a future use is being considered. Long term solutions are presented for use in the event the project has along life span and the problem is recurrent. The issue of physical security, especially with animals, cannot be overstressed. Strong, tall fences will eliminate the presence of most unwanted mammals and insure against trespassers.

     Use of a fumigant will significantly lower the densities of all microorganisms present in the soil. Sterilization is not achieved. As a result, over time, the remnants of the microbial flora will reproduce and cell densities will increase. The new population may be similar to the previous one, in terms of types and relative numbers of individual types or it may radically differ, depending on which portion of the population survived the fumigation and at what level. There is a possibility that the introduced microbe could be the dominant type. For this reason, it is recommended that the treated site be reinoculated with uncontaminated soil from the surrounding area. This will most likely result in replacement of the original indigenous microbial flora and significantly decrease the probability that the introduced microbe would flourish.

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Last Modified: June 12, 2001
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