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Infection followed by disease will depend on the microorganisms ability to multiply in the host and on the host's ability to resist or control the infection. It has proved useful to categorise all microorganisms into 4 groups which define their pathogenicity to humans; with only the first group containing non-pathogens. This categorisation applies only to the infectivity towards humans, and is of significance only, therefore, for the contained use of organisms:

Hazard Group 1 Organisms that are most unlikely to cause human disease
Hazard Group 2 Organisms capable of causing human disease and which may be a hazard to laboratory workers, but are unlikely to spread to the community. Laboratory exposure rarely produces infection and effective prophylaxis or effective treatment is usually available
Hazard Group 3 Organisms that may cause severe human disease and present a serious hazard to laboratory workers. They may present a risk of spread to the community, but there is usually effective prophylaxis or treatment available
Hazard Group 4 Organisms that cause severe human disease and are a serious hazard to laboratory workers. They may present a high risk of spread to the community, and there is usually no effective prophylaxis or treatment

The intention of this categorisation, which applies to non-modified organisms as well, is to identify appropriate containment which would be required to protect those working with the organisms. The higher the hazard group, the greater the containment required to control the organism and ensure that it does not infect those working with it.

Pathogenicity is not a simple characteristic. Many genes must interact appropriately for a microbe to cause disease. the pathogen must possess and express characteristics such as recognition factors, adhesion ability, toxigenicity and resistance to host defence systems. Single gene modifications of organisms with no pathogenic potential or history, or even the introduction of multiple genes unlikely to confer pathogenicity are unlikely to result in unanticipated pathogenicity. For example, E. coli K12 has been disabled to remove some of the factors that might be associated with pathogenicity (wild type E. coli is a group 2 pathogen). The factors which have been lost include the cell-surface K antigen, part of the LPS side chain, the adherence factor (fimbriae) that enable adherence to epithelial cells of human gut, resistance to lysis by complement and some resistance to phagocytosis. This variant of E. coli is a common host organism for genetic modifications within the laboratory.

The starting point for the risk assessment is, therefore, an assumption that the level of risk associated with the modified organism is at least as great as that of the host organism (until proved otherwise, either by direct observation, or by argument where the factors which are likely to enhance or decrease pathogenicity are considered as in the case of K12 above). Whether in the laboratory or in industry the capacity to choose a host means that in all but a few cases the host organism will have been chosen to be in hazard category 1. It is assumed that the modified organism will be used under the same containment as the host wild-type organism unless the modification inserts information which would alter the pathogenicity. The vector has also to be considered, both for its own potential for pathogenicity and for its ability to transfer the insert to other than the intended organism -- horizontal transfer of the information. Most vectors used for E. coli contain no sequences which might result in pathogenic behaviour. The presence of genes coding for antibiotic resistance might be of concern, but for most of these the antibiotic resistance is already so common in the environment that it may be discounted.

Most common E. coli vectors are transfer deficient, but the ability to transfer information either directly or with the assistance of other plasmids and the host range of the vector must be taken into account when considering the safety of the mechanism of insertion of the required genes into the host organism.

The properties of the insert are again of importance in considering the risk assessment for the modified organism. Clearly if the information encodes a toxic gene product, or one which is known to be likely to modify the pathogenicity of the organism into which it is inserted, the great the risk. If the gene product is non-toxic and is not one which may pose a risk to the people working with the organism in containment, the risk management will largely be based on the pathogenicity of the host organism.

In most instances the characteristics of the donor organisms are of less relevance to the risk assessment than those of the host. If the donor organism is merely used as a source of well characterised DNA for a selectable phenotype or a promoter or other control sequence, the characteristics of the donor are unimportant to the risk assessment. If however, the insert contains genes which are biologically active, toxins or virulence factors, then information from the donor organism are of consequence. The construction of cDNA or genomic libraries make it essential to consider all the possible hazards associated with the donor organism, and in this instance, the hazard group may well have to be the higher or the two within which the host and donor fall.

It is now possible to examine the modified organism and consider the likely risk. During the 1970's Dr. Sidney Brenner and others in the United Kingdom attempted to systematise the approach by considering three factors -- Access, Damage, and Expression. The approach was incorporated in the United Kingdom's approach to risk assessment for contained use of bacteria, and is discussed in detail in a document produced by the Advisory Committee on Genetic Modification in the United Kingdom. The latest version of the guidance was published in 1999 and provides clear guidance as to the risk assessment for the contained use of genetically modified microorganisms (including any cells in culture). The guidance note is free and may be obtained from the Health & Safety Executive in Britain. More information is available by looking at the newsletters published by the ACGM which are available on the internet on

Access is a measure of the probability that a modified micro-organism, or the DNA contained within it, will be able to enter the human body and survive there. It is a function of both host and vector. Depending on the organism being used, there are a number of routes of entry which allow access. The properties of the vector, particularly mobilisation functions need to be taken into account. In general if the organism is capable of colonising humans then access is high, whereas if the host is disabled so as to require the addition of specific nutrients not available in humans or outside of the culture media and is also sensitive to physical conditions or chemical agents present in humans, then the access factor is likely to be low.

Expression and Damage are usually associated with the insert and the gene product.

Expression is a measure of the anticipated or known level of expression of the inserted DNA; if the 'gene' inserted is intended to be expressed at a high level, for example, by deliberate in-frame insertion down-stream of a strong promoter, expression is likely to be high. If the insert is simply there to allow probes to detect the DNA, and is non-expressible DNA, i.e. with no foreseeable biological effect or gene containing introns which the host is incapable of processing, then the expression factor will be low. Examination of the final product, the modified organism itself, will determine the actual expression, which may be higher or lower than expected.

Damage is a measure of the likelihood of harm being caused to a person by exposure to the genetically modified micro-organism, and is independent of either expression or access. It is associated with the known or suspected biological activity of the DNA or of the gene product. The activity of the organism which results in any toxic, allergenic or pathogenic effect need be taken into account within this parameter. It may be that the biological activity of a protein is dependent on the host cell system in which it is expressed. An oncogene expressed in a bacterium will have no discernible effect, when present in a human cell, problems may arise. The full biological function of many gene products require post-translational modification which will not occur within a bacterial cell normally. The potential biological activity of the gene product should be considered in the context of where and how it has been expressed and the effect on its structure and activity of the mode of manufacture. The range products with 'damage' potential might include:

  • a toxic substance or pathogenic determinant that is likely to have a significant biological effect - damage is high
  • a biologically active substance which might have a deleterious effect if delivered to a target tissue
  • a biologically active substance which is very unlikely to have a deleterious effect or where it could not approach the normal body level. When cloning in E. coli the 'worst case' would be if all the E. coli in a person were replaced by the modified organism expressing a foreign polypeptide in an active form at a high rate. If all of these are absorbed in an active form and arrive at a site where they might have their maximum effect, what would be the damage?
  • a gene sequence where any biological effect is unlikely because of known properties of the protein or because of the high levels encountered in nature

Once an estimate of each of these parameters has been made (in the United Kingdom this is numerical in steps of 10-3), they may be combined. The result provides a qualitative measure of the risk, and allows a containment level to be assigned for the use of the organism in order to protect those working with the genetically modified micro-organism.

Unfortunately, this Brenner scheme is only easily applicable to a small class of experimental uses of modified micro-organisms, but the number of experiments in research laboratory environments which fit the requirements for the application of this scheme make its retention useful.

Modified organisms may be used in containment in laboratories (or pilot plants) or may be used in an industrial setting. It may be that the primary distinction here is not the size of plant or type of organism, but rather the skill and training of those working in the facility.

It is likely that a research or development laboratory will be working with organisms which pose a greater threat to either the individuals working therein or to the environment than do those organisms developed for large scale factory use. The great majority of organisms used in industrial production are well-characterised, 'familiar' organisms capable of being used under conditions of 'Good Industrial Large Scale Practice' or GILSP. Given that it is usually possible to 'choose' the parental organism into which a gene is inserted for a particular 'industrial' purpose, there would be no good reason to choose an organism likely to pose problems to either those working in the facility, or to the environment in the event of an escape.

The same logic would apply to the development stage where 'industrial' use of the modified organisms is being planned. There is a possible extra hazard in that it is at this stage that the modified genes may be inserted into the organism, and the unpredictability of insertion site may, arguably, require slightly greater care than that taken at the production facility.

In the research laboratory, organisms may be pathogenic to humans and/or to the environment, as it is here that fundamental research would be conducted. Experiments will involve organisms and /or inserts which may be injurious to the health of the workers or to those who are incidentally on site in the laboratory.

Last Modified: May 23, 2000
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