The prospect of the deliberate environmental release of genetically manipulated
microorganisms has given rise to a great deal of polemic. Amongst the rational
scientific concerns are those concerned with the fate of the released bacteria,
the fate of the recombinant genes that they carry, the selective pressures acting
upon them in different environmental situations and the long term effects on the
environment and human health. All recombinant DNA is carried by vectors (plasmids,
transposons or bacteriophage or remnants of these). Thus the way in which
recombinant constructions are made may itself lead to potential biosafety
concerns, irrespective of the host bacterium and the recombinant DNA fragment
of primary interest. The purpose of the present review is to assess progress
in improved vector design aimed at eliminating risks due to the way recombinant
vectors are constructed. Improved vector constructions include the avoidance of
the use, or removal, of antibiotic resistance genes, the use of defective
transposons rather than plasmids in order to reduce horizontal transfer and
the development of conditionally lethal suicide systems. More recently,
new site-specific recombination systems have permitted transposon vectors
to be manipulated following strain construction, but before environmental
release, so that virtually all recombinant DNA not directly involved in the
release experiment is eliminated. Such bacteria are thus pseudo-wild type in
that they contain no heterologous DNA other than the genes of interest.