the 5 39 and 3 39 flanking dna from a virulence factor encoding gene surrounding a selectable marker it also allows the production of avirulent ameba for vaccine use by knockouts of virulence genes targets for such knockouts include but are not limited to the genes encoding amebic adhesion amebic pore forming proteins and amebic proteases the dna constructs of the present invention once introduced into the host cell can exist either as chromosomal dna or as episomal dna use of rdna episome sequences in the expression vector will enable episomal maintenance and expression expression vectors such as the aforementioned plasmids containing rdna can be used to shuttle dna between various hosts for example a plasmid in accordance with the present invention which contains a foreign gene to be expressed from e histolytica can be transfected into e coli mutagenized using known techniques reisolated and subsequently transfected into e histolytica host cells which are transfected with the construct of the present invention can be screened using conventional techniques for example when the gene to be expressed is a gene which confers resistance to a particular antibiotic screening can be accomplished by gradually or immediately increasing the concentration of that particular antibiotic confirmation that gene knockout or gene complementation has occurred can be obtained by southern blots of restriction enzyme digested dna from the transformed parasite see sambrook et al 1989 34 molecular cloning a laboratory manual 34 2 ed cold spring harbor laboratory press n y in a second embodiment the present invention provides transformed enteric protozoa these enteric protozoa can be less virulent than wild type enteric protozoa in a third embodiment of the.