Genes from a bacterium, Bacillus thuringiensis (Bt), have been transferred to crop plants to replace the use of sprayable insecticides directed at some crop pests. These genes produce proteins that control the immature stages of a subset of insects (some caterpillars and beetle grubs) by creating pores in the insect gut and thus disrupting its integrity. Bt has always been consumed by people. It is a very common bacterium found naturally in soil and on plant leaves, and the Bt organism and its insecticidal proteins have been used extensively in organic farming for more than 50 years (and have been known to scientists for more than 100 years). During that time, a large number of studies have been conducted that demonstrate the human safety of this organism and these proteins, and the specificity of the Bt proteins for a subset of insects. The mechanism for this specificity has also been investigated and found to be due to specific gut receptors in these susceptible insects. These receptors are not present in other organisms, like mammals―including humans. Without these receptors, there is no way for the Bt proteins to affect mammalian guts.
In addition, many studies have been done with the specific insecticidal proteins that have been expressed in crop plants. These studies include tests in rodents, at high doses (as much as their stomachs can hold), of the active purified protein, and feeding studies where as much of the crop as possible (without causing nutritional problems) is included in the diet. Numerous studies have also been done in livestock species. None of these tests has identified any adverse effect. These results were as expected, based our knowledge of how these proteins exert their toxicity and their history of use in organic agriculture. Microscopic examination of mammalian gut membranes has confirmed no disruption (or other adverse effects) due to consumption of the Bt proteins.
In conclusion, our understanding of the mechanism of action of the Bt proteins, as well as extensive empirical evidence, supports a high level of confidence in the safety of these proteins to humans and other noninsect animals.