The digestive fate of Escherichia coli glutamate dehydrogenase deoxyribonucleic acid from transgenic corn in diets fed to weanling pigs

Corn containing genetically engineered plasmid DNA encoding an Escherichia coli glutamate dehydrogenase (gdhA) was fed to 19-d-old weanling swine to trace the digestive fate of the transgenic DNA. Eight pens of 8 pigs were fed a commercial (nongdhA) starter for 2 wk. One pig was randomly selected from each pen for 0-h control samples. The remaining 56 pigs were transitioned onto a corn-soybean meal diet and fed a diet containing 58% gdhA corn for approximately 1 wk; immediately thereafter, liver, 10th rib muscle, white blood cells, and plasma from the hepatic portal vein and ingesta from the stomach, distal ileum, and large intestine were collected. The DNA was extracted and the concentration determined via spectrophotometry. Polymerase chain reaction and gel electrophoresis were performed with primers designed to amplify 490 bp that included the plasmid's ligation site between the maize ubiquitin and the gdhA genes. The gdhA corn-derived DNA and diet served as positive assay controls, and conventional corn DNA and distilled water acted as negative assay controls. Detection limits were 0.99 fg of target DNA confounded with 500 ng of conventional corn DNA per each 20 mu L reaction. Transgenic DNA was detected in 71.43% of the stomach and 1.79% of the ileal ingesta samples from treatment animals but was not detected in the large intestine, white blood cells, plasma, liver, or muscle samples. Transgenic DNA was not detected in any sample from 0-h control animals. Stomach and ileal ingesta samples were further analyzed using real-time PCR. With an estimated limit of detection of 1.049 ag/mu L, 89.29% of the stomach ingesta samples were positive (average 1.56 fg target DNA). The proportion of transgenic DNA to total DNA differed between diet and stomach ingesta samples (P < 0.001). Despite the greater sensitivity of real-time PCR, target DNA was detected in only 1.79% of ileal ingesta. These data suggest that the gdhA transgene began degradation in the stomach and was nondetectable in the large intestine.