Stimulation of bacterial DNA transformation by cattle saliva: implications for using genetically modified plants in animal feed

To investigate the likelihood of DNA transfer from genetically modified plants (GMP) to bacteria, a rescue plasmid system for Streptococcus gordonii was modified. It was applied to monitor the DNA transformation into oral and intestinal bacteria in cattle. Transformation and recombination frequency of S. gordonii was dependent on the length of the transformed DNA. Beside horse serum, cow saliva also rendered the cells competent for DNA uptake. Competence induction was completely abolished by the addition of liquid from maize silage. Competence was partially suppressed by the addition of rumen liquid. In order to study native bacteria, 724 colonies sensitive to the antibiotics were isolated from either silage or the saliva and rumen of cows. Using horse serum, silage liquid, cow saliva or rumen liquid for competence induction, the isolates failed to integrate linearized pMK110 DNA and restore antibiotic resistance. Only 6 of the colonies obtained from the teeth of a silage-fed cow were sensitive to the antibiotics. Two isolates were related to Staphylococcus warneri. They could be transformed with the model plasmid pMK110 after induction by horse serum. DNA transformation, however, was not stimulated by incubation with cattle saliva, silage or rumen liquid. The response to competence-stimulating factors seems to vary between different bacterial species. These results suggest that the probability of DNA uptake from silage of GMPs is very low.