L. López, J. Santamaría, A. Sánchez, L. Castro, and J.L. Moreno. 2012. Presence of tetracycline resistant bacteria and genes in grassland-based animal production systems. Cien. Inv. Agr. 39(3): 411-423. This study assessed the presence of tetracycline-resistant bacteria in five different grassland-based production systems dedicated to raising dairy cattle located in the Colombian Andes. Animal (ruminal fluid and feces) and environmental (soil and water) samples were evaluated, and resistant heterotrophic bacteria were found in rumen fluid, feces, soil, and runoff water samples. The resistant bacteria were isolated and identified based on the 16S rDNA region. Subsequently, they were evaluated for the presence of tet genes, which encode ribosomal protection proteins and membrane efflux pumps. The most frequent phyla detected were Firmicutes and Proteobacteria. The most common resistance genes found were tet(W), tet(Q), and tet(M). The nucleotide sequences of the genes showed no differences in bacteria isolated from environmental samples versus ruminal fluid and feces. This result suggests that the observed environmental resistance in the evaluated grasslands is the result of horizontal gene transfer from animals to the environment.