Potential opportunities and problems for genetically altered rumen microorganisms.

Rumen microbiologists are beginning to use genetic engineering techniques, and researchers should carefully consider both the potentials and limitations of using this technology to manipulate the rumen microbial ecosystem. Despite encouraging rhetoric, it is difficult to identify specific examples where genetic engineering would enhance ruminal performance. Many practical problems (lactic acidosis, deamination, etc.) might be better served by genetic engineering approaches that delete rather than add genes. The difficulty with this approach is that a highly selective means of preventing wild types from recolonizing the rumen would be needed. The addition of specific genes is confounded by 1) the fact that the rumen microorganisms are already adapted to the rumen, 2) the diversity of species inhabiting the rumen and 3) the complexity of interactions among these species. Aspects such as increased rates of cellulose digestion and changes in amino acid composition of the microflora are particularly sensitive to these biological constraints. Genetic engineering has, however, the potential to alleviate new limitations that humans have imposed on the rumen (detoxification, resistance to low pH, the digestion of novel feed materials, etc). A particular strategy of moving acid-resistant cellulose genes into noncellulytic, but acid-resistant, rumen bacteria is described.