Deleting the msbB gene from Salmonella by genetic engineering leads to the loss of lipid A (a lipid responsible for the toxicity levels of gram-negative bacteria) and therefore reduces the toxicity of Salmonella by 10,000-fold.[9] Generating auxotrophic mutants (a strain of microorganism that will proliferate only when the medium is supplemented with some specific substance) that cannot replicate efficiently in an environment where a particular nutrient required by the mutant strain is scarce. Salmonella A1-R represents such a strain, which is auxotrophic for the amino acids leucine and arginine that are enriched in the tumor but not in normal tissues.