A genetically modified organism is any organism whose genetic material has been altered using genetic engineering techniques. The exact definition of a genetically modified organism and what constitutes genetic engineering varies, with the most common being an organism altered in a way that "does not occur naturally by mating and/or natural recombination". Genes have been transferred within the same species, across species, and even across kingdoms. New genes can be introduced, or endogenous genes can be enhanced, altered, or knocked out. A wide variety of organisms have been genetically modified, from animals to plants and microorganisms.  In 1994 the Flavr Savr tomato was released, the first commercialized genetically modified food. The first genetically modified animal to be commercialized was the GloFish and the first genetically modified animal to be approved for food use was the AquAdvantage salmon in 2015. Many objections have been raised over the development of GMOs, particularly their commercialization. Many of these involve GM crops and whether food produced from them is safe and what impact growing them will have on the environment. Other concerns are the objectivity and rigor of regulatory authorities, contamination of non-genetically modified food, control of the food supply, patenting of life, and the use of intellectual property rights. Although there is a scientific consensus that currently available food derived from GM crops poses no greater risk to human health than conventional food, GM food safety is a leading issue with critics.
  What constitutes a genetically modified organism is not always clear and can vary widely. At its broadest, it can include anything that has had its genes altered, including by nature. Taking a less broad view it can encompass every organism that has had its genes altered by humans, which would include all crops and livestock. There are examples of crops that fit this definition but are not normally considered GMOs. The genetically engineered organism  can be considered a more precise term compared to GMO when describing organisms' genomes that have been directly manipulated with biotechnology. The term GMO originally was not typically used by scientists to describe genetically engineered organisms until after usage of GMO became common in popular media. The United States Department of Agriculture considers GMOs to be plants or animals with heritable changes introduced by genetic engineering or traditional methods, while GEO specifically refers to organisms with genes introduced, eliminated, or rearranged using molecular biology, particularly recombinant DNA techniques, such as transgenesis.
 The first genetically modified crop, an antibiotic-resistant tobacco plant, was produced in 1982. China was the first country to commercialize transgenic plants, introducing a virus-resistant tobacco in 1992. In 1994 Calgene attained approval to commercially release the Flavr Savr tomato, the first genetically modified food. Also in 1994, the European Union approved tobacco engineered to be resistant to the herbicide bromoxynil, making it the first genetically engineered crop commercialized in Europe. The first genetically modified animal to be commercialized was the GloFish, a Zebrafish with a fluorescent gene added that allows it to glow in the dark under ultraviolet light. It was released to the US market in 2003. In 2015 AquAdvantage salmon became the first genetically modified animal to be approved for food use. Approval is for fish raised in Panama and sold in the US.
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 For over a century bacteria have been used in agriculture. Crops have been inoculated with Rhizobia to increase their production or to allow them to be grown outside their original habitat. Application of Bacillus thuringiensis and other bacteria can help protect crops from insect infestation and plant diseases. With advances in genetic engineering, these bacteria have been manipulated for increased efficiency and expanded host range. Markers have also been added to aid in tracing the spread of the bacteria. The bacteria that naturally colonize certain crops have also been modified, in some cases to express the Bt genes responsible for pest resistance. This led to the development of ice-minus bacteria, which have the ice-forming genes removed. When applied to crops they can compete with the non-modified bacteria and confer some frost resistance. Other uses for genetically modified bacteria include bioremediation, where the bacteria are used to convert pollutants into a less toxic form. Genetic engineering can increase the levels of the enzymes used to degrade a toxin or to make the bacteria more stable under environmental conditions.
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