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Grønvold studied the intestinal bacteria in healthy dogs undergoing an ordinary course of antibiotics. After a few days of the treatment, the E. coli bacteria in the intestines developed a resistance to several different types of antibiotics. Grønvold also discovered changes in the composition of intestinal bacteria. The use of antibiotics is thought to contribute to the proliferation of resistance to antibiotics and is also the most common cause of disturbances in intestinal flora in humans. Adult humans and animals have more bacteria in their intestines than the number of cells making up their entire bodies, and these intestinal bacteria constitute an ecological community that is essential for the health of the individual. Worldwide, nearly 50% of all antibiotics are used for veterinary purposes. Grønvold's thesis shows that the intestinal bacteria of animals are affected by antibiotics when it comes to both resistance and ecological balance.