Antibiotics have found wide nonmedical use. Some are used in animal husbandry, along with vitamin B12, to enhance the weight gain of livestock. Some authorities believe the addition of antibiotics to animal feeds is dangerous because continuous low exposure to the antibiotic can sensitize humans to the drug and make them unable to take the substance later for the treatment of infection. In addition, low levels of antibiotics in animal feed encourage the emergence of antibiotic-resistant strains of microorganisms. Drug resistance has been shown to be carried by a genetic particle transmissible from one strain of microorganism to another, and the presence of low levels of antibiotics can actually cause an increase in the number of such particles in the bacterial population and increase the probability that such particles will be transferred to pathogenic, or disease-causing, strains. In 2013 the Food and Drug Administration moved to restrict the use of antibiotics in livestock, calling for labeling changes that would bar their use to promote growth and requirements for veterinarian supervision when antibiotics are used. The use of antibiotics for disease prevention (as opposed to disease treatment) was not, however, banned. Antibiotics have also been used to treat plant diseases such as bacteria-caused infections in tomatoes, potatoes, and fruit trees. The substances are also used in experimental research.