Many bacteria can live in both humans and animals; many live in the environment as well. Some are pathogens (cause disease) in both humans and animals, and some in one but not the other. Either animals or the environment, therefore, may serve as "reservoirs" for bacteria capable of causing disease in humans. Of course, animals or the environment can act as reservoirs for non-pathogenic bacteria as well. These bacteria pose a less direct, though still significant risk to humans since they may carry and transfer to disease-causing bacteria, the genes that will make them resistant to one or more antibiotics.
Different routes by which antibiotic-resistant bacteria can move from animals to humans can be summarized as food, environment and the workplace:
Food. Resistant bacteria can contaminate animal carcasses during slaughter, winding up on raw meat that reaches the consumer. If the meat isn't cooked thoroughly, if cutting boards or knives aren't thoroughly washed before being used on other food, or if raw meat juices get splashed around the kitchen and come into contact with other food or utensils, those germs then can infect people who eat the food, or touch the utensils or household surfaces contaminated by the raw meat.
Food-borne illness is by no means rare. For example, Salmonella from poultry causes 1.4 million illnesses annually. Of course, disease-causing bacteria on meat are bad news even if they aren't resistant, but at least antibiotics can be used to treat people who get seriously sick from such infections.
Environment. Second, the nearly two trillion pounds of animal wastes produced annually in the U.S. contain significant amounts of bacteria, including resistant bacteria. Because as much as 75% of an antibiotic may pass undigested through the animal, wastes can contain antibiotics as well as antibiotic-resistant bacteria. In many cases, wastes are stored in open-air lagoons and/or spread on fields. Many of the lagoons are not lined and are prone to leaking or breaking, thereby releasing the animal wastes to surface waters and ground waters. In addition, the spread of manure on agricultural fields may contaminate vegetables and fruits, which then can transfer resistant bacteria to humans.
Workplaces. Farm workers may become infected with resistant germs in the course of caring for animals fed antibiotics, and may then pass those bacteria along to their family and other in their community. Some of the most dangerous "super bugs" have been found in children and adults who have close contact with food animal production.