Antimicrobial Resistance in Bacteria
Some bacteria have become resistant to antimicrobial drugs, and are antibiotic-resistant.
Antimicrobial resistance in bacteria is an emerging and increasing threat to human health. [2, 3] Physicians are increasingly aware that antimicrobial resistance is increasing in foodborne pathogens and that, as a result, patients who are prescribed antibiotics are at increased risk for acquiring antimicrobial-resistant foodborne infections. [3, 29] Indeed, “increased frequency of treatment failures for acute illness and increased severity of infection may be manifested by prolonged duration of illness, increased frequency of bloodstream infections, increased hospitalization or increased mortality.” 
The use of antimicrobial agents in the feed of food animals is estimated by the FDA to be over 100 million pounds per year.  It is estimated that 36% to 70% of all antibiotics produced in the United States are used in food animal feed or in prophylactic treatment to prevent animal disease. [2, 3] In 2002, the Minnesota Medical Association published an article by David Wallinga, M.D., M.P.H. who wrote:
According to the [Union of Concerned Scientists], 70 percent of all the antimicrobials used in the United States for all purposes—or about 24.6 million pounds annually—are fed to poultry, swine, and beef cattle for nontherapeutic purposes, in the absence of disease. Over half are “medically important” antimicrobials; identical or so closely related to human medicines that resistance to the animal drug can confer resistance to the similar human drug. Penicillin, tetracycline, macrolides, streptogramins, and sulfonamides are prominent examples. 
Based on recent research, it is now recommended that doctors avoid the use of commonly prescribed antibiotics, like tetracycline and ampicillin, in favor of drugs for which Shigella has not shown resistance, such as ciprofloxacin.  European countries have reduced the use of antibiotics in animal feed and have seen a corresponding reduction in antibiotic-resistant illnesses in humans.