In March 2013 the director of the U.S. Centers for Disease Control (CDC), Tom Frieden, held a press conference to discuss a “nightmare bacteria” killing patients in long-term medical care facilities. The bacteria, carbapenem-resistant enterobacteriaceae (CRE), are resistant to nearly all antibiotics, cause high mortality rates, and transfer their resistance to other bacteria. Resistance to antibiotics, substances that suppress bacterial growth or attack, makes this bacterial pathogen very dangerous because antibiotics are one of modern medicine’s main tools in the treatment of bacterial diseases.

Furthermore, it appears that CRE are spreading. In the first half of 2012 alone, nearly 200 medical facilities treated at least one patient who was infected with these bacteria. Scientists at the CDC have tracked CRE from a single health-care facility in one state in 2001 to health-care facilities in 48 states in 2015. That’s a very troubling increase, given that CRE can spread the genes that destroy the effectiveness of antibiotics to other bacteria. When this happens, people can get severe infections, for which few effective treatments exist.

Why is the spread of CRE an environmental issue? If it were a single deadly case, it would be a tragedy for the patient and the patient’s family, but it is an environmental issue because antibiotic resistant bacteria do not remain isolated. For example, penicillin, the first antibiotic to be discovered, became available for clinical use in 1944 and in 1945 the first penicillin-resistant strains of golden staph (Staphylococcus aureus) were reported. By 1959 half of the infections caused by golden staph were resistant to penicillin, so a new drug, methicillin, was introduced. Initially, methicillin killed penicillin-resistant golden staph, but within a year methicillin-resistant strains of golden staph appeared. These strains, known as MRSA (methicillin-resistant Staphylococcus aureus), originated in hospitals and then spread to the community.

Though drug resistance evolves as bacteria adapt through mutation and natural selection, human behavior is also implicated. Failure to take antibiotics as prescribed, as well as overuse of antibiotics, leads to increased prevalence of antibiotic resistance in bacterial populations. Health providers sometimes prescribe antibiotics for patients who ask for them, even when there is no medical necessity. Antibiotics will not cure a cold, for instance, because viruses cause colds and antibiotics have no effect on viruses. Regardless, many people demand them from their doctors or get them from friends in the belief that antibiotics can cure anything.

The overuse of these drugs can speed up the natural selection process for antibiotic resistance. This is because one mode of competition among microorganisms is the production and release of antibiotics. Thus, bacteria were coping with naturally occurring antibiotics billions of years before humans started to use antibiotics to treat bacterial infections. Penicillin is produced by the mold Penicillium crysogenum, and most antibiotics used in medicine are still derived from microorganisms. Thus, exposure to antibiotics and the evolution of antibiotic resistance have been part of bacterial biology throughout their evolutionary history. What is new is the use of antibiotics to treat infections, the high frequency of antibiotic resistance found in bacteria in some settings, and the increasing occurrence of bacteria that are resistant to multiple antibiotics (Figure 11.12). The consequences of these new factors have enormous significance for human health.

343

Medical misuse and overuse of antibiotics are important issues, but the use of antibiotics as growth promoters in agriculture overshadows them. Antibiotics—in many cases, the same ones prescribed to humans—are administered in food and water to cattle, hogs, sheep, and poultry to reduce the incidence of bacterial disease when animals are kept in crowded conditions. The crowding reduces both the amount of feed and the time required to bring an animal to slaughter weight. However, crowding also creates ideal conditions for the evolution and spread of antibiotic-resistant bacteria (Figure 11.13).

Meat producers in the United States administer more antibiotics per kilogram of meat produced than in any other country. A study by the Pew Charitable Trusts estimated that in 2011, 13.6 million kilograms (30 million pounds) of antibiotics were used in the production of food-producing animals in the United States, mainly as additives to feed. This accounts for approximately 80% of the total use of antibiotics in the United States (Figure 11.14 below).

It is hard to imagine a more effective method of selecting for antibiotic resistance than to expose bacteria in the guts of livestock to constant low levels of antibiotics. Resistant bacteria that originated in livestock have been found in soil, ponds, and groundwater near sites where large numbers of animals are being reared. Furthermore, the transfer of antibiotic-resistant bacteria from animal to animal and from animals to farm workers has been documented.

Antibiotic-resistant intestinal bacteria have been isolated from wild animals as well—from field mice and bank voles in England, from magpies and rabbits in Wales, from forest birds in Brazil, from Canada geese in New Jersey, and from wild frogs in New York. None of these species has direct contact with humans, and the antibiotic resistance of their bacteria probably originated in farm animals.