Introduction

Since their discovery in the 1920’s antimicrobial drugs have clearly shown their clinical worth by improving the well-being of humans and animals. However, with the emergence and selection of resistant strains of micro-organisms the clinical efficacy of some antimicrobial drugs is increasingly compromised. While it is true that antimicrobial use can select for resistant forms of bacteria, it is not clear which specific uses have the largest impact on prevalence of resistance, nor is it clear which ecological interactions are most efficient for transfer of resistant organisms or genetic resistance determinants among niches.

Antimicrobial Drugs and Mechanisms of Resistance

There are a variety of synthetic and naturally* occurring substances that are effective in limiting the growth (bacteriostatic action) or killing (bacteriocidal action) various microorganisms. While the remainder of this discussion will focus on the occurrence of antimicrobial resistance among bacterial species, similar concerns exist with regard to resistance among the viruses (e.g., HIV), and fungi. Antimicrobial drugs exert their effects in a variety of ways: inhibiting cell wall synthesis, inhibiting protein synthesis, interfering with cell membrane function, interfering with DNA/RNA synthesis, and inhibiting metabolism. Resistance to antimicrobial drugs, therefore, occurs when bacteria are capable of stopping or interfering with these actions. Some bacteria are naturally resistant to some antimicrobial agents (intrinsic resistance) because the target molecule for the antimicrobial drug is not present in that type of bacterium, or because it is not possible to achieve effective drug concentrations within the cell. Other bacteria that are normally susceptible to an antimicrobial drug can become variably resistant because of genetic modification. These modifications enable bacteria to resist antimicrobial effects in a number of ways, including reducing the amount of antibiotic that enters the bacterial cell and reaches the target (by pumping the drug out of the cell or altering the permeability of the cell to that drug), altering the drug so that it no longer has activity, modifying the target of the drug so that it is no longer affected, or by replacing the target with one that is not affected by the drug. It should also be noted that a similar manifestation of resistance to an antimicrobial drug can arise through several different modes of action. Antimicrobial resistance in a particular bacterial species can arise by one or more mutations to the genetic material of the cell or can be acquired from other cells of similar or dissimilar species or even different genera. While the short generation interval and the extraordinary number of bacteria that exist in any one ecosystem give rise to tremendous opportunity for genetic mutation to occur, this not the most likely method of acquiring new resistance. The primary mechanism by which resistance determinants move through and among populations of bacteria is through transfer of resistance genes from one bacterial cell to another.

Gene transfer can occur via transformation, transduction, or conjugation. Transformation occurs when naked DNA, which can be very stable in the environment, is picked up by a living bacterial cell and incorporated in either the chromosomal DNA or some extra-chromosomal genetic material i.e., a plasmid. Transduction occurs when genetic material is transferred among cells by a bacteriophage. Conjugation occurs when mobile genetic elements (plasmids) are transferred between bacterial cells through a sex pilus. Any of these means can result in gene transfer among cells of similar species but also even among cells of different genera. The picture of gene mobility was further complicated by the discovery of transposons and integrons. Transposons are mobile genetic elements that can freely move from the chromosomal DNA to a plasmid and back again. While resident on the plasmid, the genes are available for exchange with other cells via the methods described above. So, determining that a resistance gene is located on the chromosomal DNA does not necessarily indicate a low availability or low likelihood of transfer to other cells. Integrons are often found on plasmids and have been described as sticky flypaper for resistance genes. Because of their structure and their ready acceptance of resistance genes integrons can accumulate multiple resistance genes in close proximity. These “cassettes” of multiple resistance genes tend to be transferred together by the methods described above, conferring resistance to multiple antimicrobials after a single uptake of genetic material. Furthermore, the use of any of the antimicrobials represented in the resistance cassette is thought to select for bacterial clones that contain the entire gene cassette, therefore tending to propagate the multiple resistance attributes in bacterial populations. With such efficient and sophisticated mechanisms to distribute antimicrobial resistance genes it should be no surprise that shortly after Florey and Chain initiated clinical trials with Penicillin for the control of bacterial infections that resistant strains were identified as predicted by Fleming.

It is important to note that resistance mechanisms did not arise because of antimicrobial drug use. The genetic basis for resistance likely originated from point mutations which allowed bacteria to evade the specific detrimental effects of antimicrobial substances which were produced by competing bacterial cells. As these mutations can be thought of as genetic accidents, it is likely that clones of these resistant bacteria begin as a minority of bacterial populations. These mutations, which convey antimicrobial resistance, in most cases, would likely not result in any survival advantage if bacterial populations were not exposed to antimicrobial drugs. Evolution of resistant bacteria is theoretically inevitable when antimicrobial drugs are used because they favor evolution of bacteria that have particular survival advantages. As such, resistant bacteria are likely to become more prevalent in bacterial populations when drug use is common. However, the speed with which this resistance spreads in the host population populations undoubtedly varies depending on the particular use practices of prescribers and consumers. Some key factors affecting this evolution are the amount of antimicrobial drug used, the types of drugs used, the dosage regimens, the frequency of co-infection with resistant organisms, public behavior, and social conditions.

Interactions that potentially allow spread of bacteria, and therefore antimicrobial resistance, through ecosystems are extraordinarily complex. It has been proposed that a major method for zoonotic transfer of antimicrobial resistance is through contaminated food. However, it is important to remember that this exchange of bacteria among ecological niches is not a simple, one-way exchange. Figure 1 shows some of the complex interactions that may be related to the spread of resistance to humans and animals. There are many ways that food-producing animals could acquire resistant bacteria. Selection for resistance through antimicrobial drug use is one means, but exposure resistant bacteria that evolved in humans is another. Thus careful examination of the entire ecosystem is warranted in order to fully understand which are the most important control points for affecting the development and spread of resistance.

It has been suggested that heavy usage of antimicrobial drugs is one of the most important factors affecting the rate of resistance evolution, but usage and resistance rates also vary from country to country. It is interesting to note that the US and Japan together have about 10% of the world population but account for over 60% of the world market in antimicrobial agents. Interestingly, both countries have high rates of antimicrobial resistance in many common pathogens (SMAC). Table 1 lists common human pathogens and resistance problems that are currently encountered by physicians and health officials.

Table 1. Examples of Human Pathogenic Bacteria and Resistance Concerns (adapted from JETACAR).

The Concern about Veterinary Use of Antimicrobial Drugs

As bacterial isolates were more commonly found to be resistant, and human patients failed to respond to antimicrobial treatment with increasing frequency, the debate about the appropriateness of various types of antimicrobial drug use in animals was fueled. Early on researchers discovered that feeding small doses of antimicrobial drugs to animals resulted in improved growth and feed efficiency, although the mechanism for this effect is still unknown today. Some proposed mechanisms include control of subclinical disease, stabilizing the gut flora, or selection of specific populations of gut flora. This use has been and probably remains the most controversial use of antimicrobial drugs in animals. Numerous groups have debated the merits and potential adverse consequences of various antimicrobial uses in animals. The Swann committee from the U.K. in 1969 issued a report and called for the banning of certain antimicrobial growth promoters in animals. The FDA in the U.S. proposed a ban of Tetracycline and Penicillin for growth promotion in 1972. This proposal was never accepted and resulted in the convening of a National Academy of Sciences group to evaluate the potential human health impacts of sub-therapeutic use of antimicrobial drugs for growth promotion in animals. They concluded that there was insufficient evidence to link the feeding sub-therapeutic doses of antimicrobial drugs to animals with adverse effects on human health. Over the next 15 years many governmental groups have considered the issue of the public health impacts of antimicrobial use in animals with few definitive conclusions. More recently the emergence and epidemic spread in livestock of a multi-drug resistant form of Salmonella Typhimurium that was phage type (definitive type) DT104, and concerns about emerging fluoroquinolone-resistant Campylobacter spp. in poultry have brought the issue back into the center of interest.

Actions on the Issue of Antimicrobial Resistance

Based on concerns with the use of antimicrobials a variety of actions have been taken in numerous countries. In some countries specific uses of some antimicrobials have been banned. The primary actions that are generally being considered to help protect public health regarding antimicrobial resistance include limitations on use, monitoring and surveillance, increased research, and prudent use campaigns. Some countries have initiated or strengthened monitoring and surveillance efforts directed at antimicrobial resistance. Some countries have encouraged expanded research efforts directed at understanding the ecology of antimicrobial resistance with an emphasis on understanding the roles of various antimicrobial uses in adverse human health outcomes and identifying mitigation strategies. Finally, some countries have geared up education campaigns directed at physicians, patients, veterinarians, and clients to help assure that antimicrobials are used in a way to insure the best treatment outcomes without enhancing the development of resistance.

Antimicrobial Resistance: Animals, Food, and the Future

Antimicrobial Use in Agriculture

Antimicrobial drugs are used in both animal and crop agriculture. Streptomycin and Tetracycline are used on fruit crops to control Erwinia, the causative agent of fire-blight. In animal agriculture, antimicrobial drugs are used as therapeutics, for metaphylaxis, for prophylaxis, and for growth promotion. Metaphylaxis is the administration of antibiotics to all animals in contact within the group, to treat a symptomatic disease in some and to prevent disease in others. Prophylaxis could be illustrated by the use of antimicrobials to prevent the occurrence of clinical anaplasmosis or liver abscesses in cattle. A variety of antimicrobials are also used to improve feed efficiency and boost gain in animals. The actual amount of antimicrobial drugs used in animals has been estimated but there is not consensus on the actual total amount used nor is there consensus on the amount used by type of use. Based on extrapolations from an Institute of Medicine report, the total amount of antimicrobial production and use in the U.S. has been estimated at 50 million pounds. Levy has estimated that half of this use if for humans and half is for animal use. The Animal Health Institute has estimated that for 1998, approximately 17.8 million pounds of antimicrobials were produced for animal use. Further, they attributed 14.7 million pounds to therapeutic use and disease prevention and 3.1 million pounds to growth promotion. The Union of Concerned Scientists (UCS) has estimated that 24.6 million pounds of antimicrobials are used annually by the three major animal sectors (swine, cattle and poultry) for nontherapeutic purposes. The UCS has also estimated that approximately 3 million pounds of antimicrobials are used in human medicine annually. Some of the discrepancy in the use figures are no doubt attributable to definitions of what constitutes nontherapeutic use and indeed whether ionophores and coccidiostats ought to be counted in the total use or not. Currently, the data do not exist to definitively estimate the total use of antimicrobial drugs in agriculture, veterinary medicine, or human medicine, let alone breaking down these estimates by the type of use.

Antimicrobial Resistance and Food

There are numerous reports on the magnitude of food-borne illness in the U.S. The most comprehensive estimates were assembled by Mead et. al. and indicate that there are 76 million cases of food-borne illness annually. According to this most recent CDC estimate, most (81%) of these illnesses have an unknown etiology. Among the disease with a known or attributed etiology (13.8 million cases), approximately 4.2 million cases are attributable to bacterial causes. Some of these bacterial food-borne illnesses have been associated with agents that were phenotypically resistant to one or more antimicrobial agents. Trace-back investigations of these types of cases or outbreaks are extremely difficult for a variety of reasons including the complexity of the food distribution system, the complexity of the food processing system, the complexity of the animal production systems, and the time to diagnosis and recovery of an organism from human patients. In most cases the authors of these reports have demonstrated that food-borne transmission of pathogens can and does occur and that sometimes these pathogens possess antimicrobial resistance determinants. However, they have not been able to do more than speculate that the resistance determinant arose in the animal population because of antimicrobial drug exposure. They have often failed to consider other routes of exposure of animals to resistant organisms. The simple theory that antimicrobial use practices in animals could pose a public health risk, regardless of size of that risk, has been a stimulus to the agriculture producers and researchers to develop more data on this issue. Specifically, these groups are currently actively examining common production practices and their role in the amplification and spread of resistance in animal populations, as well as the ability to transfer resistant determinants through the food chain.

The Future

Though emergence of antimicrobial resistance is not a new issue, the lack of data assessing the societal risk has previously been an impediment to policy making, with the implied message that decisions would be made when these data were generated. With the recent expanded interest in food safety and a growing concern for antimicrobial resistance in human medicine, the issue of appropriate antimicrobial drug use in animals has once again arisen for discussion. However, the climate for governmental inaction on this issue appears to have changed, and it is possible that regulatory decisions may soon be made with or without supporting scientific information. Policy makers no longer appear to be stymied by a lack of adequate objective data. Policy makers and the public are finding it more acceptable to make decisions based on the best available data at the time and pledging to revisit decisions based on new data that may be brought to light in the future. This change of process has spurred a great deal of activity in both the public and private sectors. The future will hold 1) expanded monitoring and surveillance efforts directed at charting trends in antimicrobial resistance in bacterial populations (from animals and people) of concern, 2) expanded research efforts to more fully delineate the ecologic linkages between the various niches for antimicrobial resistance, 3) more efforts at risk assessment to define the relative magnitude of the linkages and what constitutes unacceptable consequences, 4) continued efforts to educate all those who use antimicrobials on the resistance issue and prudent use practices, and 5) continued policy making and regulatory actions.

*Antibiotics are naturally occurring substances with activity against bacteria resulting in decreased growth or killing.