What is Antimicrobial Resistance

Use of Antimicrobials in Food Animals

Medical Consequences of Antimicrobial Use in Food Animals

Resistance Monitoring

Antimicrobial is a term given to any type of chemical compound that can aid in the death or suppress the growth of microorganisms (bacteria, yeast, mycoplasma, etc). Antimicrobial compounds work by damaging the machinery inside of cells that is required to keep them functioning. The actions of antimicrobials do not always kill all the microorganisms found in the environment that they are being used. Some microorganisms can possess DNA or genes that carry the information necessary to destroy, and/or inactivate the action of antimicrobials. The DNA in bacteria capable of inactivating antimicrobials is normally present in only a very small percentage of the total bacterial population. When antimicrobials are introduced into an environment, bacteria that do not possess the DNA necessary to inactivate the antimicrobial compounds (susceptible bacteria) are killed. The elimination of the susceptible bacteria creates an opportunity for the remaining bacteria in the environment that do possess these resistance genes (resistant bacteria), to survive in greater numbers than before the introduction of antimicrobials. This was first observed over 50 years ago after penicillin was introduced to treat Staphylococcus aureus infections. It was soon noted, after they began to use penicillin, that in some patients infections could no longer be effectively treated. When a bacteria becomes resistant to a therapeutic drug that is indicated for a particular disease (first line of defense), clinicians must then search for another drug (second line of defense) to continue fighting a disease effectively. As bacterial resistance catches up with each new line of defense, clinicians and pharmaceutical companies will have to work harder to find treatment options that will continue to offer therapeutic success. Currently in hospitals there is increasing concern over certain strains of Staphylococcus aureus that possess so many resistance genes, that there is only one type of antibiotic left in the antimicrobial arsenal of over 100 drugs to fight infections. The concern is that Staphylococcus aureus will acquire resistance to the drug known as Vancomycin and there will be no treatment options left to fight this pathogen when an infection occurs with this super, resistant bacterial strain.

Antibiotic resistant strains that emerge from susceptible bacterial populations following exposure to antimicrobial compounds obtain acquired resistance. This acquired resistance may arise by genetic mutation of DNA but the primary means of becoming resistant is the acquisition of existing DNA from resistant bacteria within a population. Bacteria possess small pieces (or tiny loops) of DNA, called plasmids that are readily exchanged between individual organisms. Plasmids are the main vehicle for the spread of antimicrobial resistance in bacterial populations.

Scientific evidence clearly indicates that the use of antimicrobials can change the balance between susceptible and resistant bacterial populations. The greater the use of antibiotics, the more likely that a resistant bacteria can be selected for and increase their presence in a bacterial population. Not all bacterial populations have resistance genes present in them, so the use of antibiotics does not mean that resistance will automatically be a consequence of the use of antimicrobials. Resistant bacteria can be introduced, though, into a previously susceptible population. With the ease that we move about today with modern modes of travel, it is very easy for a person or animal to become infected with a resistant pathogen in one location and then transport this to a new area, thus facilitating the spread of antimicrobial resistance. Antibiotic resistant bacteria can be found in contaminated food, in the fecal flora of infected individuals, and harbored on individuals and animals.

The term "societal drugs" has been used to characterize antibiotics since each individual who uses antibiotics contributes to the total use of antibiotics by society and adds to the possibility that resistant bacteria are selected for. Since antibiotics are essential tools in improving and maintaining the quality of life by helping to prevent infectious diseases, we need to find ways to continue using antimicrobials that will slow down and possibly prevent the spread of antimicrobial resistant bacteria. Preserving bacterial populations that contain susceptible bacteria can be one of the goals in designing strategies for the prudent use of antimicrobials.

Use of Antimicrobials in Food Animals

In addition to the treatment of sick animals to fight infections, antimicrobials are also used at sub therapeutic levels to prophylactally prevent infections and at sub therapeutic levels as feed additives to promote growth and weight gain by increasing feed utilization. This is a practiced in the fish, cattle and poultry industries. Public health concerns have led to the question of whether the use of antimicrobials in food producing animals contributes to the increased level of antimicrobial resistance now seen in humans.

Medical Consequences of Antimicrobial Use in Food Animals

* An increase in the prevalence of resistant bacteria in animals

* The potential for the transfer of resistant bacteria from infected animals to humans through the consumption of contaminated food and water

* The transfer of resistant genes to human pathogens

* The possibility that Antimicrobial resistance can lead to treatment failures in animals and humans

Resistance Monitoring

Antimicrobial resistance manifests itself as bacterial infection that fails to respond to treatment after a drug is administered. The determination that antimicrobial resistance exists though, is determined by testing in the laboratory. Bacterial organisms suspected of antimicrobial resistance must first be isolated in the laboratory and then tested for growth in the presence of antimicrobial compounds. It is bacterial growth in the presence of antimicrobials that establishes the presence of resistance. Laboratory testing is expensive, takes several days, and requires specialized facilities. Due to these limitations, resistance testing is not always performed in cases were it is suspected. This infrequent testing has limited the knowledge about the size, distribution, stability, and trends in antimicrobial resistant bacterial populations. There are now efforts at the national and international levels that have been initiated to isolate indicator pathogens and test them for antimicrobial resistance to fill in the gaps in our understanding and improve our knowledge base on resistance.

In 1996 CDC, USDA, and FDA-CVM worked together to start NARMS, the National Antimicrobial Monitoring System, to track changes in the patterns of antibiotic susceptibility patterns for pathogens where there was a concern of transmissibility to humans (zoonosis). Bacterial isolates selected for testing in the NARM's program come from human and animal clinical cases from participating state and local health departments throughout the U.S. Species targeted for isolation and testing have been Salmonella, pathenogenic E. coli, and Campylobacter.