Antimicrobials & the Environment

Special Topics Issue:
Benchmarking Antimicrobial Use
& Antimicrobials in the Environment
Page 07 /

Antimicrobial resistance in food animals is interlinked with antimicrobial resistance in other populations, particularly in humans, and the wider environment. The interplay among these populations is especially important in the development of antimicrobial resistance, as the links between humans, animals, and the environment not only allow for the movement of bacteria but also different mobile genetic elements (which can transmit resistance to different bacteria) and antimicrobials themselves.

Exposure to Antimicrobial Resistant Bacteria in the Environment

Resistant bacteria can be introduced into the environment through a variety of routes, including but not limited to various kinds of human waste and animal feces. People may become exposed to both antimicrobial susceptible and resistant bacteria in the environment through several different routes¹, some of which also apply to exposure of livestock, pets and wildlife:

Recreation in contaminated surface water
- Previous studies have found that the risk of contracting Salmonella or Campylobacter in recreational waters is higher than or equal to the risk of contracting the organisms through chicken consumption
Consumption of contaminated drinking water
- In the United States, 9% of all outbreaks of E.coli O157 are waterborne
Consumption of fresh produce, or shellfish
- Produce can be contaminated with bacteria from soil or fertilizer (including animal manure), which is why they should always be thoroughly washed before consumption. Outbreaks of Enterobacteriaceae are commonly associated with consumption of fresh produce
- It is estimated that over 120 million cases of gastrointestinal disease occur annually as a result of eating raw or lightly cooked shellfish, which can be contaminated by bacteria found in the water from which they’re harvested.
Inhalation of aerosols
- Bacteria can also be found in aerosols, for example in very dry regions where contaminated soil and manure may become very dusty and be blown into the air by wind or machinery, or when manure sprayers are used. Aerosolized particles only travel a relatively short distance in the air, compared to airborne pathogens which can travel much further.

The natural environment is a plausible pathway for antimicrobial resistant bacteria to be transmitted to humans and farmed animals.

Development of Antimicrobial Resistance in the Environment

The minimum inhibitory concentration (MIC) refers to the concentration of an antimicrobial that is needed to inhibit the growth of, or kill, a target pathogen. However, when the concentration of the antimicrobial is below the MIC, this can lead to selection for and survival of bacteria that are resistant to certain antimicrobials. The varying concentrations of antimicrobials found in soil, sediment, and organic matter are thought to play an important role in the selection of antimicrobial resistant bacteria. In experimental studies, weekly exposure of bacteria to sub-lethal concentrations of diverse antimicrobials leads to selection and persistence of more common, frequent small-effect mutations that further increases resistance in the bacterial population.

Contamination of the Environment with Antimicrobials and Antimicrobial Resistant Bacteria

There are three major pathways identified in the literature for antimicrobial residues and resistant bacteria to enter into the environment²:

1. Municipal and industrial wastewater and sewage sludge
1. - A large amount of the antimicrobials consumed by humans are excreted in urine and feces in their biologically active form. In wastewater, these antimicrobials are either biodegraded, absorbed into sewage sludge, or exit into the wider environment unchanged. There are methods to reduce the persistence of antimicrobials by altering the salinity and temperature of the wastewater processing; however, this is not the focus of this FAAST review. For more information on this topic, check out: https://www.sciencedirect.com/science/article/pii/S0048969713000429
2. Spreading of animal manure
1. - When animals consume antimicrobials, as much as 30 to 90% is released into manure and urine, leading to antimicrobial residues entering and contaminating the environment. Animals harboring antimicrobial resistant bacteria can also shed these into the environment through bodily fluids, but especially manure.
3. Aquaculture
1. - Antimicrobials are mostly administered through feed to sick fish which could lead to the release of antimicrobials into the environment.
Also of importance: Improper disposal of antimicrobials
1. - Improper disposal of antimicrobials can also lead to the release of antimicrobial residues into the environment. Using proper disposal methods of containers that had antimicrobials is a critical way to reduce residues from entering into groundwater and other parts of the environment.

Next Page | Section 2B | Contamination Through Livestock Manure

Introduction to Livestock Medicines

Regulations on Antimicrobial Use and Access

Antimicrobial Stewardship

Antimicrobial Stewardship in the Ontario Beef Industry

Antimicrobial Stewardship in the Ontario Veal Industry

Antimicrobial Stewardship in the Ontario Dairy Industry

Antimicrobial Stewardship in the Ontario Aquaculture Industry

Exposure to Antimicrobial Resistant Bacteria in the Environment

Recreation in contaminated surface water

Consumption of contaminated drinking water

Consumption of fresh produce, or shellfish

Inhalation of aerosols

Development of Antimicrobial Resistance in the Environment

Contamination of the Environment with Antimicrobials and Antimicrobial Resistant Bacteria

1. Municipal and industrial wastewater and sewage sludge

2. Spreading of animal manure

3. Aquaculture

Also of importance: Improper disposal of antimicrobials