Background
Antimicrobial resistance (AMR) is a major global health threat in which microorganisms, including bacteria, fungi, and viruses, develop the ability to withstand the antimicrobials designed to treat them. It is projected that by 2050, AMR could become the leading cause of death worldwide, potentially resulting in up to 10 million deaths annually. In 2019 alone, bacterial AMR was directly attributed to 1.27 million deaths globally. AMR is recognised as a “One Health” issue, meaning humans, animals, and the environment are all implicated in its spread and maintenance.
Traditionally, it was believed that environmental concentrations of antimicrobials and co-selective agents were too low to exert selective pressure for antimicrobial resistance (AMR). It was believed that resistance would not increase at concentrations below the minimum inhibitory concentration (MIC), the threshold at which susceptible bacteria are either inhibited (bacteriostatic) or killed (bactericidal). However, this understanding has evolved. It is now recognised that resistance can be selected at sub-MIC concentrations, where the growth rate of susceptible bacteria is sufficiently reduced, allowing resistant strains to outcompete them. The lowest concentration at which resistant bacteria are preferentially enriched over susceptible ones is termed the minimal selective concentration (MSC). While research on defining MSCs for individual antibiotics is a growing and active field, research investigating other antimicrobials (such as antifungals) and co-selective agents, and environmentally realistic mixtures of chemicals, remain in their early stages.
Further, other chemicals released from anthropogenic pollution sources, such as non-antibiotic pharmaceuticals, personal care products, disinfectants and heavy metals are able to increase AMR through co-selection. This can occur through two key ways: 1) cross-resistance, where the same resistance mechanism is effective against the antimicrobial and the other compound (e.g., efflux pump); and 2) co-resistance, where AMR genes and resistance genes for the other compound are co-located, for example on the same plasmid. This can lead to an increased in AMR genes and organisms.
If humans interact with the environment, through either recreational (e.g., swimming in bathing and non-bathing waters), occupational (e.g., working at a wastewater treatment plant) or essential (e.g., breathing air) activities, they may be exposed to resistant microorganisms. Examples of this could include resistant E. coli in bathing waters or contact with a wastewater, or exposure to aerosolised azole resistant fungus Aspergillus fumigatus through breathing.
Recorded Lecture
Dr Isobel Stanton, an environmental molecular biologist at UKCEH, gives short overview of the issues around antibiotics and antimicrobial resistance in the environment
Key Reading
Clinical information on AMR: Global burden of bacterial antimicrobial resistance in 2019: a systematic analysis – Murray et al 2022
AMR in the environment: Antibiotic resistance in the Environment – Larsson & Flach 2022
Meta-analysis of selective concentrations: A critical meta-analysis of predicted no effect concentrations for antimicrobial resistance selection in the environment – Murray et al 2024
Overview of papers showing risk of environmental AMR on human health: Existing evidence on antibiotic resistance exposure and transmission to humans from the environment: a systematic map – Stanton et al 2022
“Beach bum” survey investigating AMR in surfers (discussed in Learn in 10 lecture): Exposure to and colonisation by antibiotic-resistant E. coli in UK coastal water users: Environmental surveillance, exposure assessment, and epidemiological study (Beach Bum Survey) – Leonard et al 2018