2 Integrating the NCA concept within a Source-Pathway-Receptor framework

2.1 Applying a Source-Pathway-Receptor Framework to AMR

Water catchment management is a growth area in the water sector through which AMR can be usefully explored. In order to standardise work on AMR, the source-pathway-receptor framework (SPR) will be applied to explore whether the NCA concept is appropriate in dealing with AMR.

2.1.1 Defining the ‘source contaminant’

In order to apply a SPR framework to AMR, we must first identify what aspect of AMR we are classifying as the ‘contaminant’ or ‘source’ for which NCA could be applied to prevent it either occurring or reaching a receptor. Through classifying the ‘contaminant’ we must also consider the appropriateness of NCA. We must ensure that by using a NCA, we are actively addressing the issue of AMR rather than simply ‘using a spider to catch a fly’ in which the chosen NCA results in a cascade effect; such as by shifting the issue of AMR in the water and environment into the ecological sector.

In order to avoid such a scenario, we must first consider the selective pressures responsible for driving the evolution and dissemination of AMR genes within populations of micro-organisms. Various studies have concluded that the drivers of AMR may be characterised under the following three classes of antimicrobials:

● Clinical antimicrobials: anti-bacterials, antifungals, antivirals and antiparasitics

● Heavy metals

● Biocides: disinfectants, surfactants, antifouling materials

It must be noted that the term ‘antimicrobials’ is often used throughout literature to describe only anti-bacterials, antifungals, antivirals and antiparasitics. However, as stated in section 1.2, the definition of antimicrobials is: any natural or synthetic substance that kills or inhibits the growth of micro-organisms. Therefore, within the context of this report, the term ‘clinical antimicrobials’ has been used to refer only to those antimicrobials which have been administered for antipathogenic purposes within clinical or veterinary environments. By contrast, the term ‘antimicrobials’ is used within this report to denote any class of antimicrobial as listed above.

In order to explore the use of NCA as an action plan for AMR, as part of water catchment management; driving agents of AMR will be considered as the ‘contaminant’ within the framework of a SPR model.

2.1.2 Defining the source-pathway-receptor

If the driving agents of AMR are to be considered the source contaminant in an S-P-R framework, then the role of micro-organisms within the SPR model must also be debated before the model may be conceptualised.

AMR genes evolve due to selective pressures within the environment; if these genes are incorporated into of a micro-organism (either within chromosomal DNA or within the cytoplasm where they may utilise protein production pathways) and expressed successfully, then that micro-organism will gain resistance. In this context, the micro-organism would be classified as the receptor of AMR.

In evolutionary terms; if a selective pressure is removed then the evolved gene will no longer confer a selective advantage, and therefore risks being outcompeted (unless exaptation occurs). For AMR to continually and successfully disseminate throughout a population of microorganisms, the same selective pressure must exist within the reservoir in which the population is being harboured.

By this logic, AMR will be expressed as a result of contamination of the micro-organism reservoir by the ‘contaminant’ i.e. the driving agents of AMR resistance. By this argument, we may therefore classify the micro-organism reservoir as the receptor within the framework.

Thus, in the context of AMR in water and the environment, the SPR framework may be defined as:

● SOURCE: the source of the driving agents entering the water and environment

● PATHWAY: the mechanism and methods by which the driving agents enter ground and surface water and are then transported within it.

● RECEPTOR: micro-organism reservoirs (both living and non-living) within water and the environment in which both driving agents and AMR genes accumulate AMR to disseminate throughout a population.

2.1.3 SPR Conceptual Model

A simplified SPR conceptual model can be seen in Table 1.