Perhaps a more optimistic start to our antimicrobial resistance (AMR) series: the annual amount of antibiotics used for treating crops is relatively low compared to the amount used in livestock production, comprising only 0.36% of total antibiotic consumption in the agricultural industry (Smalla and Tiedje, 2014). This is because there are only a relatively small amount of bacterial diseases so difficult to control, antibiotics are required to compensate for the crops’ lack of natural resistance to these diseases (FAO, 2018a; McManus, 2014). It is due to this reason that antibiotic use on crops fell out of the limelight when it comes to major efforts to reduce its usage in the agri-food industry.
Nevertheless, there remain some areas of concern. In pre-harvests, antibiotics are largely administered to plants in the form of fine mists (Zhang et al., 2017). Indirect application could also still happen via the use of soil, organic fertiliser, e.g. manure, and irrigation water already contaminated with AMR bacteria (FAO, 2018b). Unfortunately, the full effects of how antibiotics interact through these methods are complex and still relatively understudied. Antibiotics applied via the usual air blast sprays to trees planted in an orchard system, for example, were found to not always reach their intended targets since sprays may drift or become lost by runoff due to external events such as rain, thereby complicating studies (Raman et al., 2020).
Pathways of antimicrobial agents (AMA) and AMR dissemination. Movement of AMA or AMR is indicated by the overlapping circles and arrows, while colours denote group reservoirs. Stars represent AMR genes and bacteria hotspots. Asterisks represent possible AMR genes and bacteria hotspots (Thanner, Drissner, and Walsh, 2016).
Data on antibiotics in the context of plant agriculture is scarce, even more so than its aquaculture counterpart. Global estimates on the antibiotic use is virtually non-existent, and the full effects of said use, including how consuming crops contributes to the spread of antibiotic AMR genes, has yet to be fully understood. Fortunately, there is gradually mounting evidence of the potentially adverse effects on public health and the environment: vegetables grown conventionally and organically to be consumed raw facilitate the spread of AMR genes (van Hoek et al., 2015); AMR bacteria transmitted from plants to humans could asymptomatically “colonise” the intestines for an extended period of time before being discharged from the body (Maeusli et al., 2020); already contaminated crops grown as feed introduces AMR genes into the animals fed with these crops, thus accelerating the spread of AMR genes into the food chain (Marshall and Levy, 2011).
This is the fourth article of a multi-part series on the topic of antimicrobial use in the agri-food sector by Khor Reports. Read the previous posts here: Antimicrobial Resistance: Part #1 - The General Gist; Antimicrobial Resistance: Part #2 - Antimicrobials in Livestock; Antimicrobial Resistance: Part #3 - Antimicrobials in Aquaculture.