Antimicrobial resistance (AMR) is a growing concern not only in hospitals but also in our farms and fields. When pathogenic bacteria acquire AMR, antimicrobials previously used to treat them are no longer effective causing a greater burden of disease. The use of antimicrobials in livestock can lead to increasing emergence of AMR which can spread more widely in the environment when farmyard manure or slurry is used as organic fertiliser to improve soil health and productivity in grasslands.

This blog delves into a five-year project (we’re at the halfway point!), aimed at exploring how different fertilisation practices, including biosolids from sewage sludge, influence the dynamics of AMR in pasture ecosystems. The research investigates whether sewage sludge pellets (biosolids) might offer a safer alternative to traditional animal-derived fertilisers.

Farm animals near a human dwelling (Credit: Nuno Silva)

Digging into the Problem: AMR in Agricultural Ecosystems

The use of antimicrobials in livestock farming has proven essential for maintaining animal health, but are known to contain resistant bacteria due to the use of antimicrobials in livestock. As bacteria become resistant to these treatments, they can spread resistance genes into the wider environment when animal manure or slurry is used as fertiliser. Antimicrobial resistance may persist in the soil for long period of time, increasing the risk of transferring these genes to soil microbiota as well as to animal and human pathogenic bacteria. This occurs through the ability of bacteria to acquire resistance genes by horizontal gene transfer, that is, by movement of genetic material between bacteria by means that are not reproduction.

AMR and Farm Practices (Illustration by Joana Silva)

So, What About use Biosolids as a Fertiliser?

Biosolids, derived from treated sewage sludge, contain organic nutrients that can be used to enrich soils. The sludge is transformed into biosolids through two main processes: the physical removal of a significant portion of water and the biological reduction or elimination of pathogens. As a result, the biosolids generated may be used on land applications (crops lands, grasslands and forest) providing a range of nutrients to the soil. They sound like a good alternative to animal manure, right? But wait - biosolids can also harbour resistance genes just like manure and slurry, that could persist for extended periods in soil and the environment and increase the risk of development of AMR.

Comparing Fertilisation Practices- What We’re Learning So Far

Our research aims to compare whether the application of farmyard manure, slurry, and biosolids have different effects on the abundance of AMR genes and resistant bacteria in the agricultural ecosystem by testing the pasture soil, animals that graze on it and the surrounding water bodies. This project extends our prior research to assess the impact of biosolid application on the abundance of antimicrobial resistance genes in soil. To explore this, we have analysed two fields, one fertilised with biosolids, and the other with the old-fashioned inorganic fertiliser. Early results? So far, data revealed no statistically significant differences in the abundance of resistance genes before and after biosolids treatment or between the two fields.