Abstract
The increasing demand for animal-derived foods has led to intensive and large-scale livestock production with the consequent formation of large amounts of manure. Livestock manure is widely used in agricultural practices as soil fertilizer worldwide. However, several antibiotic residues, antibiotic resistance genes (ARGs) and antibiotic-resistant bacteria are frequently detected in manure and manure-amended soils. This review explores the role of manure in the persistence and dissemination of ARGs in the environment, analyzes the procedures used to decrease antimicrobial resistance in manure and the potential impact of manure application in public health. We highlight that manure shows unique features as a hotspot for antimicrobial gene dissemination by horizontal transfer events: richness in nutrients, a high abundance and diversity of bacteria populations and antibiotic residues that may exert a selective pressure on bacteria and trigger gene mobilization; reduction methodologies are able to reduce the concentrations of some, but not all, antimicrobials and microorganisms. Conjugation events are often seen in the manure environment, even after composting. Antibiotic resistance is considered a growing threat to human, animal and environmental health. Therefore, it is crucial to reduce the amount of antimicrobials and the load of antimicrobial resistant bacteria that end up in soil.
Highlights
Livestock production is an extremely dynamic activity, highly influenced by human population growth, and by competition for natural resources and by human health and environmental concerns
We will discuss the composition of manure, highlighting the most frequent antibiotics and antibiotic resistance genes (ARGs) associated with mobile genetic elements and the horizontal gene transfer (HGT) mechanisms described so far in manure, to better understand how manure can be a source of antimicrobial resistance spread, the potential impact in public health and the procedures used to reduce this risk and their limitations
The major concern about the presence of ARGs in manure is the possibility of their transfer to the soil bacteria via HGT mechanisms, which can promote the spread of antimicrobial resistance among different microbial communities [30,37,48]
Summary
Livestock production is an extremely dynamic activity, highly influenced by human population growth, and by competition for natural resources and by human health and environmental concerns. Manure gives to the soil a unique environment for the spread of antibiotic resistance genes by horizontal gene transfer (HGT) mechanisms [16,17] It is rich in nutrients and presents dense and highly diverse bacterial populations and accumulates antibiotic residues and/or metals that act as a selective pressure and may trigger the exchange of bacterial DNA. These elements mix with agricultural soil bacterial communities and irrigation water, reaching microbial communities from different ecosystems, including water streams, and spread to the environment, wildlife and human and other animal communities [18]. We will discuss the composition of manure, highlighting the most frequent antibiotics and ARGs associated with mobile genetic elements and the HGT mechanisms described so far in manure, to better understand how manure can be a source of antimicrobial resistance spread, the potential impact in public health and the procedures used to reduce this risk and their limitations
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