Abstract
The current study investigated the impact of chicken litter application on the abundance of multidrug-resistant Enterococcus spp. in agricultural soil. Soil samples were collected from five different strategic places on a sugarcane farm before and after manure application for four months. Chicken litter samples were also collected. Enterococci were enumerated using the Enterolert®/Quanti-Tray 2000® system and confirm and differentiated into species using real-time PCR. The antibiotic susceptibility profile of the isolates was determined using the disk diffusion method following the European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines. The overall mean bacterial count was significantly higher (p < 0.05) in manure-amended soil (3.87 × 107 MPN/g) than unamended soil (2.89 × 107 MPN/g). Eight hundred and thirty-five enterococci (680 from soil and 155 from litter) were isolated, with E. casseliflavus being the most prevalent species (469; 56.2%) and E. gallinarum being the least (16; 1.2%). Approximately 56% of all the isolates were resistant to at least one antibiotic tested, with the highest resistance observed against tetracycline (33%) and the lowest against chloramphenicol (0.1%); 17% of E. faecium were resistant to quinupristin-dalfopristin. Additionally, 27.9% (130/466) of the isolates were multidrug-resistant, with litter-amended soil harbouring more multidrug-resistant (MDR) isolates (67.7%; 88/130) than unamended soil (10.0%; 13/130). All isolates were susceptible to tigecycline, linezolid and gentamicin. About 7% of the isolates had a multiple antimicrobial resistance index > 0.2, indicative of high antibiotic exposure. Although organic fertilizers are regarded as eco-friendly compared to chemical fertilizers for improving soil fertility, the application of untreated animal manure could promote the accumulation of antibiotics and their residues and antibiotic-resistant bacteria in the soil, creating an environmental reservoir of antimicrobial resistance, with potential human and environmental health risks.
Highlights
Poor soil fertility is a significant challenge for small and large-scale farming systems in sub-Saharan Africa, and chemical and organic fertilizers are frequently added to soil to improve its quality, texture, and crop yield [1]
Organic fertilizers are regarded as eco-friendly compared to chemical fertilizers for improving soil fertility, the application of untreated animal manure could promote the accumulation of antibiotics and their residues and antibiotic-resistant bacteria in the soil, creating an environmental reservoir of antimicrobial resistance, with potential human and environmental health risks
The use of antibiotics in livestock farming has proven to be beneficial for economic reasons, their use as growth promoters for prophylaxis, metaphylaxis and treatment establishes a reservoir of antibiotic-resistant bacteria (ARB), including multidrug-resistant (MDR) ones and antibiotic resistance genes (ARGs) in the gastrointestinal tract of livestock, and subsequently their waste [7]
Summary
Poor soil fertility is a significant challenge for small and large-scale farming systems in sub-Saharan Africa, and chemical and organic fertilizers are frequently added to soil to improve its quality, texture, and crop yield [1]. The use of antibiotics in livestock farming has proven to be beneficial for economic reasons, their use as growth promoters for prophylaxis, metaphylaxis and treatment establishes a reservoir of antibiotic-resistant bacteria (ARB), including multidrug-resistant (MDR) ones and antibiotic resistance genes (ARGs) in the gastrointestinal tract of livestock, and subsequently their waste [7]. The addition of such animal waste as manure to the soil, without treatment, may contribute to the transmission of antibiotic resistance to soil bacteria and pose serious environmental risks [8]. This agricultural practice has resulted in the contamination of soil, surface water, groundwater, and the food chain with antibiotic residues and ARB, posing a severe public health concern associated with farm produce such as raw vegetables [5,9]
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