Antibiotic residues in swine fecal matter: effect on edaphic bacteria counts

Abstract

The use of antibiotics in animal production has raised concerns about environmental residues and persistence. In intensive pig farming, antibiotics are widely used, leading to reduced effectiveness and resistant strains that can transfer to other animals or humans. These compounds may not be fully absorbed or metabolized, allowing them to enter the environment through excreta. Significant concentrations of antibiotics have been detected in manure, potentially contaminating groundwater, surface waters, and soil, altering its microbiome. Despite these risks, systematic monitoring is lacking. This study aimed to assess the impact of residual amoxicillin (AMOXI) and oxytetracycline(OTC) in pig feces on soil total aerobic mesophilic bacteria (TAMB). Fecal samples (100 g) from pigs not treated with antimicrobials were fortified with known concentrations of AMOXI and OTC to simulate post-administration levels. Untreated feces were included as a control. Samples were placed outdoors, for a 30 day-period, on natural grassland at the CIVETAN experimental unit in Tandil, Bs. As., Argentina. Post-experiment soil samples (0-10 cm depth) were collected beneath each fecal deposit and analyzed using serial dilution, plating on nutrient agar, and incubation at 28°C for 24-48 hours. A one-way ANOVA was conducted to detect differences between groups. Results showed that soils beneath antibiotic-supplemented feces had higher average TAMB counts (AMOXI: 2.14 x 10⁷ CFU/g; OTC: 2.5 x 10⁷ CFU/g) compared to controls (1.29 x 10⁷ CFU/g), with statistically significant differences (p<0.01). These findings align with previous studies by Binh et al. (2007) and Chen et al. (2013), indicating that AMOXI and OTC increase total bacterial counts in soils exposed to these antibiotics. This likely occurs at the expense of other microbial populations targeted by these drugs. In conclusion, antibiotics in pig feces can reach the soil and alter its microbial composition.