Abstract
Diffuse environmental antibiotic and antibiotic resistance gene contamination is increasing human and animal exposure to these emerging compounds with a consequent risk of reduction in antibiotic effectiveness. The present work investigated the effect of the antibiotic sulfamethoxazole (SMX) on growth and antibiotic resistance genes of a microbial community collected from an anaerobic digestion plant fed with cattle manure. Digestate samples were used as inoculum for concentration-dependent experiments using SMX at various concentrations. The antibiotic concentrations affecting the mixed microbial community in terms of growth and spread of resistant genes (sul1, sul2) were investigated through OD (Optical Density) measures and qPCR assays. Moreover, SMX biodegradation was assessed by LC-MS/MS analysis. The overall results showed that SMX concentrations in the range of those found in the environment did not affect the microbial community growth and did not select for antibiotic-resistant gene (ARG) maintenance or spread. Furthermore, the microorganisms tested were able to degrade SMX in only 24 h. This study confirms the complexity of antibiotic resistance spread in real matrices where different microorganisms coexist and suggests that antibiotic biodegradation needs to be included for fully understanding the resistance phenomena among bacteria.
Highlights
Antibiotic resistance (AR) in bacteria is a natural phenomenon [1]
The present study evaluated the antibiotic SMX effects on growth and AR spread of a mixed natural microbial community collected from an anaerobically digested cattle manure
Few data are currently available on its amount in digestate [30,31]; a recent study reports comparable SMX amounts in cattle manure samples collected from an anaerobic digestion (AD) plant-feeding tank [31]
Summary
Antibiotic resistance (AR) in bacteria is a natural phenomenon [1]. antibiotic abuse in human and veterinary medicine has caused a worldwide increase and spread in antibiotic multi-resistant bacteria among humans, animals, and the environment [2,3,4].The role of various anthropogenic sources (e.g., wastewater treatment plant effluents and biosolids, reclaimed water, organic amendments for agricultural practices, farm runoff) as key points in the diffusion of resistant bacteria and genes in soil and water ecosystems has been recognized [3,5]. Several ecological aspects of AR spread in natural microbial communities need to be clarified Microorganisms interact in both intra- and inter-specific relationships, modulated by site-specific environmental conditions (e.g., temperature, humidity, multi-contamination), which make each ecosystem a unique one. Most studies aimed at evaluating the MSC showed that environmental antibiotic residues (e.g., in the range of 100 pg/mL to ng/mL, depending on the antibiotic and the type of resistance mutation examined) can select resistant bacteria [12,13] Most of these works did not take into account the complexity of microbial community interactions, including possible horizontal gene transfer mechanisms (HGT-transfer among different bacterial species, taxa, and habitats) [14]
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