The Effect of Clarithromycin Toxicity on the Growth of Bacterial Communities in Agricultural Soils

Temporal dynamics of the soil resistome and microbiome irrigated with treated wastewater containing clarithromycin

Clarithromycin, a common antibiotic found in domestic wastewater, persists even after treatment and can transfer to soils when treated wastewater (TWW) is used for irrigation. This residual antibiotic may exert selection pressure, promoting the spread of antibiotic resistance. While Predicted No Effect Concentrations (PNECs) are used in liquid media to predict resistance risks, PNEC values for soils, especially for clarithromycin, are lacking. Thus, this study aimed to assess clarithromycin's fate and its concentration threshold affecting soil microbial communities and macrolide resistance genes. The study used a soil microcosm approach with TWW containing clarithromycin at concentrations of 0, 0.01, 0.1, 0.5, and 1 mg/kgdry soil over a three-month period. Results showed clarithromycin persisted with limited degradation, likely due to strong adsorption to soil particles. Two transformation products were identified: decladinose-CLA (abiotic degradation) and phosphate-CLA (bacterial phosphotransferase activity). Soil bacterial communities were more influenced by TWW than by clarithromycin itself, as its antimicrobial effect was reduced due to adsorption. While clarithromycin did not significantly affect the abundance of resistance genes like intl1, mphA, and ereA, concentrations above 0.01 mg/kg increased the ermB gene abundance during the first week. The mefA gene (macrolide efflux pump) showed a hormetic effect: low doses (<0.1 mg/kg) increased gene abundance, while higher doses (>0.5 mg/kg) inhibited gene transfer or the bacteria carrying it. This study performed under controlled conditions provided insights into antibiotic resistance dynamics in soils exposed to clarithromycin, highlighting key concentration thresholds influencing resistance spread in soils.

Influence of soil type on bacterial growth and tolerance to experimentally added human antibiotics

The human antibiotics cefuroxime (CXM) and azithromycin (AZI) are among the most commonly prescribed. A significant portion of both are excreted and has been detected in sewage treatment plant effluents. The increasing use of such effluents in crops for irrigation and as fertilisers poses a threat to soil microbiota because of the presence of antibiotics. The lack of studies on CXM and AZI in soils hinders our understanding of their potential toxic effects on soil bacterial communities and ecosystem services. This study significantly contributes to the literature by quantifying the toxicity of CXM and AZI at varying concentrations in 12 different crop soils and tracking their evolution over time. The study also examined whether antibiotic pressure led to the development of more tolerant bacterial communities. The results of this study are the values of the logarithm of the antibiotic concentration at which 50 % of bacterial growth is inhibited (Log IC50) and indicate that both antibiotics are toxic to soil bacteria. The direct toxicity of CXM (1 day after contamination) was higher (Log IC50: 0.9 = 7.9 mg kg-1) than that of AZI (Log IC50: 3.4 = 2362 mg kg-1). However, bacterial growth was less affected by CXM over time, whereas AZI remained toxic in some soils until day 42 (Log IC50: 3.2 = 1533 mg kg-1 and 3.4 = 2291 mg kg-1, respectively). The overall results indicate that selective pressure exerted by antibiotics generates antibiotic tolerance in soils, even at the lowest antibiotic concentration studied (7.8 mg kg-1). The general trend was to increase tolerance to higher antibiotic concentrations up to the highest concentration studied (2000 mg kg-1). However, the degree of tolerance developed was highly dependent on soil type. More studies should be conducted to quantitatively assess the toxic and tolerance-developing effects of antibiotics in soils. Such information will be valuable for identifying which antibiotics pose a threat to the soil microbiota and consequently to human health.

Effects of ciprofloxacin, trimethoprim, and amoxicillin on microbial structure and growth as emerging pollutants reaching crop soils

The presence of emerging pollutants, and specifically antibiotics, in agricultural soils has increased notably in recent decades, causing growing concern as regards potential environmental and health issues. With this in mind, the current study focuses on evaluating the toxicity exerted by three antibiotics (amoxicillin, trimethoprim, and ciprofloxacin) on the growth of soil bacterial communities, when these pollutants are present at different doses, and considered in the short, medium, and long terms (1, 8 and 42 days of incubation). Specifically, the research was carried out in 12 agricultural soils having different physicochemical characteristics and was performed by means of the leucine (³H) incorporation method. In addition, changes in the structure of soil microbial communities at 8 and 42 days were studied in four of these soils, using the phospholipids of fatty acids method for this. The main results indicate that the most toxic antibiotic was amoxicillin, followed by trimethoprim and ciprofloxacin. The results also show that the toxicity of amoxicillin decreases with time, with values of Log IC₅₀ ranging from 0.07 ± 0.05 to 3.43 ± 0.08 for day 1, from 0.95 ± 0.07 to 3.97 ± 0.15 for day 8, and from 2.05 ± 0.03 to 3.18 ± 0.04 for day 42, during the incubation period. Regarding trimethoprim, 3 different behaviors were observed: for some soils the growth of soil bacterial communities was not affected, for a second group of soils trimethoprim toxicity showed dose-response effects that remained persistent over time, and, finally, for a third group of soils the toxicity of trimethoprim increased over time, being greater for longer incubation times (42 days). As regards ciprofloxacin, this antibiotic did not show a toxicity effect on the growth of soil bacterial communities for any of the soils or incubation times studied. Furthermore, the principal component analysis performed with the phospholipids of fatty acids results demonstrated that the microbial community structure of these agricultural soils, which persisted after 42 days of incubation, depended mainly on soil characteristics and, to a lesser extent, on the dose and type of antibiotic (amoxicillin, trimethoprim or ciprofloxacin). In addition, it was found that, in this research, the application of the three antibiotics to soils usually favored the presence of fungi and Gram-positive bacteria.

Special Issue on “Soil and Sustainable Development: Challenges and Solutions”

Food production is increasing year by year, with modern agriculture occupying high-fertility soils [...]