β-Lactams and Florfenicol Antibiotics Remain Bioactive in Soils while Ciprofloxacin, Neomycin, and Tetracycline Are Neutralized

Abstract

It is generally assumed that antibiotic residues in soils select for antibiotic-resistant bacteria. This assumption was tested by separately adding 10 different antibiotics (>200 ppm) to three soil-water slurries (silt-loam, sand-loam, and sand; 20% soil [wt/vol]) and incubating mixtures for 24 h at room temperature. The antibiotic activity of the resultant supernatant was assessed by culturing a sensitive Escherichia coli strain in the filter-sterilized supernatant augmented with Luria-Bertani broth. We found striking differences in the abilities of supernatants to suppress growth of the indicator E. coli. Ampicillin, cephalothin, cefoxitin, ceftiofur, and florfenicol supernatants completely inhibited growth while bacterial growth was uninhibited in the presence of neomycin, tetracycline, and ciprofloxacin supernatants. High-performance liquid chromatography (HPLC) analysis demonstrated that cefoxitin and florfenicol were almost completely retained in the supernatants, whereas tetracycline and ciprofloxacin were mostly removed. Antibiotic dissipation in soil, presumably dominated by adsorption mechanisms, was sufficient to neutralize 200 ppm of tetracycline; this concentration is considerably higher than reported contamination levels. Soil pellets from the tetracycline slurries were resuspended in a minimal volume of medium to maximize the interaction between bacteria and soil particles, but sensitive bacteria were still unaffected by tetracycline (P ‫؍‬ 0.6). Thus, residual antibiotics in soil do not necessarily exert a selective pressure, and the degree to which the pharmaceutical remains bioactive depends on the antibiotic. Efforts to control antibiotic contamination would be better directed toward compounds that retain biological activity in soils (e.g., cephalosporins and florfenicol) because these are the antibiotics that could exert a selective pressure in the environment.

Joint antibacterial activity of soil-adsorbed antibiotics trimethoprim and sulfamethazine

“…Previous studies also reported that some soil-adsorbed antibiotics such as tetracycline, tylosin, oxytetracycline and ofloxacin retained antibacterial activity (Chander et al, 2005;Goetsch et al, 2012;Peng et al, 2014). But Subbiah et al (2011) found that β-lactams and florfenicol antibiotics remain bioactive in soils while ciprofloxacin, neomycin, and tetracycline are neutralized. This indicates that residual antibiotics in soil do not necessarily exert a selective pressure, and the degree to which antibiotic remains bioactive depends on the physiochemical properties and biological potency of this chemical.…”

“…However, knowledge about the relationship between antibiotic sorption behavior and antibacterial activity in soil environments is still scarce (Chander et al, 2005;Goetsch et al, 2012;Halling-Sørensen et al, 2003;Subbiah et al, 2011). Furthermore, although soil adsorbed antibiotics are expected to have the potential to exert biological effects against soil microorganisms, this aspect has not been well-elucidated.…”

Use of Aliphatic n -Alkynes To Discriminate Soil Nitrification Activities of Ammonia-Oxidizing Thaumarchaea and Bacteria

“…For instance, "Candidatus Nitrososphaera viennensis," isolated from neutral soil and demonstrating optimal growth at pH ϳ7.5, required 500 M ATU to stop the majority of nitrification (34). Second, although we have had success in measuring the relative contributions of AOA and AOB to NP in soil slurries by using the RNP assay with or without antibiotics (19), there are several challenges with extending this method into whole soil, including (i) difficulty in distributing aqueous solutions of antibiotics uniformly throughout an unsaturated soil, (ii) antibiotic effectiveness being negated by binding to soil particles (35,36), (iii) variable antibiotic resistance among AOB (this study), and (iv) incomplete removal of acetylene by degassing.…”

Comparison of the prevalence of antibiotic-resistant Escherichia coli isolates from commercial-layer and free-range chickens in Arusha district, Tanzania

“…To determine the prevalence of antibiotic resistance, E. coli isolates were tested against a panel of 11 antibiotics that belonged to seven different classes (β-lactams, cephalosporins, amphenicols, tetracyclines, sulfonamides, aminoglycosides and fluoroquinolones) by using a breakpoint assay (Subbiah et al, 2011). Briefly, MAC agar plates (150 mm, diameter) were prepared with each antibiotic at a fixed concentration (given below) that was guided by the Clinical Laboratory Standard Institute (CLSI) recommended minimum inhibitory concentration for E. coli (NCCLS, 2007).…”