Binding of ciprofloxacin by humic substances: a molecular dynamics study

Occurrence and sorption of fluoroquinolones in poultry litters and soils from São Paulo State, Brazil

Animal production is one of the most expressive sectors of Brazilian agro-economy. Although antibiotics are routinely used in this activity, their occurrence, fate, and potential impacts to the local environment are largely unknown. This research evaluated sorption-desorption and occurrence of four commonly used fluoroquinolones (norfloxacin, ciprofloxacin, danofloxacin, and enrofloxacin) in poultry litter and soil samples from São Paulo State, Brazil. The sorption-desorption studies involved batch equilibration technique and followed the OECD guideline for pesticides. All compounds were analyzed by HPLC, using fluorescence detector. Fluoroquinolones' sorption potential to the poultry litters (K(d) ≤65 L kg(-1)) was lower than to the soil (K(d) ~40,000 L kg(-1)), but was always high (≥69% of applied amount) indicating a higher specificity of fluoroquinolones interaction with soils. The addition of poultry litter (5%) to the soil had not affected sorption or desorption of these compounds. Desorption was negligible in the soil (≤0.5% of sorbed amount), but not in the poultry litters (up to 42% of sorbed amount). Fluoroquinolones' mean concentrations found in the poultry litters (1.37 to 6.68 mg kg(-1)) and soils (22.93 μg kg(-1)) were compatible to those found elsewhere (Austria, China, and Turkey). Enrofloxacin was the most often detected compound (30% of poultry litters and 27% of soils) at the highest mean concentrations (6.68 mg kg(-1) for poultry litters and 22.93 μg kg(-1) for soils). These results show that antibiotics are routinely used in poultry production and might represent one potential source of pollution to the environment that has been largely ignored and should be further investigated in Brazil.

Effect of sterilized human fecal extract on the sensitivity of Escherichia coli ATCC 25922 to enrofloxacin

The ingestion of antimicrobial residues in foods of animal origin has the potential risk of exposing colonic bacteria to small concentrations of antibiotics and inducing resistance in the colonic bacteria. To investigate whether human intestinal contents would influence resistance development in bacteria, Escherichia coli ATCC 25922 (MIC of enrofloxacin <0.03 μg ml(-1)) was exposed to 0.01 to 1 μg ml(-1) of enrofloxacin in media supplemented with glucose, sucrose, sodium acetate or sterilized human fecal extract. In the first passage, only the medium containing sterilized fecal extract supported the growth of E. coli at an enrofloxacin concentration equal to the MIC. In the second and third passages following exposure to sub-inhibitory concentrations of the drug, the bacteria in media containing sterilized fecal extract grew at 0.1 μg ml(-1) of enrofloxacin. The efflux pump inhibitors, reserpine and carbonyl cyanide-m-chlorophenylhydrazone (CCCP), increased the sensitivity of bacteria to 0.1 μg ml(-1) of enrofloxacin in the medium containing sucrose, but their effect was not observed in the medium supplemented with 2.5% sterilized fecal extract. The proportions of unsaturated and saturated fatty acids in E. coli grown in the medium with 2.5% sterilized fecal extract differed from those grown in the medium alone. Fecal extract may contain unknown factors that augment the ability of E. coli to grow in concentrations of enrofloxacin higher than MIC, both in the presence and absence of efflux pump inhibitors. This is the first study showing that fecal extract affects the level of sensitivity of E. coli to antimicrobial agents.

Adsorption of aromatic carboxylate ions to black carbon (biochar) is accompanied by proton exchange with water

We examined the adsorption of the allelopathic aromatic acids (AA), cinnamic and coumaric, to different charcoals (biochars) as part of a study on bioavailability of natural signaling chemicals in soil. Sorption isotherms in pH 7 buffer, where the AAs are >99% dissociated, are highly nonlinear, give distribution ratios as high as 10(4.8) L/kg, and are insensitive to Ca(2+) or Mg(2+). In unbuffered media, sorption becomes progressively suppressed with loading and is accompanied by release of OH(-) with a stoichiometry approaching 1 at low concentrations, declining to about 0.4-0.5 as the pH rises. Sorption of cinnamate on graphite as a model for charcoal was roughly comparable on a surface area basis, but released negligible OH(-). A novel scheme is proposed that explains the pH dependence of adsorption and OH(-) stoichiometry and the graphite results. In a key step, AA(-) undergoes proton exchange with water. To overcome the unfavorable proton exchange free energy, we suggest AA engages in a type of hydrogen bond recognized to be of unusual strength with a surface carboxylate or phenolate group having a comparable pK(a). This bond is depicted as [RCO(2)···H···O-surf](-). The same is possible for AA(-), but results in increased surface charge. The proton exchange pathway appears open to other weak acid adsorbates, including humic substances, on carbonaceous materials.

Sorption of the cyanobacterial toxins cylindrospermopsin and anatoxin-a to sediments

The occurrence of the cyanobacterial toxins anatoxin-a (ATX) and cylindrospermopsin (CYN) in surface waters has been reported throughout the world. Beside degradation, sorption is an important pathway for toxin elimination if these resources are used for drinking water production via sediment passage. However, to date studies that systematically investigated sorption of these toxins onto sediments are lacking. Therefore, the aim of our work was (i) to determine the adsorption coefficients of ATX and CYN according to the Freundlich and Langmuir model for sediments of various textures and (ii) to derive sorption-relevant sediment characteristics. We determined sorption parameters in air-dried samples of eight differently textured sediments using batch experiments. Results for both toxins showed best fits with the Langmuir model. Organic C proved to be the main sediment parameter determining CYN sorption. There was no or little CYN sorption on sandy and silty sediments (0-39 μg kg(-1)), respectively, presumably due to charge repulsion from the negatively charged surfaces. Sorption of ATX (max. sorbent loading ranging from 47 to 656 μg kg(-1)) was much stronger than that of CYN (max. sorbent loading ranging from 0 to 361 μg kg(-1)) and predominantly controlled by clay and to a minor degree also by organic C and silt. While ATX sorption to most sediments occurred mainly through cation exchange this mechanism played only a minor role in CYN sorption to organic C. Hence, high mobility for CYN and moderate mobility for ATX during sediment passage has to be expected.