Effect of Oxytetracycline and Chlortetracycline on Bacterial Community Growth in Agricultural Soils

Abstract

Toxicity on soil bacterial community growth caused by the antibiotics oxytetracycline (OTC) and chlortetracycline (CTC) was studied in 22 agricultural soils after 1, 8 and 42 incubation days. The leucine incorporation method was used with this aim, estimating the concentration of each antibiotic which caused an inhibition of 50% in bacterial community growth (log IC50). For OTC, the mean log IC50 was 2.70, 2.81, 2.84 for each of the three incubation times, while the values were 2.05, 2.22 and 2.47 for CTC, meaning that the magnitude of OTC toxicity was similar over time, whereas it decreased significantly for CTC with incubation time. In addition, results showed that the toxicity on bacterial community growth due to CTC is significantly higher than when due to OTC. Moreover, the toxicity on bacterial community growth due to both antibiotics is dependent on soil properties. Specifically, an increase in soil pH and silt content resulted in higher toxicity of both antibiotics, while increases in total organic carbon and clay contents caused decreases in OTC and CTC toxicities. The results also show that OTC toxicity can be well predicted by means of specific equations, using the values of pH measured in KCl and those of effective cation exchange capacity as input variables. CTC toxicity may be predicted (but with low precision) using pH measured in KCl and total organic carbon. These equations may help to predict the negative effects caused by OTC and CTC on soil bacteria using easily measurable soil parameters.

The Toxicity Exerted by the Antibiotic Sulfadiazine on the Growth of Soil Bacterial Communities May Increase over Time

“…However, in soil 2 (Figure 2), the bacterial community growth decreased for the higher SDZ doses, achieving significant reductions in relation to the control for the doses of 500 and 2000 mg•kg −1 . However, the magnitude of decrease for the highest SDZ dose (2000 mg•kg −1 ) was relatively low (32%), if compared with results previously reported for other antibiotics, such as tylosin [33,41], tetracyclines [22,28,29,33], or streptomycin [22]. It is worth noting that, despite the low adsorption previously reported for SDZ onto soils [42], the short-term effect of SDZ on bacterial community growth was relatively low.…”

“…These results can be considered somehow surprising, since organic compounds (including antibiotics) generally decrease their toxicity with time. As examples, decreases in toxicity of organic compounds were previously found for atrazine when examining effects on microbial biomass [46], as well as for fomesafen regarding effects on urease activity [47], or tetracycline regarding its effect on bacterial community tolerance [28,29]. This decrease is reasonable, since the concentrations of these organic compounds were reduced with time due to degradation [48], or suffered immobilization in the soil via ageing processes [49], or microbial communities developed tolerance to the organic compound [50].…”

“…The effect of time on toxicity can differ depending on the type of toxic compound, soil characteristics, or the toxicity test employed. Usually, toxicity will decrease with time due to dissipation of the compound [23,[27][28][29]. However, an increased toxicity with time is also possible, as suggested for the herbicide mecoprop using soil respiration as toxicity test in an acid soil [30], and recently confirmed for terbutryn [31].…”

Soil Enzymatic Activities and Microbial Community Structure in Soils Polluted with Tetracycline Antibiotics

“…The final antibiotic concentrations (for CTC, TC, and OTC) in each soil were 0.00, 0.49, 1.95, 7.81, 31.25, 125, 500, and 2000 mg•kg −1 of soil. The concentrations were previously selected [12] in order to obtain almost complete inhibition of soil bacterial growth, thus offering estimates of to-xicity indices in a more reliable way [16]. The antibiotics were added to the soils using inert talc powder as a carrier for equalizing the amount of dry material added to each soil sample, as well as to facilitate mixing of the substances into soil [17].…”

“…Santás-Miguel et al [11], in a recent laboratory study, added with different concentrations of tetracycline (TC) to 22 soils incubated under controlled conditions of moisture and temperature for 42 days, and they reported that, initially, tetracycline negatively affected bacterial activity measured by means of the cellular incorporation of labeled leucine, whereas this effect was drastically reduced at the end of the incubation time. Subsequently, Santás-Miguel et al [12] studied the toxicity of oxytetracycline (OTC) and chlortetracycline (CTC) toward bacterial communities for 42 days and observed a negative effect of both antibio-tics on the growth of bacterial communities. The effect of CTC decreased with incubation time; however, the effect of OTC on bacterial communities persisted over time.…”