Soil Bacterial Community Tolerance to Three Tetracycline Antibiotics Induced by Ni and Zn

A laboratory work has been carried out to determine the tolerance of soil bacterial communities to Ni and Zn and co-tolerance to tetracycline antibiotics (chlortetracycline (CTC), oxytetracycline (OTC) and tetracycline (TC)) in soils individually spiked with five different concentrations of Ni or Zn (1,000, 750, 500, 250, and 125 mg kg−1), and an uncontaminated (0 mg kg−1) control soil. The PICT parameter (pollution-induced community tolerance) was estimated for the bacterial community using the tritium (3H)-labeled leucine incorporation technique, and the values corresponding to log IC50 were used as toxicity index. The mean log IC50 values observed in the uncontaminated soil samples indicate that Zn (with log IC50 = −2.83) was more toxic than Ni (log IC50 = −2.73). In addition, for the soil with the lowest carbon content (C = 1.9%), Ni-contaminated samples showed increased tolerance when the Ni concentrations added were ≥500 mg kg−1, while for the soils with higher carbon content (between 5.3% and 10.9%) tolerance increased when Ni concentrations added were ≥1,000 mg kg−1. Regarding the soils contaminated with Zn, tolerance increased in all the soils studied when the Zn concentrations added were ≥125 mg kg−1, regardless of the soil carbon content. The co-tolerance increases obtained after exposure of the bacterial suspension to TC, OTC and CTC showed an identical behavior within these tetracycline antibiotics. However, it was dependent on the heavy metal tested (Ni or Zn). In the case of soils 1 (C = 1.1%) and 2 (C = 5.3%), the soil bacterial communities showed increases in co-tolerance to TC, OTC and CTC for Ni concentrations added of ≥125 mg kg−1, while for soil 3 (with C = 10.9%) co-tolerance took place when Ni was added at ≥1,000 mg kg−1. However, in soils contaminated with Zn, increases in co-tolerance to CTC, OTC and TC occurred at Zn concentrations added of ≥125 mg kg−1 for the 3 soils tested. These results can be considered relevant when anticipating possible environmental repercussions related to the simultaneous presence of various types of pollutants, specifically certain heavy metals and antibiotics.