Abstract
A Pseudomonas fluorescens strain ZY2, isolated from swine wastewater, was used to investigate the synergistic effects of five heavy metals (Pb, Cu, Zn, Cr(VI) and Hg) on bacterial resistance to antibiotics. Results indicate that the combined effects of antibiotic type, heavy metal type and concentration were significant (p < 0.01). Cross-resistance to Hg and antibiotics was the most noticeable. Moreover, the resistance to Hg and cefradine or amoxicillin, and Cr and amoxicillin were synergistic for low heavy metal concentrations, and turned antagonistic with increasing concentrations, while the resistances to Cr or Cu and cefradine, Pb or Cu and amoxicillin, Cu and norfloxacin showed reverse effects. In addition, resistance to Zn and amoxicillin were always synergetic, while resistance to Pb and cefradine or norfloxacin, Cr or Hg and norfloxacin as well as all the heavy metals and tetracycline were antagonistic. These results indicate that bacterial resistance to antibiotics can be affected by the type and concentration of co-exposed heavy metals and may further threaten people’s health and ecological security severely via horizontal gene transfer.
Highlights
Antibiotics and heavy metals are two common types of typical environmental pollutants from effluent and industrial activities, and both are hazardous to public health and ecological safety [1]
We studied the resistances of Pseudomonas fluorescens ZY2 to four antibiotics and five heavy metals (Pb, Cu, Zn, Cr and Hg) used with different concentrations
Hg was the only heavy metal of which the minimum inhibitory concentrations (MICs) decreased with the addition of antibiotic discs onto the media, for example, tetracycline decreased the MIC of Hg from 25 to 10 mg/L, which was supported by its special physicochemical properties and extreme toxicity [33]
Summary
Antibiotics and heavy metals are two common types of typical environmental pollutants from effluent and industrial activities, and both are hazardous to public health and ecological safety [1]. Though their individual effect on test organisms and the environment has been studied for more than thirty years [2,3], very little information is available on their combined effects. Bacterial resistance to heavy metals and antibiotics can be developed after a low-level and prolonged exposure to these two types of common environmental pollutants [4]. The results will be helpful in further revealing the ecological harm and human health hazard of complex pollutants
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