• The effect of three commonly used antibiotics amoxicillin, cefixime and metronidazole on the abundance of microorganisms in soils with different concentrations of heavy metals
  • Ziba Najafzadeh Nobar,1,* Ali Akbar Safari Sinegani,2
    1. Department of Soil Science, Faculty of Agriculture, Bu-Ali Sina University, Hamedan, Iran.
    2. Department of Soil Science, Faculty of Agriculture, Bu-Ali Sina University, Hamedan, Iran.


  • Introduction: It has been clarified that the simultaneous toxic effect of antibiotic and heavy metal on microorganisms is higher than their separate effect. On the other hand, the simultaneous pollution of antibiotics and heavy metals increases antibiotic resistance genes, bacterial resistant and horizontal transfer of antibiotic resistance genes and has consequences on the diversity and abundance of microorganisms. On the other hand, reports show that more than 50% of lands around the world are contaminated with heavy metals (Lormohammadi et al., 2022; Yang et al., 2022; Zhou et al., 2022). Therefore, the transformation of the soil microorganism community against metal and antibiotics pollution is the main goal of this research, in which to investigate the frequency of some soil microorganisms in response to the application of three antibiotics amoxicillin, cefixime and metronidazole in three soils: agricultural, rangeland soil and mining wastes with amounts various heavy metals were collected from Hamadan city. Therefore, the changes in the abundance of fungi and bacteria and the special group of enterobacteria and Pseudomonas in the treatment of the mentioned antibiotics were investigated in these soils. The findings of this research show the difference between groups of microorganisms and how they metabolize in response to the mentioned antibiotics, which can be used in selecting the treatment method for people and livestock, as well as the stability of microorganisms in different habitats.
  • Methods: Three soil samples were selected from mining wastes, rangeland and agricultural soil then concentrations of 100 and 200 mg.kg-1 antibiotics were applied to the soils. In three time periods: 0 to 7 days (short-time incubation), 15 and 30 days (medium-time), 60 and 90 days (long-time), the abundance of four groups of soil microorganisms (fungi, bacteria, pseudomonas and enterobacteria) was counted. Factorial experiment was performed with a completely randomized design in three replications.
  • Results: Data analysis showed that in the short time, the abundance of four groups of microorganisms in agricultural soil decreased in amoxicillin treatment (up to 5.92 percent) but were more resistant in contaminated mine and rangeland soils and increased to 11.91 % against their control. The fungi of two mine and rangeland soils against amoxicillin decreased to 6.73 %. Also in this period, the population of all microorganisms in rangeland soil and agricultural soil (except agricultural pseudomonas) decreased to 11.52 % and also bacteria and pseudomonas of mining soil against Cefixime decreased to 13.5 %. At start up to 7 days of incubation, mine soil bacteria, pseudomonas of agricultural soil and enterobacteria of two agricultural and rangeland soils showed resistance against metronidazole. The data analysis showed that the long-time use of all three antibiotics in rangeland and mine soils led to a positive response in the abundance of all bacteria, pseudomonas and enterobacteria in the soil compared to the control treatment. On the other hand, it should be said that the addition of antibiotics to soils with high concentrations of heavy metals can increase the abundance of enterobacteria in the soil. In agricultural soil, at the end of 90 days of incubation, the abundance response of all bacteria, pseudomonas, fungi and enterobacteria to the application of all three antibiotics was negative. In the long-time period, the lowest number of enterobacteria was obtained with the use of 200 mg. kg-1 metronidazole and the lowest number of pseudomonas with the use of 200 mg. kg-1 amoxicillin. Amoxicillin (200 mg. kg-1) showed the lowest number of fungi in medium-time incubation. The bad effect of 200 mg concentration of metronidazole on the abundance of microorganisms was intensified in the high concentration of heavy metals in the soil.
  • Conclusion: The response of microorganisms in different soils to each of the used antibiotics is not the same. Although the abundance of microorganisms in soils contaminated with heavy metals is less, but their ability to live against antibiotics is higher. Overall, the resistance of bacteria in metal-contaminated soils was higher against antibiotics, and the harmful consequences of antibiotics in the medium and long time, especially in soils contaminated with heavy metals (rangeland and mine) disappeared.
  • Keywords: Amoxicillin, Cefixime, Heavy metals, Metronidazole, Population of Soil Microorganisms