| As an emerging contaminant,antibiotic resistance genes(ARGs)have attracted wide attention worldwide.ARGs can be transmitted and disseminated between the same genera or different genera through the horizontal transfer mechanism.Heavy metals have a selective effect on bacterial community and can promote the transfer and spread of ARGs.Iron is a common metal element in life,but the research on the impact of iron on the environmental behavior of ARGs is mainly focused on nano-iron,iron oxide and zero-valent iron,but few studies are performed to investigate the effect of Fe(?)and Fe(?)on the abundance and horizontal transfer of ARGs.In this study,Escherichia coli DH5?carrying RP4 plasmid was used as the donor bacteria and Escherichia coli HB101 was used as the recipient.The effects of Fe(?)and Fe(?)on the conjugative horizontal transfer of RP4 plasmid were investigated by three-dimensional fluorescence spectroscopy,scanning electron microscopy,transmission electron microscopy,and plate counting.In addition,Serratia nematodiphila encoding streptomycin resistance isolated from agricultural soils was also used as the recipient,then the effects of Fe(?)and Fe(?)on the conjugative horizontal transfer of RP4 plasmid to the environmental indigenous strains were investigated.This study compares and analyzes the difference in the effect of Fe(?)and Fe(?)on the transfer efficiency of RP4 plasmid to different types of strains.The main conclusions are as follows:(1)In the plasmid conjugation horizontal transfer system using Escherichia coli HB101 and Serratia nematodiphila as the recipient bacteria,the changes in the fluorescence intensity of extracellular polymer substances(EPS)are mainly related to the microbial metabolites and humic acid-like substances.When the concentration of Fe(?)or Fe(?)is 10?500?g·L-1,the fluorescence intensity of EPS in the horizontal transfer system is stronger than that with Fe(?)or Fe(?)concentrations of5?200 mg·L-1.When the concentration of Fe(?)or Fe(?)is greater than500?g·L-1,the fluorescence intensity of the horizontal transfer system decreases with an increase of Fe(?)or Fe(?)concentration.In general,the fluorescence intensity of the horizontal transfer system with Serratia nematodiphila as the recipient bacteria is weaker than that in the horizontal transfer system using E.coli HB101 as the recipient bacteria.(2)The exposure of Fe(?)and Fe(?)can affect the morphology and cell structure of bacteria in the horizontal transfer system,but the influencing degree is not significantly related to the exposure concentrations of Fe(?)and Fe(?).Nevertheless,Fe(?)and Fe(?)exposure will change the surface morphology and permeability of the bacterial cells.When the concentration of Fe(?)and Fe(?)is in the range of 50?200 mg·L-1,a large amount of solids can be found in the horizontal transfer system,which are bound to the bacterial surfaces.This may be the potential reason causing changes in the bacterial cell structures.(3)Exposure to different concentrations of Fe(?)and Fe(?)has a certain effect on the abundance of E.coli HB101,of which,Fe(III)is greater.When the concentration of Fe(?)and Fe(?)is 10?g·L-1?5 mg·L-1,their influence on the abundance of Serratia nematodiphila is not significant.In contrast,when the concentration of Fe(?)and Fe(?)is 50 mg·L-1and 100 mg·L-1,the growth and reproduction of Serratia nematodiphila are promoted.(4)In the horizontal transfer system with Escherichia coli HB101 as the recipient bacteria,there is no significant regularity in the conjugative efficiency of RP4 plasmid by the exposure of different concentrations of Fe(?).The conjugative effecieny is in the range of 9.53×10-5?2.64×10-3after 24 h.In contrast,the Fe(?)exposure has an inhibitory effect on the conjugative horizontal transfer of RP4 plasmid,and the conjugative efficiency ranges from 1.3×10-5to 3.02×10-4after 24h.In the horizontal transfer system with Serratia nematodiphila as the recipient bacteria,the effect of Fe(?)or Fe(?)on the conjugative efficiency of RP4 plasmid exhibits a trend of low-promoting and high-inhibition.When the concentration of Fe(?)or Fe(?)is in the range of 10?g·L-1?100mg·L-1,the conjugative h efficiency of RP4 plasmid after 24 h ranges from 4.6×10-8to 5.07×10-7and from 5×10-9to 4.9×10-8,respectively.As a result,the conjugation horizontal transfer of RP4plasmid is more likely occur between the same bacterial species. |
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