Environmental Behavior Of Typical Antibiotic-resistance Bacteria And Genes Onto The Mineral Interface Under The Light Irradiation

The widespread use of antibiotics has produced a reservoir of antibiotic-resistance bacteria?ARB?and antibiotic-resistance genes?ARGs?which persist and spread within various environmental systems.Further,they can pose as a potential threat to public health.However,most of the related researches were mainly focused on the pollution profiles of them in the environment,but very few works were carried out to probe the mechanism for ARGs transfer in the environmental systems.Light and minerals are often present in the environment with various microorganisms,which may affect the environmental formation of ARB and the migration and transformation of ARGs.Therefore,in this study,we firstly examined the conjugative transfer of genes encoding resistance to cephalosporin(?77?(6_?3?and polymyxin?mcr-1?from two antibiotic-resistant donor strains?E.coli DH5??CTX?,E.coli DH5??MCR??to a streptomycin-resistant receptor strain?E.coli C600?Sm??under different stimulus including visible light?VL?,simulated sunlight?SS?,ultraviolet light?UV?and natural sphalerite?NS??ZnS as the control?.Furthermore,we studied the mineral and mineral under light stimuli for the occurrence of ARGs transfer in bacterial strains,by investigating the stress response of bacteria,the conjugation and transfer efficiency of donor and recipient bacteria,the effects of external stimuli on the conjugation and transfer of plasmids.Finally,the mechanism of light and mineral effects on the spread and diffusion of ARGs was explored.The main results of this thesis are as follows:?1?First,to understand and explore the transfer of ARGs and the mechanism of bacterial antibiotic resistance transmission under different light irradiation,we studied the activity of ARB and ARGs,oxidative stress,the expression of related genes and the changes of transfer efficiency after exposure with various lights.Results showed that the conjugative transfer frequency was not affected by VL irradiation,while the transfer frequency was slightly improved?2-10 folds?by SS irradiation and extremely accelerated?up to 100 folds?by UV irradiation.Furthermore,the study also explored the link between ARG transfer and stress conditions.This was done by studying the physiological and biochemical change,oxidative stress,and functional genes expression of co-cultured antibiotic-resistance-E.coli strains under stress response.When they were correlated with the results of the transfer frequency change,we found that VL irradiation neither affects bacterial physiological and biochemical conditions,nor induces the oxidative stress and gene expression.For SS irradiation,the oxidative stress occurred slowly,with a slight increase in the expression of target genes in bacterial cells.In contrast,for UV irradiation,the bacteria were rapidly inactivated,the degree of oxidative stress was very severe and the expression of the target stress genes was markedly up-regulated.?2?Furthermore,we found that the process of NS exposure could elicit bacterial stress responses that induce some mechanisms such as cell membrane permeability and altering the expression of conjugation-related genes,leading to the increased conjugative transfer rates,where ARGs show more persistence under environmental systems.The results of NS without light irradiation demonstrated that NS induced concentration-dependent increases in conjugative transfer rates as compared with the controls.The mechanisms by which NS promotes the transfer of ARGs may involve the damage of bacterial membranes by oxidative stresses,an enhancement of the expression of stress-related genes and the altered mRNA expression of membrane protein genes.?3?Finally,to study the transfer of ARGs by NS with light irradiation,the conjugative transfer was comparatively studied under different light irradiation conditions including visible light?VL?,simulated sunlight?SS?and ultraviolet light(UV_254nm and UV_365nm).Results show that it was observably improved?2-12 folds and 2-10 folds?by NS under VL and UV_254nm54nm irradiation,while the conjugative transfer frequency was not affected by NS with SS and UV_365nm irradiation.Furthermore,the ARG transfer showed strong links with stress condition such as Reactive oxygen radicals?ROSs?formation;oxidative stress response;and functional gene expression of ARB strains.When they were correlated with the transfer frequency results,we found that NS with VL irradiation did slightly affect the ROS formation of the bacteria or induce oxidative stress and gene expression.For NS with SS irradiation,oxidative stress occurred slowly,with a slight increase in the expression of target stress genes in the bacterial cells.In contrast,NS with UV_254nm irradiation rapidly inactivated the bacteria,the degree of oxidative stress was increase and the expression of the target genes was markedly up-regulated,depending to the effect of UV_254nm irradiation.Moreover,NS with UV_365nm irradiation extremely inactivated the bacteria,the degree of oxidative stress increased and the expression of the target genes was markedly up-regulated.Above all,our study explored the effect of light irradiation,mineral,and mineral with light stimulation on the conjugation and transfer mechanism of ARGs,and found that NS could promote the occurrence of conjugation and transfer between E.coli by stimulating the up-regulation of oxidative stress genes and conjugation genes in bacteria.This thesis provides a research idea how to comprehensively explore the promotion of other external stimuli on the horizontal transfer of ARGs.In addition,this thesis studied the effect of the naturally occurred NS under the stimulation of the external environment factors such as light on the transfer of ARB and ARGs.We further put forward the conclusion that the environmental factors could promote the spread of bacterial antibiotic resistance in water environment,endanger the aquatic environment and public health.