Study On Antibacterial Properties And Antibacterial Mechanism Of Cu-bearing Stainless Steel

The Cu-bearing stainless steel(SS),as a new type of structural/functional integrated material,has broad-spectrum antibacterial properties on the basis of maintaining the original mechanical properties,processabilities and corrosion resistance of the common SS.Thus,its usage has great potential to reduce the environmental microbial risks,presenting broad application prospects.Previous studies have shown that Cu-bearing SS can kill the bacteria by releasing Cu ions,and then the formation of biofilm can be inhibited.However,the majority of these reports are foused on the acute toxicity of Cu to the individual cells.The environmental effects of Cu-bearing SS on microbial populations under low or even non-toxic conditions have not been reported yet.In addition,a growing number of studies have indicated that there is controversy regarding the release of Cu as the antibacterial mechanism of Cu-bearing SS.It was found that Cu-bearing SS usually kills the bacteria only in contact with it,and the release of Cu ions is far less than the minimum inhibitory concentration of Cu.What’s more,attributing the bactericidal behavior to Cu ions release completely,while the bactericidal effect led by the structure and properties of material has been neglected.Therefore,this study aimed at the antibacterial properties of Cu-bearing stainless steel in typical medical and food fields,two grades of stainless steel commonly used in these two environments,namely 420-Cu SS and 304L-Cu SS,were selected as the object,and the acute bactericidal mechanism of medical 420-Cu SS against the individual microbial cells from the perspectives of materials and biology was systematically studied and explored.Furthermore,the non-toxic stress mechanism of food-contacted 304L-Cu SS on the biofilm development was elucidated from the perspective of bacterial quorum sensing.The main research work carried out in this study and the main conclusions obtained are as follows:(1)Two bacterial inoculation methods,dry and wet,were establised to control the contact degree between Escherichia coli(E.coli)and the surface of a 420-Cu SS.It was found that under the conditions of dry inoculation,the bacteria could have a full contact with the material,and 420-Cu SS executed its bactericidal performance quickly and efficiently.Further,the material structure,surface elements,as well as the release level of bactericidal Cu elements of 420-Cu SS were analysed.Besides,the bacterial cell membrane,respiratory metabolism and ROS expression level were detected.The results showed that the inactivation mechanism of Cu-bearing SS on individual bacteria is a chain reaction induced by the direct contact between bacteria and the steel.Herein,the contact potential difference between the Cu-rich phase and the matrix in the Cu-bearing SS could induce a charge transfer reaction with bacteria,destroying the respiratory metabolism of bacterial cells,and promoting the Cu toxicity,damaging the bacterial membrane.(2)Based on the acute toxicity mechanism of 420-Cu SS against individual bacterial cells,the effect of surface roughness on bactericidal performance of 420-Cu SS was investigated.It was found that the bactericidal efficiency of 420-Cu SS increased with decrease of the surface roughness.In the meantime,the changes in surface properties such as wettability and volt potential caused by the roughness treatment were analyzed,as well as the changes in physiological properties of E.coli after incubating with the 420-Cu SS with different surface roughness.The results indicated that the macroscopic roughness of the surface polished by diamond abrasive paste could help reducing the bactericidal burden of the surface.The microscopic roughness could promote a full contact between bacterial cells and the material surface,which was conducive to the induction of contact bactericidal reactions.(3)The most commonly used food contact materials,304L SS,and the newly developed 304L-Cu SS were comparatively studied to simulate their contact scenes with typical food.By detecting the quality changes of the foods during incubating with different materials,it was preliminarily confirmed that the Cu-bearing SS could lag the proliferation of microorganisms in the foods contact with the steel,delaying the deterioration of food quality.What’s more,a bacteriostatic atmosphere could be formed around the food.(4)304L-Cu austenitic SS was selected and subjected to solution and aging treatments,respectively.And then the samples were put in the nutrient broth suspension of E.coli for incubation.The results showed that the aged 304L-Cu SS could significantly retard the structure devolopment of E.coli biofilm on its surface,and this retarding growth effect did not come from the acute bactericidal effect of 304L-Cu SS.Considering the changes in the quorum sensing signal account for regulating the development of biofilms during the bacterial growth cycle,the activity of quorum sensing signal in different incubation systems was investigated.The results confirmed that the material structure of aged 304L-Cu SS could induce the overexpression of ROS in the incubation system,which could significantly quench the quorum sensing signal in the corresponding environment,and finally lead to the retardation of biofilm development.This work detailed the regulation behavior of Cu-bearing SS against individual microorganisms and microbial populations,clarified the potential complex interaction between Cu-bearing SS and microorganisms,and proposed the acute toxicity mechanism of Cu-bearing SS against individual bacteria.Besides,it is confirmed that the Cu-bearing SS can excute low-toxic or non-toxic stress effects on the bacterial population in the nutritional environment where is not conducive to its acute toxicity,manifesting the retardation of biofilm development.The environmental effect of Cu-bearing SS confirmed its application superiority in typical fields,and also provided a new theoretical basis for the optimization design and promotion of Cu-bearing antibacterial SS.