Joint Effects Of Nanomaterials And Antibiotics To Bacteria: Antibacterial Properties And Mechanism

Nanomaterials rarely appear in the environment since the real environment is extremely complex. It is well-known that simultaneous exposures to different toxicants may induce unexpected biological effects to organisms, such as synergism and antagonism. To better understand the bioeffects of nanomaterials, the studies emphasized in complex systems are of great importance. Ag NPs are widely used in a broad range of products, such as bandages, surface coatings, medical equipment, food packaging, household appliances and cosmetics because of its excellent antibacterial properties. Meanwhile, antibiotics are widely used and widespread in the eco-system. It is considered to be a new type of environmental pollutants and easily induces the generation of antibiotics-resistant bacteria, which might threat the human beings and the environment. Ag NPs and antibiotics are likely to coexist in the environment。Herein, we selected the typical nanomaterials(Ag-PVP NPs) and representative antibiotics as models, to investigate the joint effects of nanomaterials and antibiotics to bacteria(E.coli, S.aureus, gentamicin-resistant E.coli), and explore the related mechanisms of synergism and antagonism. Based on the results, we explored the applications of the synergistic effects of nanomaterials and antibiotics to kill antibiotics-resistant bacteria.Firstly, the antibacterial susceptibilities of Ag-PVP NPs and antibiotics were determined by measuring the minimal inhibition concentrations and the corresponding fractional inhibitory concentration indices. The survival rates as well as the growth curves of bacteria exposed to Ag-PVP NPs and antibiotics were measured to investigate the joint antibacterial effects quantitatively. Then, live/dead staining, cell morphology investigation, inductively coupled plasma mass spectrometry and UV-vis techniques were used to study the interactions of Ag-PVP NPs and gentamicin towards different antibacteria. Moreover, UV-vis spectra were used to study the interaction between Ag-PVP NPs and ampicillin. The major achievements are listed as following:(1) Simultaneous exposure to Ag-PVP NPs and antibiotics induced different biological effects to the bacteria, including synergism effects and antagonism effects. In detail, gentamicin increased the antibacterial activity of Ag NPs whereas ampicillin decreased the antibacterial activity and penicillin did not affect the antibacterial activity. The combinations of Ag-PVP NPs and gentamicin delayed the exponential phase and shortened the exponential phase of bacterial growth. In addition, the combinations inhibited the growth of gentamicin-resistant E. coli synergistically. On the contrary, bacterial growth was less inhibited after exposure to Ag-PVP NPs in the presence of ampicillin.(2) The presence of gentamicin changes the properties of Ag-PVP NPs dramatically, including promotion of the dissolution of Ag-PVP NPs and enhancement of the positive charge of the NPs, which further increase the concentration of silver ions in the media and facilitate the attachment of Ag NPs onto the surface of bacteria, correspondingly. As a result, higher silver accumulation appears in bacteria, leading to the bacterial growth inhibition and death. Meanwhile, gentamicin chelates with the dissolved silver, inhibiting the antibacterial activity of released silver ions. In other words, gentamicin plays a dual role in the antibacterial activity of Ag-PVP NPs.(3) Ampicillin may absorb onto the surface of Ag-PVP NPs, leading to the decrease in the concentration of free ampicillin, thus weakens its antibacterial effects, causing antagonism effects.Furthermore, the Zn O/GO composites were prepared by a facile one-pot reaction, and the antibacterial activity as well as the cytotoxicity of the Zn O/GO composites was measured. The synergistic effects of GO and Zn O NPs led to the superior antibacterial activity of the composites. GO helped the dispersion of Zn O NPs, slowed the dissolution of Zn O, acted as the storage site for the dissolved zinc ions, and enabled the intimate contact of E. coli with Zn O NPs and zinc ions as well. The close contact enhanced the local zinc concentration pitting on the bacterial membrane and the permeability of the bacterial membrane and thus induced bacterial death. In addition, the Zn O/GO composites were found to be much less toxic to He La cells, compared to the equivalent concentration of Zn O NPs in the composites.Our results provide not only the valuable data for the bioeffects and mechanisms researches of nanomaterials in combined system, but also the possible strategies of utilizing synergism effect in the development of novel antimicrobial.