Characterization and evolution of antibiotic resistance of Salmonella in municipal wastewater treatment plants.

The objective of this study was to investigate the evolution of antibiotic resistance phenotypes, antibiotic resistance genes (ARGs) and Class 1 integron of Salmonella in municipal wastewater treatment plants (WWTPs). A total of 221 Salmonella strains were isolated from different stages of three WWTPs. After the susceptibility testing, high frequency of resistance was observed for tetracycline (TET, 47.5% of isolates) and sulfamethoxazole (SMZ, 38.5%), followed by ampicillin (AMP, 25.3%), streptomycin (STP, 17.6%), chloramphenicol (CHL, 15.4%), and gentamicin (GEN, 11.3%). Low prevalence of resistance was detected for norfloxacin (0.45%), ciprofloxacin (0.9%), and cefotaxime (0.9%). The tetA and sul3 genes were most frequently detected among the Salmonella isolates. Statistically significant correlations among AMP, CHL, GEN, and STP resistances were observed. High detection frequencies of Class 1 integron were observed in double antibiotic-resistant and multiple-antibiotic-resistant (MAR) Salmonella, which were 94.3% and 85.7%, respectively. The proliferation of MAR Salmonella and transfer of ARGs occurred in WWTPs. Class 1 integron plays a crucial role in the evolution of MAR Salmonella during WWTPs.

Occurrence of tetracycline-resistant fecal coliforms and their resistance genes in an urban river impacted by municipal wastewater treatment plant discharges.

Antibiotic resistance of fecal coliforms in an urban river poses great threats to both human health and the environment. To investigate the occurrence and distribution of antibiotic resistant bacteria in an urban river, water samples were collected from the Chanhe River in Xi'an, China. After membrane filtration of water samples, the tetracycline resistance rate of fecal coliforms and their resistance genes were detected by plating and polymerase chain reaction (PCR), respectively. We found that fecal coliforms were generally resistant to tetracycline and saw average resistance rates of 44.7%. The genes tetA and tetB were widely detected, and their positive rate was 60%-100% and 40%-90%, respectively. We found few strains containing tetC, tetK, tetQ and tetX, and we did not identify any strains containing tetG, tetM or tetO. The prevalence of tetA and tetB over other genes indicated that the main mechanism for resistance to tetracycline is by changes to the efflux pump. Our analysis of the types and proportion of tetracycline resistance genes in the Chanhe River at locations upstream and downstream of the urban center suggests that the increased number of tetracycline-resistant fecal coliforms and spatial variation of tetracycline resistance genes diversity were related to municipal wastewater treatment plant discharge.

Dewatering behavior and regulation mechanism of montmorillonite nanosheet in aqueous solution.

Two-dimensional montmorillonite nanosheet (MMTNS) is desirable building block for fabricating multifunctional materials as due to its extraordinary properties. In practical applications, however, the concentration of MMTNS prepared by exfoliation is normally too low to be used for material assembling. The general thermal-concentration method is effective, however, it can be time-consuming and require a lot of energy. In this case, the remarkable dispersion stability of MMTNS is worth noting. Herein, the extraordinary dispersion stability of MMTNS derived from electrostatic and hydration repulsion was firstly revealed by molecular dynamics (MD) simulation, which caused the poor dewatering of MMTNS. Further, based on the surface and structural chemistry of MMTNS, a series of strategies, involving charge and cross-linked structure regulation on the edge surface, as well as electrical double-layer modulation and calcification modification based on the electrolytes, were proposed to inhibit the dispersion and enhance the aggregation of MMTNS. Intriguingly, a novel chemical, Tetraethylenepentamine (TEPA) was applied in the dewatering of MMTNS. The TEPA not only act as a cross-linker to bond with MMTNS into an easy-to-dewatering 3D network structure, but also act as a switch for effortless viscosity tuning. Meanwhile, the dual function of electrolytes for electrical double layer compression and calcification modification of MMTNS was investigated by DLVO theory and structural analyses. This work offers explicit directions for improving the dewatering performance of MMTNS to meet the requirements of practical implementation.