Objective To investigate molecular epidemiology and antimicrobial susceptibility of Salmonella spp. isolates recovered from the stool samples of children with diarrhea. Methods Seventy-two isolates of Salmonella spp. were collected from children with diarrhea. The serum type of Salmonella spp.was determined by serology agglutinating method. Antimicrobial susceptibility was determined by K-B disk diffusion method and MICs of cefotaxime and ceftazidime were measured by agar dilution method for Salmonella spp. isolates. PCR and DNA sequencing were used for detecting ESBL, ISEcpl and AmpC genes; The transfer of cefotaxime resistance was determined by conjugation experiments. PFGE was performed for determining the homogeneity of the S. typhimurium isolates. Results A total of 72 isolates of Salmonella spp. were collected, among which S. typhimurium accounted for 86 % (62/72) and was the main serum type. S. typhimurium isolates and S. thompson isolates were often resistant to most of clinically used antimicrobial agents. Resistance of S. thompson isolates to ampicillin was the highest (90%, 56/62),followed by tetracycline (81%, 50/62), trimethoprim/sulfamethoxazole (74%, 46/62) and chloramphenicol (66%, 41/62). Seventeen S. typhimurium isolates (27%, 17/62) and two S. thompson isolates were resistant to cefotaxime. Forty-nine S. typhimurium isolates and two S. thompson isolates were positive for blaTEB-1b and resistant to ampicillin. Thirteen ESBL-producing S. typhimurium isolates (21%, 13/62) were positive for blaCTX-M (eight for blaCTX-M-14, three for blaCTX-M-15, one for blaCTX-M-55, one for both blaCTX-M-14 and blaCTX-M-55). All isolates harboring blaCTX-M genes were positive for upstream insert sequence ISEcpl. blaDHA-1was detected in a cefoxitin-resistant S. thompson isolate. Two main clones (PFGE type A and D) accounting for 19% (12/62) and 50% (31/62) respectively were found among 62 S. typhimurium isolates. Seven CTXM-producing isolates belonged to PFGE type D. Conclusions The multi-resistance to antimicrobial agents and high prevalence of blaCTX-M genes are found among S. typhimurium and S. thompson clinical isolates. blaCTX-M-55 is first found in S. typhimurium isolates and blaDHA-1 is found in S. thompson isolates. Clonal spread is responsible for the dissemination of S. typhimurium isolates.

Chiral amines are important building blocks for the synthesis of pharmaceutical products and fine chemicals. Highly stereoselective synthesis of chiral amines compounds through asymmetric amination has attracted more and more attention. ω-transaminases (ω-TAs) are a promising class of natural biocatalysts which provide an efficient and environment-friendly access to production of chiral amines with stringent enantioselectivity and excellent catalytic efficiency. Compared with (S)-ω-TA, the research focused on (R)-ω-TA was relatively less. However, increasing demand for chiral (R)-amines as pharmaceutical intermediates has rendered industrial applications of (R)-ω-TA more attractive. Improving the thermostability of (R)-ω-TA with potential biotechnological application will facilitate the preparation of chiral amines. In this study, the dynamic surface loop with higher B-factor from Aspergillus terreus (R)-ω-TA was predicted by two computer softwares (PyMOL and YASARA). Then mutant enzymes were obtained by deleting amino acid residues of a dynamic surface loop using site-directed mutagenesis. The results showed that the best two mutants R131del and P132-E133del improved thermostability by 2.6 ℃ and 0.9 ℃ in T₅₀¹⁰ (41.1 ℃ and 39.4 ℃, respectively), and 2.2-fold and 1.5-fold in half-life (t1/2) at 40 ℃ (15.0 min and 10.0 min, respectively), compared to that of wild type. Furtherly, the thermostability mechanism of the mutant enzymes was investigated by molecular dynamics (MD) simulation and intermolecular interaction analysis. R131del in the loop region has lower root mean square fluctuation (RMSF) than the wild type at 400 K for 10 ns, and mutant enzyme P132-E133del increases four hydrogen bonds in the loop region. In this study, we obtain two stability-increased mutants of (R)-ω-TA from A. terreus by deleting its dynamic surface loop and also provide methodological guidance for the use of rational design to enhance the thermal stability of other enzymes.