The hydrogenation of carbon dioxide
catalyzed by half-sandwich
transition metal complexes (M = Co, Rh, and Ir) was studied systematically
through density functional theory calculations. All metal complexes
are found to process a similar mechanism, which involves two main
steps, the heterolytic cleavage of H2 and the hydride transfer.
The heterolytic cleavage of H2 is the rate-determining
step. The comparison of three catalytic systems suggests that the
Ir catalyst has the lowest activation free energy (13.4 kcal/mol).
In contrast, Rh (14.2 kcal/mol) and Co (18.3 kcal/mol) catalysts have
to overcome relatively higher free energy barriers. The different
catalytic efficiency of Co, Rh, and Ir is attributed to the back-donation
ability of different metal centers, which significantly affects the
H2 heterolytic cleavage. The highest activity of an iridium
catalyst is attributed to its strong back-donation ability, which
is described quantitatively by the second order perturbation theory
analysis. Our study indicates that the functional group of the catalyst
plays versatile roles on the catalytic cycle to facilitate the reaction.
It acts as a base (deprotonated) to assist the heterolytic cleavage
of H2. On the other hand, during the hydride transfer,
it can also serve as Brønsted acid (protonated) to lower the
LUMO of CO2. This ligand assisted pathway is more favorable
than the direct attack of hydride to CO2. These finds highlight
that the unique features of the metal center and the functional ligands
are crucial for the catalyst design in the hydrogenation of carbon
dioxide.
Staphylococcus aureus is the most common etiological pathogen of bovine mastitis. The resistant strains make the disease difficult to cure. The aim of this study was to characterize the genetic nature of the antimicrobial resistance in S. aureus cultured from bovine mastitis in Northwest China in 2014. A total of 44 S. aureus were isolated for antimicrobial resistance and resistance-related genes. Antimicrobial resistance was determined by disc diffusion and the corresponding resistance genes were detected by PCR. Phenotype indicated that S. aureus isolates were resistant to penicillin (84.09%), erythromycin (20.45%), tetracycline (15.91%), gentamicin (9.09%), tobramycin (6.82%), kanamycin (6.82%) and methicillin (2.27%). 9.09% of the S. aureus isolates were classified as multidrug resistant. In addition, genotypes showed that the isolates were resistant to rifampicin (100%, rpoB), penicillin (95.45%, blaZ), tetracycline (22.73%, tetK, tetM, alone or in combination), erythromycin (22.73%, ermB or ermC), gentamicin/tobramycin/kanamycin (2.27%, aacA-aphD), methicillin (2.27%, mecA) and vancomycin (2.27%, vanA). Resistance to tetracycline was attributed to the genes tetK and tetM (r=0.558, P<0.001). This study noted high-level geno-and phenotypic antimicrobial resistance in S. aureus isolates from bovine mastitis cases in Northwest China.
This study aimed to investigate the antimicrobial resistance and virulence genes of Enterococcus faecalis isolated from subclinical bovine mastitis cases in China. Enterococcus faecalis isolates were identified by 16S rRNA amplification and sequencing. Antimicrobial susceptibility was determined by the disc diffusion method. Antimicrobial resistance and virulence genes were tested by PCR. Overall, E. faecalis was recovered from 81 of 1,787 (4.5%) mastitic milk samples. The isolates showed high resistance against tetracycline (87.7%) and erythromycin (79.0%). The most prevalent resistance genes found in the E. faecalis were tetK (96.3%), tetL (79.0%), and tetM (87.7%) for tetracycline and ermC (97.5%) for erythromycin. Moreover, gelE (70.4%), esp (85.2%), efaA (91.4%) were the most common virulence genes. This is the first report to characterize E. faecalis recovered from subclinical bovine mastitis cases in China.
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