Use of critically important antimicrobial classes early in life may adversely impact bacterial resistance profiles during adult years: potential co-selection for plasmid-borne fluoroquinolone and macrolide resistance via extended-spectrum beta-lactam use in dairy cattle

The transfer of antimicrobial resistance genes commonly occurs via vertical and horizontal gene transfer, as such genes are often found on the same mobile genetic element. This occurrence can lead to the co-selection of resistance to antimicrobials without their application. Dairy cattle located in the south-western United States were enrolled in a matched-pair longitudinal study to evaluate the effects of a two-dose ceftiofur treatment for metritis on levels of third-generation cephalosporin resistance among faecal Escherichia coli temporally. Escherichia coli chosen for further investigation were isolated on selective media, harboured extended-spectrum beta-lactam, fluoroquinolone and macrolide resistance genes. This combination has previously been unreported; importantly, it included genes encoding for resistance to antibiotics that can only be used in dairy cattle less than 20 months of age. Fluoroquinolones, macrolides and third and higher generation cephalosporins are considered critically important and highest priority for human medicine by the World Health Organization.     

The slow dissociation rate of K-1602 contributes to the enhanced inhibitory activity of this novel alkyl–aryl-bearing fluoroketolide

Ketolides belong to the latest generation of macrolides and are not only effective against macrolide susceptible bacterial strains but also against some macrolide resistant strains. Here we present data providing insights into the mechanism of action of K-1602, a novel alkyl–aryl-bearing fluoroketolide. According to our data, the K-1602 interacts with the ribosome as a one-step slow binding inhibitor, displaying an association rate constant equal to 0.28?×?10^4?M^?1 s^?1 and a dissociation rate constant equal to 0.0025?min^?1. Both constants contribute to produce an overall inhibition constant K_i equal to 1.49?×?10^?8?M, which correlates very well with the superior activity of this compound when compared with many other ketolides or fluoroketolides.