Quinolones: pharmacology

Interplay between intrinsic and acquired resistance to quinolones in Stenotrophomonas maltophilia

To analyse whether the mutation-driven resistance-acquisition potential of a given bacterium might be a function of its intrinsic resistome, quinolones were used as selective agents and Stenotrophomonas maltophilia was chosen as a bacterial model. S. maltophilia has two elements - SmQnr and SmeDEF - that are important in intrinsic resistance to quinolones. Using a battery of mutants in which either or both of these elements had been removed, the apparent mutation frequency for quinolone resistance and the phenotype of the selected mutants were found to be related to the intrinsic resistome and also depended on the concentration of the selector. Most mutants had phenotypes compatible with the overexpression of multidrug efflux pump(s); SmeDEF overexpression was the most common cause of quinolone resistance. Whole genome sequencing showed that mutations of the SmeRv regulator, which result in the overexpression of the efflux pump SmeVWX, are the cause of quinolone resistance in mutants not overexpressing SmeDEF. These results indicate that the development of mutation-driven antibiotic resistance is highly dependent on the intrinsic resistome, which, at least for synthetic antibiotics such as quinolones, did not develop as a response to the presence of antibiotics in the natural ecosystems in which S. maltophilia evolved.

In vitro activity and mode of action of fluoroquinolones

The in vitro activity of nine fluoroquinolones has been determined by agar dilution method against 1100 clinical isolates of different microorganisms: 412 Enterobacteriaceae, 93 non-fermenters, 398 enteropathogens, 140 Gram-positive cocci, 31 Haemophilus and 26 gonococci. The bactericidal activity tested in broth against different representative isolates was generally similar to the inhibitory concentration. Killing kinetics studied in different isolates of Enterobacteriaceae, in Pseudomonas aeruginosa, Vibrio cholerae, Staphylococcus aureus and Streptococcus faecalis showed a very rapid bactericidal effect, after 2 to 8 h of contact, in all species except in Streptococcus faecalis.

Mechanism of antibacterial action: electron transfer and oxy radicals

Most of the main categories of bactericidal agents, namely, aliphatic and heterocyclic nitro compounds, metal derivatives and chelators, quinones, azo dyes, and iminium-type ions, are proposed to exert their action by a unified mechanism. The toxic effect is believed to result generally from the catalytic production of reactive oxygen radicals that usually arise via electron transfer. Cyclic voltammetry was performed on a number of these agents. Reductions were for the most part reversible, with potentials in the favorable range of -0.20 to -0.58 V.