Abstract
The antibacterial efficacy of the tetracycline antibiotics has been greatly reduced by the development of resistance, hence a decline in their clinical use as antibiotics. The hok/sok locus is a type I toxin/antitoxin plasmid stability element, often associated with multi-drug resistance plasmids, especially ESBL-encoding plasmids. It enhances host cell survivability and pathogenicity in stressful growth conditions, and particularly increases bacterial tolerance to β-lactam antibiotics. The hok/sok locus forms dsRNA by RNA:RNA interactions of the toxin and antitoxin, and doxycycline has been reported to bind and inhibit dsRNA cleavage/processing. This study investigated the antibacterial efficacy of doxycycline in hok/sok host bacteria cells, the effect on hok/sok-induced growth changes and the potential mechanism of the observed changes. Different strains of E. coli with growth characteristics affected by the hok/sok locus were transformed with hok/sok plasmids, and assessed for doxycycline susceptibility and growth changes. The results show that the hok/sok locus increases bacterial susceptibility to doxycycline, especially in strains with more pronounced hok/sok growth effects. The increased doxycycline susceptibility occurs despite β-lactam resistance imparted by hok/sok. Doxycycline was found to induce bacterial death in a manner phenotypically characteristic of Hok toxin expression, suggesting that it inhibits the toxin/antitoxin dsRNA degradation, leading to Hok toxin expression and cell death. In this way, doxycycline could be used to counteract the multi-drug resistance plasmid maintenance/propagation and pathogenicity mechanisms associated with the hok/sok locus. This has great potentials in the global war to contain the rise in antimicrobial resistance.
