ObjectivesTo gain a more detailed understanding of endogenous (mutational) and exogenous (horizontally acquired) resistance to silver in Gram-negative pathogens, with an emphasis on clarifying the genetic bases for resistance.MethodsA suite of microbiological and molecular genetic techniques was employed to select and characterize endogenous and exogenous silver resistance in several Gram-negative species.ResultsIn Escherichia coli, endogenous resistance arose after 6 days of exposure to silver, a consequence of two point mutations that were both necessary and sufficient for the phenotype. These mutations, in ompR and cusS, respectively conferred loss of the OmpC/F porins and derepression of the CusCFBA efflux transporter, both phenotypic changes previously linked to reduced intracellular accumulation of silver. Exogenous resistance involved derepression of the SilCFBA efflux transporter as a consequence of mutation in silS, but was additionally contingent on expression of the periplasmic silver-sequestration protein SilE. Silver resistance could be selected at high frequency (>10−9) from Enterobacteriaceae lacking OmpC/F porins or harbouring the sil operon and both endogenous and exogenous resistance were associated with modest fitness costs in vitro.ConclusionsBoth endogenous and exogenous silver resistance are dependent on the derepressed expression of closely related efflux transporters and are therefore mechanistically similar phenotypes. The ease with which silver resistance can become selected in some bacterial pathogens in vitro suggests that there would be benefit in improved surveillance for silver-resistant isolates in the clinic, along with greater control over use of silver-containing products, in order to best preserve the clinical utility of silver.

Bacteriophage (phage), viruses that can infect and kill bacteria, are being investigated through phage therapy as a potential solution to the threat of antimicrobial resistance (AMR). In reality, however, phage are also natural drivers of bacterial evolution by transduction when they accidentally carry nonphage DNA between bacteria.

In bacteria, nucleotide excision repair (NER) plays a major role in repairing DNA damage from a wide variety of sources. Therefore, its inhibition offers potential to develop a new antibacterial in combination with adjuvants, such as UV light. To date, only one known chemical inhibitor of NER is 2-(5-Amino-1,3,4-thiadiazol-2-ylbenzo(f)chromen-3-one) (ATBC) exists and targets Mycobacterium tuberculosis NER. To enable the design of future drugs we need to understand its mechanism of action. To determine the mechanism of action, we used in silico structure-based prediction, which identified the ATP binding pocket of E. coli UvrA as a probable target. Growth studies in E. coli showed it was non-toxic alone, but able to impair growth when combined with DNA damaging agents, and as we predicted it reduced by ~70% UvrA’s ATPase rate. Since UvrA’s ATPase activity is necessary for effective DNA binding, we used single molecule microscopy to directly observe DNA association. We measured a ~7-fold reduction in UvrA molecules binding to a single molecule of dsDNA suspended between optically trapped beads. These data provide a clear mechanism of action for ATBC, and show that targeting UvrA’s ATPase pocket is effective and ATBC provides an excellent framework for the derivation of more soluble inhibitors that can be tested computationally for activity.

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-Leucinyl benzenesulfonamides have been discovered as a novel class of potent inhibitors of leucyl-tRNA synthetase. The binding of inhibitors to the enzyme was measured by using isothermal titration calorimetry. This provided information on enthalpy and entropy contributions to binding, which, together with docking studies, were used for structure-activity relationship analysis. Enzymatic assays revealed that-leucinyl benzenesulfonamides display remarkable selectivity for leucyl-tRNA synthetase compared to and human orthologues. The simplest analogue of the series, -leucinyl benzenesulfonamide (R = H), showed the highest affinity against leucyl-tRNA synthetase and also exhibited antibacterial activity against Gram-negative pathogens (the best MIC = 8 μg/mL, ATCC 25922), which renders it as a promising template for antibacterial drug discovery.