Investigating the effects of nitric oxide upon antibiotic susceptibility in pathogenic Escherichia coli

A 30-year void in antibiotic discovery coupled with rampant antimicrobial resistance (AMR) among multi-drug resistant pathogens has created what is known as the Antimicrobial Resistance Crisis. Recent evidence strongly suggests aerobic respiration is required for bactericidal antibiotics to effectively kill bacteria via promoting generation of excess reactive oxygen species. Given that the immune system releases the potent respiratory inhibitor nitric oxide (NO) to combat pathogens, it is crucial to study how clinically relevant bacteria respond to an environment where a respiratory inhibitor (NO) is present along with an antibiotic. Herein, four hypotheses were tested to investigate how antibiotic susceptibility in respiratory mutant Escherichia coli strains were influenced by NO. To investigate the antagonistic effects of a bactericidal aminoglycoside and NO, viability assays and oxygen consumption measurements were undertaken in the presence and absence of GSNO, a NO-donor, and gentamicin. Results clearly show E. coli exhibited protection from the lethal effects of gentamicin when oxidative phosphorylation was halted. Furthermore, the NO-tolerant respiratory oxidase cytochrome bd-I (encoded by cydAB) was shown to sensitize E. coli to antibiotics in the presence of GSNO. To investigate if GSNO offered relief from excess reactive oxygen species stemming from gentamicin-mediated hyperactivation of respiration, reactive oxygen species were carefully measured after treatment with gentamicin in the presence and absence of GSNO. Interestingly, GSNO stimulated excess reactive oxygen species rather than diminished; therefore, an alternative hypothesis is offered: aminoglycosides utilize the generation of the proton motive force to enter the cell to perform the primary mode of action to kill bacteria. NO-mediated respiratory inhibition may diminish the proton motive force thus gentamicin is unable to efficiently enter the cell and kill. This work supports the link between aerobic respiration and bactericidal antibiotic function through a reactive oxygen species-independent pathway and demonstrates the role the immune system may have in antibiotic efficacy.