Impact of Zinc Pre-exposure on de novo Antibiotic Resistance Development

Antimicrobial resistance (AMR) is a global health crisis that is currently predicted to worsen. While the impact of improper antibiotics is an established driver, much less is known on the impacts of metal supplements. Here, we specifically probe the impact of zinc (Zn) on AMR. In conflict settings where diarrhea disease cases are high, Zn is both given as a supplement for treatment of these diseases prior to use of antibiotics such as ciprofloxacin and is associated with weapons of war. In this study, we find that the order with which E. coli is exposed to zinc impacts resistance development, with increasing pre-exposure time leading to accelerated ciprofloxacin resistance, while combined exposure of zinc with ciprofloxacin delays ciprofloxacin resistance. We did not find evidence that zinc pre-exposure leads to genetic changes or change in antibiotic tolerance, though it does increase both the lag phase and doubling time of E. coli, suggesting the mechanism may be due to changes in gene expression. While the zinc phenotype behavior is not permeant and would no longer be observed if ciprofloxacin exposure did not occur right after zinc pre-exposure, the elevated MIC phenotype resulting from the zinc pre-exposure was more stable than the zinc phenotype. These results are important as they highlight the need to reexamine the clinical role of zinc in treating diarrheal diseases and assess if changes in resistance development observed in vitro are also observed in vivo. ImportanceAntimicrobial resistance (AMR) is a global problem. According to a 2014 Review on Antimicrobial Resistance, it is projected to result in several million deaths by 2050 (Review on Antimicrobial Resistance, Tackling a Crisis for the Health and Wealth of Nations, 2014). While the improper usage of antibiotic treatments is an accepted driver of AMR, little work has focused on how non-antibiotic medication, such as supplements, might impact this when combined with antibiotics. One supplement of interest is the heavy metal zinc which is used in conjunction with ciprofloxacin to treat diarrheal diseases in children. We find that the order and duration of zinc exposure has significant impact on resistance development. More specifically, although the combined presence of zinc and ciprofloxacin delays the onset of resistance, when used successively as they often are in practice, zinc pre-exposure followed by ciprofloxacin exposure results in faster resistance development.