Bystander selection --the selective pressures for resistance exerted by antibiotics on microbes that are not the target pathogen of treatment --is critical to understanding the total impact of broad-spectrum antibiotic use on pathogenic bacterial species that are often carried asymptomatically. However, to our knowledge, this effect has never been quantified. We quantify bystander selection for resistance for a range of clinically relevant antibiotic-species pairs as the proportion of all antibiotic exposures received by a species for conditions in which that species was not the causative pathogen ("proportion of bystander exposures"). Data sources include the 2010-2011 National Ambulatory Medical Care Survey and National Hospital Ambulatory Medical Care Survey (NAMCS/NHAMCS), the Human Microbiome Project, and additional carriage and etiological data from existing literature. For outpatient prescribing in the United States, we find that this proportion over all included antibiotics is over 80% for 8 of 9 organisms of interest. Low proportions of bystander exposure are often associated with infrequent bacterial carriage or concentrated prescribing of a particular antibiotic for conditions caused by the species of interest. Applying our results, we roughly estimate that pneumococcal conjugate vaccination programs result in nearly the same proportional reduction in total antibiotic exposures to S. pneumoniae , S. aureus , and E. coli , despite the latter two organisms not being targeted by the vaccine. These results underscore the importance of considering antibiotic exposures to bystanders, in addition to the target pathogen, in measuring the impact of antibiotic resistance interventions.
Significance StatementBystander selection, defined as the inadvertent pressures imposed by antibiotic treatment on microbes other than the targeted pathogen, is hypothesized to be a major factor in the propagation of antibiotic resistance, but its extent has not been characterized. We estimate the proportion of bystander exposures across a range of antibiotics and organisms and describe factors driving variability of these proportions. Impact estimates for antibiotic resistance interventions, including vaccination, are often limited to effects on a target pathogen. However, the reduction of antibiotic treatment for conditions caused by one pathogen may have the broader potential to decrease bystander selection pressures for resistance on many other organisms.