Antimicrobial resistance genes are enriched in aerosols near impacted urban surface waters in La Paz, Bolivia

Abstract

Antibiotic resistance poses a major global health threat. Understanding emergence and dissemination of antibiotic resistance in environmental media is critical to the design of control strategies. Because antibiotic resistance genes (ARGs) may be aerosolized from contaminated point sources and disseminated more widely in localized environments, we assessed ARGs in aerosols in urban La Paz, Bolivia, where wastewater flows in engineered surface water channels through the densely populated urban core. We quantified key ARGs and a mobile integron (MI) via ddPCR and E. coli spp. as a fecal indicator by culture over two years during both the rainy and dry seasons in sites near wastewater flows. ARG targets represented major antibiotic groups-tetracyclines (tetA), fluoroquinolines (qnrB), and beta-lactams (bla_TEM)-and an MI (intI1) represented the potential for mobility of genetic material. Most air samples (82%) had detectable targets above the experimentally determined LOD: most commonly bla_TEM and intI1 (68% and 47% respectively) followed by tetA and qnrB (17% and 11% respectively). ARG and MI densities in positive air samples ranged from 1.3 × 10¹ to 6.6 × 10⁴ gene copies/m³ air. Additionally, we detected culturable E. coli in the air (52% of samples <1 km from impacted surface waters) with an average density of 11 CFU/m³ in positive samples. We observed decreasing density of bla_TEM with increasing distance up to 150 m from impacted surface waters. To our knowledge this is the first study conducting absolute quantification and a spatial analysis of ARGs and MIs in ambient urban air of a city with contaminated surface waters. Environments in close proximity to urban wastewater flows in this setting may experience locally elevated concentrations of ARGs, a possible concern for the emergence and dissemination of antimicrobial resistance in cities with poor sanitation.

Keywords: Antibiotic resistance; Bioaerosol; Dissemination; E. coli; Mobile integron; Wastewater.

Figure 1.

(A) Location of La Paz, Bolivia. (B) La Paz Municipality boundaries with all sampling sites and control sites. (C) Sampling locations in relation to covered and uncovered OWCs.

Figure 2.

Left: average ARG and MI densities with mean standard error bars for the distribution in gene copies per cubic meter of air, where targets were detected at levels equal to or above the LOD. Right: mean culturable E. coli per cubic meter of air with mean standard error bars for the distribution in coliforming units per cubic meter of air.

Figure 3.

ARG density in gene copies per m³ of air in relation to the distance from OWCs. The blue line indicates the linear regression calculation for each data set and the data is not separated by time of day. Each plot includes the corresponding regression line equation, R-squared value, and p-value.