Quantitative Microbial Risk Assessment of Antibiotic-Resistant E. coli, Legionella pneumophila, and Mycobacteria in Nonpotable Wastewater Reuse Applications

Abstract

Antibiotic-resistant bacteria (ARB) have become a major threat to public health and modern medicine. A simple death kinetics-based dose-response model (SD-DRM) was incorporated into a quantitative microbial risk assessment (QMRA) to assess the risks of exposure to reclaimed wastewater harboring antibiotic-resistant E. coli, Legionella pneumophila, and Mycobacterium avium for multiple exposure scenarios. The fractions of ARB and trace antibiotics present in the body were incorporated to demonstrate their impact on infection risks. Both ARB and antibiotic susceptible bacteria, ASB, are assumed to have the same dose-response in the absence of antibiotics but behave differently in the presence of residual antibiotics in the body. Annual risk of L. pneumophila infection exceeded the EPA 10^-4 pppy (per person per year) benchmark at concentrations in reclaimed water greater than 10³-10⁴ CFU/L, depending on parameter variation. Enteropathogenic E. coli infection risks meet the EPA annual benchmark at concentrations around 10⁵-10⁶ total E. coli. The results illustrated that an increase in residual antibiotics from 0 to 40% of the minimum inhibitory concentration (MIC) reduced the risk by about 1 order of magnitude for E. coli but was more likely to result in an untreatable infection.

Keywords: Monte Carlo simulation; dose−response; exposure assessment; simple death; sustainable water management.

Figure 1

Annual infection risk for toilet flushing for M. avium (a,b) and L. pneumophila (c,d). For (a) and (c), the residual antibiotic concentration in human host was kept constant at 5% MIC while varying ARB fractions, f_r. For (b) and (d), the ARB f_r was held constant at 0.05 while varying the residual antibiotic concentration from 0, 50 to 100% of MIC. The black risk curve represents an infection outcome more likely treatable by antibiotic, the red line represents an untreatable infection is more likely, and the blue line is for the EPA risk benchmark/target of 1 in 10 000 infections per person per year.

Figure 2

Annual infection risk of EPEC through accidental ingestion at a reclaimed water-irrigated golf course or public park. The concentration (x-axis) represents the concentration of total E. coli in the irrigation water, which is reduced by a fraction for EPEC risk. For (a), the AB concentration was kept constant at 5% MIC while varying ARB fractions f_r in the total dose. The f_r in (b) was held constant at 0.05 while increasing the antibiotic concentration from 0 to 40% of MIC. The black, red, and blue lines in the graph are the same as described in Figure 1.

Figure 3

Annual infection risk of EPEC through the consumption of irrigated lettuce. The concentration represents the concentration of total E. coli in the irrigation water, which is reduced by a fraction for EPEC risk. For (a), the AB concentration was kept constant at 5% MIC while displaying different ARB fractions, f_r. The f_r in (b) was held constant at 0.05. The black, red, and blue lines in the graphs are the same as described in Figure 1.

Figure 4

Sensitivity analysis for exposure and ARB dose–response parameters. Bars are color-coded to represent each pathogen assessed.

Figure 5

Illustration of the binary outcomes of infection (AB treatable or untreatable) by varying f_r and C for constant doses of each pathogen. AB concentration is expressed as the fraction of MIC from 0 to 10%.