were confirmed as ESBL E. coli . For the ESBL CBT method, 76.9% of the presumptively positive isolates were confirmed as ESBL E. coli . Lastly, for the GEL ESBL method, 60.2% of the presumptively positive isolates were confirmed as ESBL E. coli. These field meth- ods likely are suitable for field applications in settings with limited resources or infrastruc- ture, as they gave comparable results to the standard method, which is not easily usable in the field because it requires additional materials and equipment. Continued and widespread monitoring of ESBL E. coli in environmental waters is a useful monitoring and surveillance approach for antimicrobial resistance, and as such is recommended by WHO. Our research sug- gests the need, however, to further adapt and simplify the current Tricycle protocol to more easily and broadly detect ESBL E. coli in envi- ronmental waters by field testing. Acknowledgments: Support for this project was provided by Aquagenx. The funders, however, had no role in the data collection and analy- sis components of this work. We acknowledge Amina Bash, Lauren Burke, and Elina Thomas for their expertise in processing laboratory samples. We thank Dr. Mark D. Sobsey of the University of North Carolina for his helpful review of this manuscript. Corresponding Author: Emily S. Bailey, Assis- tant Professor, Department of Public Health, College of Pharmacy & Health Sciences, Campbell University, 4350 U.S. Highway 421
TABLE 2
Results of Isolate Confirmation Testing by Assay Method
Method
CTX Streak Plating # (%)
Indole Positive # (%)
ESBL Positive # (%)
Membrane filtration ( n = 117)
110 (94.0) 88 (96.7) 86 (87.8) 287 (92.8)
97 (82.9) 83 (91.2) 82 (83.7) 262 (85.6)
65 (55.6) 70 (76.9) 59 (60.2) 197 (64.4)
ESBL CBT ( n = 91) GEL ESBL ( n = 98)
Total ( N = 306)
Note. CBT = compartment bag test; CTX = cefotaxime; ESBL = extended-spectrum beta-lactamase.
the three methods directly for our evalua- tion. This method of direct comparison has been previously evaluated and although con- centrations dier when microorganisms are detected by each method for a variety of rea- sons, when these methods are used on field samples or in field settings, the results appear to be equivalent (Eckner, 1998; Gronewold & Wolpert, 2008). As such, in our evaluation, microbial concentrations based on CFU and MPN units are treated as equivalent, as previ- ously documented by Bailey et al. (2017). The ESBL CBT and GEL ESBL methods are portable and easy to use and would be par- ticularly applicable when used in field condi- tions. For the GEL ESBL method, however, there is a learning curve and the manufac- turer’s instructions are cumbersome to a new user. Therefore, for more consistent results, it could be helpful to provide additional visual aids for the use of this method. These methodological limitations, in addi- tion to the limited number of samples ( n =
100), are important considerations when comparing the various experimental meth- ods. Despite these limitations, the results from our evaluation of 306 presumptive ESBL E. coli isolates examined across the three methods would be comparable with results one would expect with testing using the Tricycle protocol. Additionally, although it was not a direct focus of our evaluation, the quality of the surface water included in our comparison of the three methods is a relevant variable that would be interesting to consider in future comparisons. Conclusion Our evaluation provides quantitative evidence that the three dierent culture methods we compared can detect statistically similar lev- els of ESBL E. coli in surface water samples. We found no statistically significant dier- ence in the three methods for detecting ESBL E. coli. For the membrane filtration method, 55.6% of the presumptively positive isolates
South, Lillington, NC 27546. Email: ebailey@campbell.edu
References
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November 2024 • Journal of Environmental Health
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