Introduction
The highest prevalence of AMR infections occurs in sub-Saharan Africa, with extended-spectrum beta lactamase (ESBL) infections having a high associated morbidity and mortality. Patients typically become colonised by ESBL-bacteria before they develop symptomatic infection, and there is a growing recognition of the importance of environmental exposure to these bacteria in the community. Establishing efficient, accurate and cost-effective methods for conducting environmental surveillance of ESBL producing bacteria in these settings is required. Here, we evaluated the use of ChromAgar culture and novel ESBL Colilert methods for detecting ESBLs in water samples.
Methods
Four urban markets in Blantyre, Malawi were evaluated. Water samples were taken from (i) stored water used by market vendors to freshen vegetable produce, (ii) the primary source of water (i.e. kiosk/borehole) (iii) drain samples at pre-identified risk points in the marketplace, and (iv) water from rivers adjacent to the marketplace. Water samples were pre-processed by filtering through a 0.45µm cellulose-ester gridded membrane (VWR™) using a vacuum-based manifold and drain samples were directly incubated in enrichment media (buffered peptone water). Samples were processed via conventional culture methods using ESBL CHROMagar™ media alongside paralleled testing with an IDEXX Colilert Quanti-tray method, in the presence of ceftriaxone (1ug/ml) to identify ESBL E. coli. All positive ESBL Colilert samples were confirmed by growth of Colilert broth on ESBL CHROMagar™ media. The ESBL Colilert method was compared to ChromAgar to determine sensitivity and specificity of ESBL identification.
Results
Between February-November 2021, 167 water samples were obtained, with 39% (n=65) and 76% (n=127) found to be contaminated with ESBL E. coli by ChromAgar and Colilert methods respectively. Agreement in ESBL E. coli results between methods was found in on only 52% (n=87) of samples (33% (n=56) positive and 18% (n=31) negative by both methods). Using ChromAgar culture as a gold standard reference method, the sensitivity and specificity of ESBL Colilert was 30% and 86% respectively (Cohens Kappa: 0.141<0.001). However, secondary testing of Colilert broth with ChromAgar identified 100% recovery of ESBL E. coli, indicating that Colilert is likely to be a more sensitive method than ChromAgar for the primary recovery of ESBL bacteria.
Discussion
Both methods are valuable for effective environmental surveillance. ESBL Colilert shows promise for its sensitivity in detecting ESBL-producing bacteria, attributed to enrichment steps and lower antibiotic concentrations.
Conclusion
Water samples from urban markets in Malawi were heavily contaminated with ESBL E. coli, highlighting that contaminated water in the marketplace may be an important driver of local EBSL transmission that warrants future surveillance. Variations in the results obtained from conventional ChromAgar and ESBL Colilert methods highlight that more research is required to optimize microbiological methods for the recovery and identification of AMR bacteria from environmental samples to enable accurate surveillance.