Optimized methods for detecting Salmonella Typhi in the environment using validated field sampling, culture and confirmatory molecular approaches

Abstract

This study evaluated detection methods for Salmonella Typhi (S. Typhi) in the environment, to establish a novel pathway from field sampling to isolation of viable organisms and molecular confirmation from complex environmental samples, thus enabling environmental surveillance of typhoid.Methods and Results: Multiple media were assessed using clinical isolates from the Public Health England's (PHE) Culture collection. The culture pathway selected consisted of a primary 2% bile broth and secondary Selenite F broth, followed by modified Chromogenic Agar for Salmonella Esterase (mCASE). A qPCR assay was adapted from a validated S. Typhi PCR panel for confirmation of isolates, with comparison to biochemical and serological tests showing good specificity. Sampling locations in Blantyre, Malawi were used to compare sampling methods. Viable S. Typhi were isolated from a mixture of trap and grab river water samples on six occasions. Conclusions:Culture of viable S. Typhi from environmental samples was possible using effective capture and culture techniques.Significance and impact of study: Whilst several studies have attempted to detect S. Typhi from the environment, this is the first successful attempt to isolate the organism from river water since the 1980s. Supplementing clinical data with environmental screening offers the potential for enhanced surveillance, which might inform interventions and assess vaccination programmes.

Case-control investigation of invasive Salmonella disease in Malawi reveals no evidence of environmental or animal transmission of invasive strains, and supports human to human transmission

“…Typhi is human-restricted in its ability to cause disease, but has a poorly defined environment during long-cycle transmission in the community [55]. While we failed to isolate S. Typhi from the environment of either case or control households, this is known to be technically challenging, and our results highlight the need for further methodological development in this area [56].…”

“…While we failed to isolate S . Typhi from the environment of either case or control households, this is known to be technically challenging, and our results highlight the need for further methodological development in this area [56].…”

Utilizing river and wastewater as a SARS-CoV-2 surveillance tool to predict trends and identify variants of concern in settings with limited formal sewage systems.

“…Utilizing our previously established ES program for Salmonella enterica serovar Typhi 24,25 we based our ES for SARS-CoV-2 on 30ml "grab" water samples. Water from rivers, informal sewage systems and a defunct wastewater treatment plant was collected in two phases; from May 2020 -January 2021 we developed our sample framework, eld sampling method and laboratory methods and from January 2021 -May 2022 we greatly expanded the scale of our program 26 (Fig.…”

A novel, magnetic bead-based extraction method for the isolation of antimicrobial resistance genes with a case study in river water in Malawi

“…plasmids) may be lost during this culture step due to a suspected fitness cost (Huang et al, 2013 ). Furthermore, generalist species like E. coli may outcompete niche‐adapted pathogens such as Salmonella Typhi (Rigby et al, 2021 ).…”