# Reliable Differentiation of a Bivalent Live Salmonella Vaccine and Field Strains: Multi-Supplier Validation of a Disc Diffusion Method

## TL;DR

This paper validates a cost-effective disc diffusion method to distinguish a live Salmonella vaccine from field strains, offering an alternative to PCR testing.

## Contribution

The study validates a disc diffusion method across multiple suppliers for reliably differentiating a bivalent Salmonella vaccine from field strains.

## Key findings

- Discs from four out of eight suppliers reliably differentiated vaccine and field strains.
- The disc diffusion method is a cost-effective alternative to real-time PCR for this purpose.

## Abstract

Zoonotic Salmonella, particularly Salmonella Enteritidis (SE) and Salmonella Typhimurium (ST), represents a major cause of foodborne infections associated with poultry and eggs. Vaccination of breeding flocks and laying hens using live vaccines administered via drinking water has significantly reduced human salmonellosis and prevalence in the poultry sector. However, the detection of vaccine strains in feces from recently vaccinated birds requires reliable methods for distinguishing them from field strains. This study validates a disc diffusion method based on the antimicrobial resistance profiles of vaccine strains from a live bivalent vaccine against SE and ST. Discs from eight suppliers were evaluated, of which those from four proved suitable. This approach constitutes a simple and cost-effective alternative to real-time PCR for appropriately equipped laboratories.

Zoonotic Salmonella is one of the most important foodborne zoonoses worldwide, with Salmonella Enteritidis (SE) and Salmonella Typhimurium (ST) being the most important serovars, which may be transmitted via poultry meat and eggs. Vaccination of breeding- and laying-hen flocks using live vaccines administered via drinking water has long been a well-recognized tool and contributed significantly to decreasing cases of salmonellosis in humans and its prevalence in the chicken sector. However, if a vaccine strain is detected in feces from recently vaccinated birds, it is imperative to have methods for reliably distinguishing between field and vaccine strains. In this study, we validated a disc diffusion method based on the antimicrobial resistance patterns of the vaccine strains contained in a live bivalent vaccine conferring protection against SE and ST. Discs from eight suppliers were sourced and tested for their suitability for reliably distinguishing between field and vaccine strains, a requirement fulfilled by discs from four suppliers. This method serves as an alternative approach for laboratories equipped to perform disc diffusion assays, and it is also a more cost-effective solution than real-time PCR assays.