The effects of acidifying agents on the growth, antimicrobial resistance, and plasmid genome of Salmonella Infantis

Authors: Cho, Sohyun; CHUNG, TAEJUNG - Oak Ridge Institute For Science And Education (ORISE); ZOCK, GREGORY - Oak Ridge Institute For Science And Education (ORISE); Oladeinde, Adelumola; Rothrock Jr, Michael; Li, Xiang; Plumblee Lawrence, Jodie; WALID, AL HAKEEM - Oak Ridge Institute For Science And Education (ORISE); Lewis, Micah

Submitted to: Letters in Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2025
Publication Date: 9/10/2025
Citation: Cho, S., Chung, T., Zock, G., Oladeinde, A.A., Rothrock Jr, M.J., Li, X., Plumblee Lawrence, J.R., Walid, A., Lewis, M.A. 2025. The effects of acidifying agents on the growth, antimicrobial resistance, and plasmid genome of Salmonella Infantis. Letters in Applied Microbiology. https://doi.org/10.1093/lambio/ovaf115.
DOI: https://doi.org/10.1093/lambio/ovaf115

Interpretive Summary: Salmonella is a zoonotic pathogen that threatens food safety, and reducing Salmonella contamination in poultry can reduce the risk to the human food supply. Salmonella Infantis has become one of the main serotypes found in chicken products in the US and across the globe. S. Infantis strains that harbor the pESI plasmid are prevalent in litter samples from broiler farms, indicating that pre-harvest management practices cannot efficiently eradicate S. Infantis. In this study, we examined to see if acidifying agents that are extensively used to acidify litter and water for poultry house ammonia control and for pathogen reduction have a deleterious effect on the survival and growth of Salmonella by exposing Salmonella Infantis to sodium bisulfate (SBS) and acidified copper sulfate (ACS). We also investigated if there are fitness effects of the pESI plasmid on S. Infantis when grown in the presence of the acidifiers and examined any changes in antimicrobial resistance phenotypes and pESI plasmid genome composition. SBS and ACS had a bacteriostatic effect on S. Infantis at concentrations recommended by the manufacturers, while ACS at double the recommended concentration had a bactericidal effect, suggesting that SBS and ACS prepared at the recommended concentrations to be used in the poultry industry do not prevent the growth of Salmonella. The growth studies showed that the growth of the pESI-carrying S. Infantis strains was similar to that of the pESI-negative strains in LB media, suggesting that there was a minimal benefit or burden imposed by the pESI. However, when exposed to a double concentration of SBS, both the strains exhibited reduced growth with a significantly long lag phase time. In addition, pESI-carrying isolates lost either sulfamethoxazole (SOX) or trimethoprim-sulfamethoxazole (SXT) resistance phenotypes as well as the associated genes on the plasmid. The media containing SBS posed a selective pressure on S. Infantis to drop SOX and SXT resistance, and further studies are needed to determine why SOX or SXT resistance was lost in the presence of SBS.

Technical Abstract: Salmonella is a zoonotic pathogen that threatens food safety, and reducing Salmonella contamination in poultry can reduce the risk to the human food supply. In this study, we examined to see if acidifying agents that are extensively used to acidify litter and water for poultry house ammonia control and for pathogen reduction have a deleterious effect on the survival and growth of Salmonella by exposing Salmonella Infantis to sodium bisulfate (SBS) and acidified copper sulfate (ACS). We also investigated if there are fitness effects of the pESI on S. Infantis when grown in the presence of acidifiers and examined any changes in antimicrobial resistance (AMR) phenotypes and pESI plasmid genome composition. SBS and ACS had a bacteriostatic effect on S. Infantis at concentrations recommended by the manufacturers, while ACS at double the recommended concentration had a bactericidal effect. The growth studies showed that the growth of the pESI-carrying S. Infantis strains was similar to that of the pESI-negative strains in LB media, suggesting that there was a minimal benefit or burden imposed by the pESI. Additionally, several evolved isolates of the pESI-carrying strains either gained or lost phenotypic resistance to certain antimicrobial drugs. Susceptibility to sulfamethoxazole (SOX) seems to be more favorable in the media containing SBS since the addition of SBS posed a selective pressure on S. Infantis to drop SOX resistance. Further studies are needed to determine why SOX or SXT resistance and the sul1 or dfrA14 genes were undetected in the presence of SBS.