Low-Temperature Transcriptional Responses in Selected pESI-Positive Salmonella Infantis Isolates with Contrasting Antimicrobial Resistance Phenotypes: An Exploratory Study

Refrigeration and freezing are commonly used preservation methods in the control of foodborne pathogens. However, the transcriptional responses of Salmonella Infantis to low-temperature conditions remain incompletely understood. In this study, the effect of low-temperature stress on the expression of pESI-related repA, ipf, and K88-like genes, as well as cspA, involved in cold shock response, and ompF, associated with environmental stress adaptation, was investigated in three pESI-positive, antibiotic-susceptible and three pESI-positive multi-antibiotic-resistant (MDR) S. Infantis isolates. Isolates were exposed to 4 °C and −20 °C conditions for 24 h, while isolates incubated at 37 °C were considered as the control group. Relative expression levels of genes were determined using the SYBR Green-based RT-qPCR method. Mean transcriptional patterns differed between the two selected isolate groups, most prominently at 4 °C. Relative to their respective 37 °C controls, all five target genes showed decreased mean expression in susceptible isolates but increased mean expression in MDR isolates at 4 °C. At −20 °C, group-mean expression was increased across all five targets in the MDR group, whereas the susceptible group showed decreased mean expression of the pESI-associated targets repA, ipfA, and klf, but modest increases in ompF and cspA. A significant temperature-by-phenotype interaction was detected for ipfA (p = 0.029). Between-group comparisons identified significant differences for the K88-like fimbrial locus (klf) and ompF at 4 °C. However, larger isolate groups and functional validation studies are needed to elucidate the biological significance of the identified expression changes.