Characterization of Antibiotic Resistance Genes, Plasmids, Biofilm Formation, and In Vitro Invasion Capacity of Salmonella Enteritidis Isolates from Children with Gastroenteritis

Objectives: The primary aim of this study was to investigate the presence of antibiotic resistance genes (ARGs) and plasmid replicon types for 75 multidrug-resistant (MDR) Salmonella Enteritidis isolates from children with gastroenteritis. We also evaluated the association among biofilm formation, in vitro invasion capacity, and antibiotic resistance phenotypes. Materials and Methods: Twenty-two ARGs and 18 different plasmid incompatibility types were investigated using polymerase chain reaction (PCR) and DNA sequencing. In vitro invasion capacity of S. Enteritidis isolates possessing different antibiotic resistance patterns was assessed using the Caco2 human intestinal epithelial cell line and biofilm formation was performed in a 96-well polystyrene well format using crystal violet detection. Results: The presence of plasmid-mediated quinolone resistance genes and beta-lactamase genes was established using PCR amplification. All the tested S. Enteritidis isolates that were fluoroquinolone resistant possessed gyrA mutations and 50% also possessed mutations in parC. MDR S. Enteritidis isolates containing three (29/75) or four (21/75) plasmid replicon types were predominant and 71/75 carried both FIIs and FIC replicon-type plasmids. MDR isolates were strong or moderate biofilm producers and a significant positive association (p < 0.05) between antibiotic resistance and biomass of biofilms was observed in the strains assayed. A ceftiofur-resistant strain was significantly more invasive (p < 0.01) than the other isolates. Conclusions: We observed a high incidence of ARGs and diversity of plasmids in S. Enteritidis isolates from children. Biofilm formation and invasion capacity highlight a significant hazard to public health.