In vitro assessment of the synergistic effects of cefotaxime, colistin, and fosfomycin combinations against foodborne resistant Escherichia coli and Salmonella isolates

The emergence of multidrug-resistant pathogens, particularly β-lactam, colistin, and fosfomycin-resistant Escherichia coli and Salmonella, is a significant public health concern. This study evaluated the in vitro synergistic effects of antimicrobial combinations against these resistant isolates. Ten isolates that originated from retail chicken meat, including five E. coli and five Salmonella isolates, were tested against cefotaxime (CTA), fosfomycin (FOS), and colistin (COL), both individually and in combinations. Antimicrobial susceptibility was assessed using the broth microdilution method, and synergistic interactions were evaluated using checkerboard and time-killing assays. All isolates were multidrug-resistant (MDR) and were resistant to CTA, COL, and FOS. The checkerboard assay showed varying levels of synergy: two out of five E. coli isolates exhibited synergy with FOS-COL, while one E. coli isolates out of four isolates showed synergy with CTA-COL. No E. coli isolates showed synergy with FOS-CTA. For Salmonella, two out of five isolates exhibited synergy with both FOS-CTA and FOS-COL, while three out of four isolates showed synergy with CTA-COL. The time-killing assay confirmed these results, with the FOS-COL combinations showing synergy against both E. coli and Salmonella strains. Notably, the FOS-COL combination demonstrated bactericidal effects against E. coli, and all three combinations were bactericidal against Salmonella. The study highlights the potential of antimicrobial combinations, particularly FOS-COL, in combating MDR E. coli and Salmonella. These findings support the use of combination therapy as a promising strategy to in effectively treating multi-drug-resistant foodborne infections, ensuring better medical outcomes and enhanced food safety, warranting further investigation into their mechanisms and clinical applications.