Synergistic effects of a lytic Salmonella bacteriophage cocktail and selected antimicrobial agents on Salmonella enterica serovar Enteritidis.

Abstract

Salmonella Enteritidis is a major causative agent of gastroenteritis and foodborne illnesses, posing significant therapeutic challenges due to the rise of multidrug-resistant (MDR) strains. The increasing prevalence of resistant isolates highlights the need for alternative strategies to improve treatment outcomes. Lytic bacteriophage therapy has emerged as a promising complementary approach to antimicrobials. This study aimed to investigate the synergistic bacteriostatic and bactericidal effects of a newly formulated cocktail comprising three distinct lytic bacteriophages combined with selected antimicrobials against the standard strain of Salmonella Enteritidis (ATCC 13076). Three distinct bacteriophages were isolated from poultry farm wastewater, purified based on their different plaque morphologies, and characterized by transmission electron microscopy (TEM), revealing short non-contractile tailed caudoviruses. Phage stability was evaluated across a pH range of 3 to 11 and temperatures from 4 to 50 °C. The minimum inhibitory concentration (MIC) was determined using the macrodilution (tube dilution) method, and viable bacterial counts (CFU/mL) were measured at 0, 6, 12, and 24 h to assess the bacteriostatic and bactericidal effects on the bacterial strain. The phage cocktail combined with ciprofloxacin and ceftriaxone showed significant synergistic activity, resulting in reductions of 5.8 and 4.9 log CFU/mL, respectively, corresponding to up to a 75% reduction in MIC. In contrast, combinations with ampicillin and erythromycin demonstrated the least efficacy. The phage cocktail alone achieved a 3.2 log reduction in CFU/mL. The phage cocktail demonstrated enhanced antibacterial activity when combined with ciprofloxacin or ceftriaxone, achieving greater reductions in Salmonella Enteritidis counts compared to individual treatments. Minimal synergy was observed with ampicillin and erythromycin. Overall, the evaluated phage-antimicrobial combinations exhibited superior antibacterial effects within the scope of this study.