Oregano–Rosemary Essential Oil Combinations as Potential Growth-Promoting Alternatives in Poultry: Evidence from Bacterial Time–Kill Curves

The widespread use of antibiotics as growth promoters in poultry production has contributed to the emergence of antimicrobial resistance and the dissemination of zoonotic pathogens, underscoring the urgent need for effective non-antibiotic alternatives. This study aimed to evaluate the in vitro antibacterial activity of oregano (Origanum vulgare) essential oil (OEO), rosemary (Salvia Rosmarinus) essential oil (REO), and their combination (OR) against Escherichia coli and Salmonella typhimurium strains of poultry origin.

Minimum inhibitory and bactericidal concentrations were determined, and the optimal OR combination was selected using checkerboard assays. Time–kill curves were performed using ATCC reference strains (E. coli ATCC 25922 and S. Typhimurium ATCC 14028), as well as poultry field isolates (n = 2 per species), exposed to individual essential oils and their combination at different multiples of the MIC, under pH conditions simulating the avian gastrointestinal tract (pH 5.0 and 7.4). Bacterial viability was monitored over time based on viable cell counts (CFU/mL), and antibacterial activity was quantified using the antibacterial effect index (E). Results were interpreted according to established cut-off values for bacteriostatic activity (E = 0), bactericidal activity (E ≤ −3), and virtual eradication (E ≤ −4).

Both essential oils exhibited rapid and concentration-dependent antibacterial activity. At 2× and 4× MIC, OEO and REO induced immediate bacterial killing, achieving virtual eradication (E ≤ −4) within 10–20 minutes of exposure. The OR combination consistently enhanced antibacterial activity, reaching bactericidal and eradication thresholds faster and at lower concentrations compared to individual oils. Comparable killing dynamics were observed across pH conditions and between reference strains and poultry field isolates for both E. coli and S. typhimurium.

These findings demonstrate the potent and rapid antimicrobial action of OEO and REO, particularly when used in combination, supporting their potential application as non-antibiotic growth-promoting alternatives in poultry production within a One Health framework.