Combined effects of vitamin C & food-derived ingredients against multidrug-resistant Staphylococcus aureus and Pseudomonas aeruginosa

Abstract

The growing interest in food-derived antimicrobial therapy is driven by the expectation that natural compounds with antibacterial properties might serve as adjunctive strategies against antimicrobial resistance. This study examined whether common food-derived compounds can modify the activity of ciprofloxacin against multidrug-resistant Staphylococcus aureus and Pseudomonas aeruginosa, both listed in the 2024 WHO Bacterial Priority Pathogens List. Vitamin C, turmeric, black seed, garlic, gallic acid and vanillin were prepared as 2 g/10 ml stocks and tested by disc diffusion at 5–15 μl per disc, giving total loads of roughly 300–3000 μg of natural compound. Ciprofloxacin (30 μg per disc) was used as the antibiotic control. On their own, the food-derived agents showed little or no inhibition at doses comparable to normal intake, and clear zones appeared only when vitamin C, gallic acid or vanillin were pushed into the upper hundreds of micrograms per disc. Ciprofloxacin was then combined with vitamin C plus either gallic acid or vanillin at stock ratios of 4:6, 3:7 and 2:0, keeping the ciprofloxacin dose at 30 μg while varying the total phenolic load between about 300 and 3000 μg per disc. In S. aureus, several gallic-acid–vitamin-C mixtures at 4:6 and 3:7 gave zones up to about 40 mm, compared with 37–38 mm for ciprofloxacin alone, indicating a modest synergistic effect. In contrast, vanillin-rich mixtures and many P. aeruginosa combinations produced smaller zones, around 25–30 mm where the ciprofloxacin control was 37–39 mm, consistent with antagonism. Some intermediate ratios were essentially indifferent. Overall, vitamin C and food-derived phenolics showed both synergistic and antagonistic interactions with ciprofloxacin, depending on dose and ratio, highlighting the need for careful optimisation before considering such combinations as antimicrobial adjuvants. Although the study is limited to an in vitro model, the results underscore the importance of further evaluating how antioxidant-rich supplements and dietary phenolics may enhance or complement antibiotic function. These observations support continued investigation in mammalian cell lines and in vivo systems to clarify their safety, pharmacokinetic behavior, and potential clinical benefits when used alongside conventional antibiotic therapy.