Study Traces How Common Preservatives Kill Bacteria, Opening Path to Smarter Formulations

Scientists have for the first time captured the sequence of events by which two widely used preservatives bring bacteria down, a finding that could inform how preservation systems are designed across food, home and personal care products.

The work, carried out by researchers at the Institute of Nano Science & Technology (INST) Mohali — an autonomous institute under the Department of Science and Technology — in collaboration with Unilever R&D Bangalore, examined how sodium benzoate (SB) and phenoxyethanol (POE) act on bacterial cells. The findings were published in Letters in Applied Microbiology.

Both compounds have long histories. Sodium benzoate has protected foods such as pickles, ketchup and fizzy drinks since the early 1900s and was the first food preservative approved by the US FDA in 1908. Phenoxyethanol is a more recent addition, used for decades in shampoo, moisturiser, sunscreen and some vaccines. Despite this extensive use, the molecular basis of their antibacterial action had remained poorly understood.

To close that gap, the team combined transmission electron microscopy with biochemical assays, studying responses in two representative pathogens: the Gram-positive Staphylococcus aureus and the Gram-negative Pseudomonas aeruginosa.

The results point to a coordinated, multi-target attack rather than a single mode of action. The bacterial cell envelope is physically compromised while reactive aldehydes and oxygen-derived species build up inside the cell, disrupting proteins, genetic material and other essential functions.

Imaging showed that the two preservatives take different routes to inactivation. Sodium benzoate caused cells to shrink and collapse, whereas phenoxyethanol led to membrane expansion and eventual rupture. The researchers also found that sodium benzoate becomes markedly more potent in acidic conditions, with roughly a sixteen-fold increase in activity, while phenoxyethanol maintained consistent efficacy across a broad pH range.

The experiments spanned inhibition zone and MIC assays to gauge growth suppression, electron microscopy to observe structural breakdown, a dye leakage test to detect membrane damage, and two biochemical assays (MBTH and DCFDA) to measure internal chemical disruption — all conducted under both neutral and acidic conditions.

The authors of the study are Ishani Sharma, S M Rose, Madhu Lata, Somnath Das, Nagaraja IS Acharya, Maheshwara Naik, Samiran Mahapatra and Sharmistha Sinha.

According to the researchers, understanding these mechanisms can help industry select the appropriate preservative for a given product format — sodium benzoate for acidic formulations and phenoxyethanol where pH varies — use accurate amounts rather than estimates, reduce spoilage and waste, and stay ahead of bacteria that might otherwise develop resistance.