Utilization of short-chain carboxylic acids (SCCA) is a feasible approach to reduce food waste by preventing microbial spoilage. However, the antimicrobial mode of action of SCCA has not been fully uncovered. This study aimed to evaluate the effects of structurally different SCCA with 3 carbons in the backbone on the antimicrobial activity against Salmonella enterica, including hydrophobicity (log K_ow) as a factor of compound structure that is often ignored.

The minimum inhibitory concentrations (MIC) of propionic (log K_ow = 0.33), lactic (log K_ow = –0.72), 3-phenylpropionic (log K_ow = 1.84) and 3-phenyllactic acid (log K_ow = 1.18) were tested using a high-throughput broth dilution assay at pH 4.5, and the OD₆₀₀ was recorded after 24 h incubation. To determine microbial response, Salmonella cells were harvested at log phase (OD₆₀₀ 0.2~0.3), RNA was extracted, reverse transcribed and sequenced. DeSeq2 was used to compare the transcriptional profiles of treated cells and controls that grew without subinhibitory levels of SCCA.

The antimicrobial activity of SCCA increased with increasing compound hydrophobicity: 3-PP > 3PL > propionic > lactic acid. 3-PP inhibited the growth of S. enterica the most and reduced the final cell density to 50% (MIC₅₀) at 1.8 mM, followed by 3PL (5.0 mM), propionic acid (5.2 mM), and lactic acid (28.2 mM). RNA-seq indicated higher expression of lldPRD and nap operon encoding proteins related to lactate uptake and nitrate reductase, which has been linked to acid stress responses. When subjected to propionic acid and 3PP, eut and pdu genes that are responsible for ethanolamine and glycerol/1,2-propanediol utilization were upregulated, respectively.

In conclusion, we identified 3-PP as a potential biopreservative to inhibit Salmonella with hydrophobicity as a determining factor. Gene expression responses differed between compounds suggesting that compound structure is an important contributor to the inhibitory activity of SCCA.