When I was running a flavor and fragrance company, we made preservatives with microbes. We fermented sugar into propionic acid and grew cultures whose fermentates carried antimicrobial peptides, and we sold both. We also showed we could make benzoic acid by converting benzaldehyde with microbes. Using microbes to restrain other microbes makes sense once you’ve spent time with the rumen. The community that taught us so much about cooperation is also full of organisms competing for the same food.
Researchers have pulled several of those weapons out of rumen bacteria. Chen and colleagues isolated albusin B from cultures of Ruminococcus albus 7 and showed it inhibited Ruminococcus flavefaciens, another fiber-degrading rumen bacterium. [1] Whitford and colleagues tested 35 rumen streptococci, found seven that inhibited other streptococci, and purified one of the inhibitors, bovicin 255. [2] In both cases, the target was a close relative competing for the same food.
Our two kinds of preservative attack cells in different ways. Propionic and benzoic acid are weak acids. Their uncharged forms can cross cell membranes. Inside the cell, where the pH is higher than in the food, they release protons and leave charged acid anions that cross the membrane less readily. The cell has to manage both the acidity and the accumulating anions. Lowering the food’s pH increases the proportion of uncharged acid available to enter cells. [3] Nisin, a well-studied bacteriocin from outside the rumen, binds lipid II, a precursor used to build the bacterial cell wall, and helps form pores in the membrane. It interferes with wall construction and compromises the cell’s ion gradients at the same time. [4]
A fermentate is a mixture. Ours carried acids, peptides, leftover sugars, and other products of the culture, so identifying one active compound didn’t tell us what the ingredient would do. A peptide that stops one bacterium may do nothing to a spoilage yeast or mold, and a food’s pH, fat, and processing can change the result. The activity had to be tested in the food it was meant to protect, against the organisms that actually spoil it.
The propionic acid and the fermentates both made it to market. The benzoic acid was a clean-label project. We wanted to develop an apple-derived cultured ingredient that could do the preservation work of sodium benzoate. We got it through proof of concept. Then we sold the company.
Sources
[1] Chen, Stevenson, and Weimer (2004). Albusin B, a bacteriocin from the ruminal bacterium Ruminococcus albus 7 that inhibits growth of Ruminococcus flavefaciens. Applied and Environmental Microbiology.
https://doi.org/10.1128/AEM.70.5.3167-3170.2004
[2] Whitford et al. (2001). Identification of bacteriocin-like inhibitors from rumen Streptococcus spp. and isolation and characterization of bovicin 255. Applied and Environmental Microbiology.
https://doi.org/10.1128/AEM.67.2.569-574.2001
[3] Seekles et al. (2022). Natural variation and the role of Zn2Cys6 transcription factors SdrA, WarA and WarB in sorbic acid resistance of Aspergillus niger. Microorganisms. Background on weak-acid stress and resistance.
https://doi.org/10.3390/microorganisms10020221
[4] Wiedemann et al. (2001). Specific binding of nisin to the peptidoglycan precursor lipid II combines pore formation and inhibition of cell wall biosynthesis for potent antibiotic activity. Journal of Biological Chemistry.
https://doi.org/10.1074/jbc.M006770200