I noted in class that yogurt has a long shelf-life, usually many weeks longer than indicated on the labels “use-by” dates. After all, yogurt is made from milk that has received a high-temperature heat process. Most yogurts are made in a hygienic environment with minimal exposure. Importantly, the final product has a pH less than 4.5, sometimes even as low as pH 4.0.
Despite these conditions, spoilage can still occur, usually by acid-tolerant fungi, including both yeasts and molds. These microbes are either environmental contaminants or are introduced post-fermentation via fruits or other ingredients. Adding sorbic acid or other antimicrobials is generally not a viable option, because these products are usually marketed on their “natural” ingredients.
Although many lactic acid bacteria produce bacteriocins, these agents do not inhibit fungi. Nonetheless, there are other ways lactic acid bacteria can inhibit spoilage microbes. In this study, a team of scientists led by Chr. Hansens R & D group described a novel application of bioprotective cultures based on the principle of competitive exclusion. The latter occurs when organisms in the same environment or niche find themselves competing for limited availability of a carbon source or a particular nutrient. Whichever gets that nutrient first is the “winner”, effectively excluding the competitor.
In this case, the researchers discovered that two strains of Lactobacillus, L. paracasei and L. rhamnosus, had the unique ability to rapidly transport manganese. Indeed, so effective was their sequestration of manganese that milk quickly became depleted in this essential element. Importantly, fungi need manganese as a co-factor in enzymes essential for their growth.
Experiments revealed that adding these strains to milk inhibited yeast and mold, supporting the hypothesis. The researchers were also able to identify genes for manganese transport in these bacteria and measure expression via transcriptional profiling (i.e., mRNA for the relevant genes). Of the two transporters, one was an ABC system active at neutral pH, and the other was a proton-driven symporter active at acidic pH. The latter, encoded by the mntH gene, was considered more responsible for scavenging the manganese.
Because they knew which genes were involved, they were able to make deletion mutants, where the mntH gene was inactivated. As predicted, inhibition activity was eliminated. Likewise, if they used the manganese accumulating Lactobacillus strain but added back extra manganese, growth of the fungi was restored.
So successful was this research, Chr. Hansen now markets this as a bioprotective ingredient call “FRESHQ”, promoting it as a natural shelf-life extender.
Once again, another great example of physiology and genetics having a very applied outcome.