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Transcriptional gene fusions with the Escherichia coli ␤-glucuronidase gene (gusA) were used to study the medium-and growth-dependent expression of the divergently transcribed genes involved in proteinase production (prtP and prtM) of Lactococcus lactis SK11. The results show that both the prtP and prtM genes are controlled at the transcriptional level by the peptide content of the medium and, to a lesser extent, by the growth rate. A more than 10-fold regulation in ␤-glucuronidase activity was observed for both prtP and prtM promoters in batch and continuous cultures. The level of expression of the prtP and prtM promoters was high in whey permeate medium with relatively low concentrations of peptides, whereas at increased concentrations the expression of the promoters was repressed. The lowest level of expression was observed in peptide-and amino acid-rich laboratory media, such as glucose-M17 and MRS. The addition of specific dipeptides, such as leucylproline and prolylleucine, to the growth medium negatively affected the expression of the prtP-gusA fusions. The repression by dipeptides was not observed in mutants defective in the uptake of di-tripeptides, indicating that the internal concentration of dipeptides or derivatives is important in the regulation of proteinase production.

BackgroundThe lactic acid bacterium Lactobacillus rhamnosus GG is the most studied probiotic bacterium with proven health benefits upon oral intake, including the alleviation of diarrhea. The mission of the Yoba for Life foundation is to provide impoverished communities in Africa increased access to Lactobacillus rhamnosus GG under the name Lactobacillus rhamnosus yoba 2012, world’s first generic probiotic strain. We have been able to overcome the strain’s limitations to grow in food matrices like milk, by formulating a dried starter consortium with Streptococcus thermophilus that enables the propagation of both strains in milk and other food matrices. The affordable seed culture is used by people in resource-poor communities.ResultsWe used S. thermophilus C106 as an adjuvant culture for the propagation of L. rhamnosus yoba 2012 in a variety of fermented foods up to concentrations, because of its endogenous proteolytic activity, ability to degrade lactose and other synergistic effects. Subsequently, L. rhamnosus could reach final titers of 1E+09 CFU ml−1, which is sufficient to comply with the recommended daily dose for probiotics. The specific metabolic interactions between the two strains were derived from the full genome sequences of L. rhamnosus GG and S. thermophilus C106. The piliation of the L. rhamnosus yoba 2012, required for epithelial adhesion and inflammatory signaling in the human host, was stable during growth in milk for two rounds of fermentation. Sachets prepared with the two strains, yoba 2012 and C106, retained viability for at least 2 years.ConclusionsA stable dried seed culture has been developed which facilitates local and low-cost production of a wide range of fermented foods that subsequently act as delivery vehicles for beneficial bacteria to communities in east Africa.Electronic supplementary materialThe online version of this article (doi:10.1186/s12934-015-0370-x) contains supplementary material, which is available to authorized users.

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A fluorescence method to monitor lysis of cheese starter bacteria using dual staining with the LIVE/DEAD BacLight bacterial viability kit is described. This kit combines membrane-permeant green fluorescent nucleic acid dye SYTO 9 and membrane-impermeant red fluorescent nucleic acid dye propidium iodide (