In this article we show that microbial growth in isolation leads to the selection of strains with increased metabolic efficiency but slower growth rates. To our knowledge we show here for the first time the selection of cells with increased yield at the cost of growth rate. Such yield-selection is not possible in a suspension culture where faster growing cells will out-compete slower, but more efficient ones. We solved this problem through culturing single cells in emulsion droplets which leads to an increase of mutants that have a higher number of offspring. Serial propagation in such a system allowed eventually the identification of strains with increased efficiency. The results are relevant to numerous fundamental questions in evolutionary biology but also to biotechnological applications like the increase of biomass yield.
L. lactis is mainly isolated from plants and the dairy environment. Literature suggests that dairy isolates have evolved from plant isolates. In this paper we mimicked the transition from the plant to the dairy environment by propagating a L. lactis plant isolate in milk for 1000 generations. Evolved strains showed faster growth rates and increased fitness in milk. The transcriptome of evolved strains converged towards that of a dairy isolate. We showed that changes in nitrogen metabolism and the down-regulation of genes which are dispensable in the dairy environment are the main adaptations.
Cooperative behavior is widely spread in microbial populations. An example is the expression of an extracellular protease by the lactic acid bacterium Lactococcus lactis, which degrades milk proteins into free utilizable peptides that are essential to allow growth to high cell densities in milk. Cheating, protease-negative strains can invade the population and drive the protease-positive strain to extinction. By using multiple experimental approaches, as well as modeling population dynamics, we demonstrate that the persistence of the proteolytic trait is determined by the fraction of the generated peptides that can be captured by the cell before diffusing away from it. The mechanism described is likely to be relevant for the evolutionary stability of many extracellular substrate degrading enzymes.
Lactococcus lactis is one of main bacterial species found in mixed dairy starter cultures for the production of semi-hard cheese. Despite the appreciation that mixed cultures are essential for the eventual properties of the manufactured cheese the vast majority of studies on L. lactis were carried out in laboratory media with a pure culture. In this study we applied an advanced recombinant in vivo expression technology (R-IVET) assay in combination with a high-throughput cheese-manufacturing protocol for the identification and subsequent validation of promoter sequences specifically induced during the manufacturing and ripening of cheese. The system allowed gene expression measurements in an undisturbed product environment without the use of antibiotics and in combination with a mixed strain starter culture. The utilization of bacterial luciferase as reporter enabled the real-time monitoring of gene expression in cheese for up to 200 h after the cheesemanufacturing process was initiated. The results revealed a number of genes that were clearly induced in cheese such as cysD, bcaP, dppA, hisC, gltA, rpsE, purL, amtB as well as a number of hypothetical genes, pseudogenes and notably genetic elements located on the non-coding strand of annotated open reading frames. Furthermore genes that are likely to be involved in interactions with bacteria used in the mixed strain starter culture were identified.