“Another possible form of commensalism takes place in starters for Gouda cheese, where PrtP- L. lactis strains benefit from the peptides that are released from milk protein through the action of extracellular proteases (PrtP) produced by PrtP+ strains while the PrtP+ strains do not seem directly affected. In milk, PrtP+ strains produce more biomass than their isogenic PrtP- variants lacking plasmids containing the protease gene. In pure cultures of PrtP+ strains grown in milk, PrtP- variants rapidly occur. The outcome of the long-term propagation of PrtP+ and PrtP- strains in a protein-containing medium like milk is that the strain that makes the least use of the resources in the medium, namely, the PrtP- strain, will become dominant. In this case, the immediate gain for the PrtP- strain is traded for the long-term community benefit. This particular example is also known as the ‘prisoner’s dilemma’ in evolutionary game theory. The population dynamics of PrtP+ and PrtP- isolates are highly dependent on the growth conditions that influence the costs and benefits of proteolytic phenotype.”
Sieuwerts, M. De Bok, Hugenholtz, et al., “Unraveling Microbial Interactions in Food Fermentations: from Classical to Genomics Approaches,” Applied and Environmental Biology, Aug 2008.
Lb sanfranciscensis is PrtP- and prefers the uptake of peptides, while most other lactic-acid bacteria recovered from wheat- and/or rye-based fermentations are PrtP+. Notice the relationship?