Phenospace and Mutational Stability of Shared Antibiotic Resistance

Experiments have demonstrated that in an antibiotic-polluted environment two bacteria strains, one being sensitive and one being resistant to the antibiotic, can coexist. The resistant bacteria produce enzymes that degrade the antibiotic and thereby protect the sensitive bacteria. Since the sensitive strain does not invest energy in producing these enzymes, it can reproduce faster than the resistant strain. This requires, however, coexistence with enzyme-producing bacteria. Hence their coexistence is an evolutionarily stable state, where even a high concentration of antibiotic cannot remove the sensitive strain. Still, it remains unknown why some strains can coexist while others result in a monoculture. We study a model that predicts which phenotypes are able to coexist. It shows that such a coexistence is not mutationally stable and it allows to determine the required phenotype for a third (not necessarily resistant) strain to invade and replace such two coexisting strains.