Migration alters oscillatory dynamics and promotes survival in connected bacterial populations

Migration influences population dynamics on networks, thereby playing a vital role in scenarios ranging from species extinction to epidemic propagation. While low migration rates prevent local populations from becoming extinct, high migration rates enhance the risk of global extinction by synchronizing the dynamics of connected populations. Here, we investigate this trade-off using two mutualistic strains of E. coli that exhibit population oscillations when co-cultured. In experiments, as well as in simulations using a mechanistic model, we observe that high migration rates lead to synchronization whereas intermediate migration rates perturb the oscillations and change their period. Further, our simulations predict, and experiments show, that connected populations subjected to more challenging antibiotic concentrations have the highest probability of survival at intermediate migration rates. Finally, we identify altered population dynamics, rather than recolonization, as the primary cause of extended survival.National Institutes of Health (U.S.) (Grant R01 GM102311-01)National Science Foundation (Award PHY-1055154)National Institutes of Health (U.S.) (Award GM085279-02)Alfred P. Sloan Foundation (Award BR2011-066)National Institutes of Health (U.S.) (Award DP2

Oscillatory dynamics in a bacterial cross-protection mutualism

Cooperation between microbes can enable microbial communities to survive in harsh environments. Enzymatic deactivation of antibiotics, a common mechanism of antibiotic resistance in bacteria, is a cooperative behavior that can allow resistant cells to protect sensitive cells from antibiotics. Understanding how bacterial populations survive antibiotic exposure is important both clinically and ecologically, yet the implications of cooperative antibiotic deactivation on the population and evolutionary dynamics remain poorly understood, particularly in the presence of more than one antibiotic. Here, we show that two Escherichia coli strains can form an effective crossprotection mutualism, protecting each other in the presence of two antibiotics (ampicillin and chloramphenicol) so that the coculture can survive in antibiotic concentrations that inhibit growth of either strain alone. Moreover, we find that daily dilutions of the coculture lead to large oscillations in the relative abundance of the two strains, with the ratio of abundances varying by nearly four orders of magnitude over the course of the 3-day period of the oscillation. At modest antibiotic concentrations, the mutualistic behavior enables long-term survival of the oscillating populations; however, at higher antibiotic concentrations, the oscillations destabilize the population, eventually leading to collapse. The two strains form a successful cross-protection mutualism without a period of coevolution, suggesting that similar mutualisms may arise during antibiotic treatment and in natural environments such as the soil.National Institutes of Health (U.S.) (Grant R01 GM102311-01)National Science Foundation (U.S.) (CAREER Award PHY- 1055154)Pew Charitable Trusts (Pew Scholars in the Biomedical Sciences Program Grant 2010-000224-007)National Institutes of Health (U.S.) (R00 Pathways to Independence Award GM085279-02)Alfred P. Sloan Foundation (Fellowship BR2011-066)Paul G. Allen Family Foundation (Allen Distinguished Investigator Award)National Institutes of Health (U.S.) (New Innovator Award DP2)National Science Foundation (U.S.). Graduate Research Fellowship Program (Grant 064596