Cell distribution and habitat fragmentation affecting the spread of plasmids in soil bacterial populations

Abstract

Aims and Objectives. Transfer of conjugative plasmids among soil bacteria is an important evolutionary driver for fostering adaptation to environmental stresses such as antibiotic or xenobiotic contamination. However, plasmids impose a metabolic burden on host cells and the reason for their persistence over evolutionary times remains unclear. Here, we hypothesize that soil environments favor plasmid maintenance due to habitat fragmentation and nutrient spatial heterogeneity that promote plasmid transfer rates and reduce competition in microniches. We aim to test that hypothesis and disentangle the contributions of transmission and selection. Materials and Methods. The soil bacterium Pseudomonas putida was used as donor and recipient of the conjugative plasmid pIPO2tet (conferring resistance to tetracycline). A tagging system with fluorescent proteins allowed us to visually discriminate recipients, donors and transconjugants using microscope image analysis and plating on selective media. Bacteria were grown in controlled systems of varying complexity, from agar surfaces to micromodels, and in presence or absence of tetracycline as a selective agent. Results. Experiments on homogeneous agar surfaces showed that transfer rate and the final size of the plasmid-carrying population increased with cell density, while competition (as measured by selection coefficient) tended to decrease. The presence of antibiotics at sub-inhibitory concentrations also affected plasmid transfer rate and selection. To address the role of spatial isolation of microhabitats, we have designed micromodels that allow us to observe local variations in the spread of bacterial plasmids, with results still pending. Conclusions. Local (microscale) conditions such as cell density and spatial confinement can enhance or suppress plasmid transfer among bacterial populations, as well as affect the selective effects of carrying a plasmid. The study offers new insights linking soil microhabitats to ecological and evolutionary adaptations of soil bacteria.