Project description:Prochlorococcus is found throughout the euphotic zone in the oligotrophic open ocean. Deep mixing and sinking in aggregates or while attached to particles can, however, transport cells below this sunlit zone, depriving them of light for extended periods of time and influencing their circulation via ocean currents. Viability of these cells over extended periods of darkness could shape the ecology and evolution of the Prochlorococcus collective. We have shown that when co-cultured with a heterotrophic microbe and subjected to repeated periods of extended darkness, Prochlorococcus cells develop a heritable dark-tolerant phenotype – through an apparent epigenetic mechanism – such that they survive longer periods of darkness. Here we examine this adaptation at the level of physiology and metabolism in co-cultures of dark-tolerant and parent strains of Prochlorococcus, each grown with the heterotroph Alteromonas under diel light:dark conditions. The relative abundance of Alteromonas is higher in dark-tolerant than parental co-cultures, while dark tolerant Prochlorococcus cells are also larger, contain less chlorophyll, and are less synchronized to the light:dark cycle. Meta-transcriptome analysis of the cultures further suggests that dark-tolerant co-cultures undergo a coupled shift in which Prochlorococcus uses more organic carbon and less photosynthesis, and Alteromonas uses more organic acids and fewer sugars. Collectively, the data suggest that dark adaptation involves a loosening of the coupling between Prochlorococcus metabolism and the light:dark cycle and a strengthening of the coupling between the carbon metabolism of Prochlorococcus and Alteromonas.
Project description:Virulent bacteriophages (or phages) are viruses that specifically infect and lyse a bacterial host. When multiple phages co-infect a bacterial host, the extent of lysis, dynamics of bacteria-phage and phage-phage interactions are expected to vary. The objective of this study is to identify the factors influencing the interaction of two virulent phages with different Pseudomonas aeruginosa growth states (planktonic, an infected epithelial cell line, and biofilm) by measuring the bacterial time-kill and individual phage replication kinetics. A single administration of phages effectively reduced P. aeruginosa viability in planktonic conditions and infected human lung cell cultures, but phage-resistant variants subsequently emerged. In static biofilms, the phage combination displayed initial inhibition of biofilm dispersal, but sustained control was achieved only by combining phages and meropenem antibiotic. In contrast, adherent biofilms showed tolerance to phage and/or meropenem, suggesting a spatiotemporal variation in the phage-bacterial interaction. The kinetics of adsorption of each phage to P. aeruginosa during single- or co-administration were comparable. However, the phage with the shorter lysis time depleted bacterial resources early and selected a specific nucleotide polymorphism that conferred a competitive disadvantage and cross-resistance to the second phage. The extent and strength of this phage-phage competition and genetic loci conferring phage resistance, are, however, P. aeruginosa genotype dependent. Nevertheless, adding phages sequentially resulted in their unimpeded replication with no significant increase in bacterial host lysis. These results highlight the interrelatedness of phage-phage competition, phage resistance and specific bacterial growth state (planktonic/biofilm) in shaping the interplay among P. aeruginosa and virulent phages.