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Single-cell versus population-level reproductive success of bacterial immigrants to pre-colonized leaf surfaces

Authors

  • Mitja N. P. Remus-Emsermann,

    Corresponding authorCurrent affiliation:
    1. Institute of Microbiology, Eidgenössische Technische Hochschule Zürich, Zürich, Switzerland
    • Department of Microbial Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Wageningen, The Netherlands
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  • George A. Kowalchuk,

    1. Department of Microbial Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Wageningen, The Netherlands
    2. Department of Ecological Science, VU University Amsterdam, Amsterdam, The Netherlands
    3. Institute of Environmental Biology, Faculty of Science, Utrecht University, Utrecht, The Netherlands
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  • Johan H. J. Leveau

    1. Department of Microbial Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Wageningen, The Netherlands
    2. Department of Plant Pathology, University of California, Davis, CA, USA
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For correspondence. E-mail remus-emsermann@micro.biol.ethz.ch; Tel. (+41) 44 632 38 30; Fax (+41) 44 633 94 43.

Summary

We assessed how preemptive inoculation of plant leaves with bacteria affected the establishment of secondary colonizers. We quantified the latter in two ways: (i) at the population level, i.e. as counts of colony-forming units and (ii) at the level of single cells by tracking the reproductive success of individual bacteria. Both analyses showed that the ability of secondary immigrants to establish on the leaf was negatively correlated with the level of pre-population by primary colonizers. This effect was best described by an inverse dose–response curve with an apparent half-point inhibition efficacy of approximately 106 cells of primary colonizers per gram leaf. This efficacy was the same whether calculated from population- or average single-cell data. However, single-cell data revealed that even under conditions of heavy pre-population with primary colonizers, a small fraction of secondary immigrants still produced offspring, although the corresponding population measurement showed no increase in total population size. This observation has direct relevance for biocontrol strategies that are based on the principle of preemptive exclusion of foliar bacterial pathogens: even at seemingly saturating levels of primary inoculum, some secondary colonizers may still be able to reproduce and possibly reach a quorum to trigger behaviours that enhance survival or virulence.

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