Bacteriocin-mediated interactions within and between coexisting species

Abstract

Bacteriocins are bacteriocidal toxins released by almost all bacteria. They are thought to have a narrow range of killing, but as bacteriocin-mediated interactions have been rarely studied at biologically relevant scales, whether this narrow range of action falls mostly within or mostly between coexisting species in natural communities is an open question with important ecological and evolutionary implications. In a previous study, we systematically sampled Xenorhabdus bacteria along a hillside and found evidence for genotypic variability and bacteriocin-mediated interactions within Xenorhabdus bovienii and X. koppenhoeferi colonies that were collected only a few meters apart. In contrast, colonies that were isolated from the same soil sample were always genetically similar and showed no inhibitions. Here, we conducted pairwise growth-inhibition assays within and between seven X. bovienii and five X. koppenhoeferi colonies that were isolated from different soil samples; all seven X. bovienii colonies and at least three of the X. koppenhoeferi have been distinguished as distinct genotypes based on coarse-grain genomic markers. We found signatures for both conspecific and heterospecific bacteriocin inhibitions in this natural community of Xenorhabdus bacteria, but intraspecific inhibitions were significantly more common than interspecific inhibitions. These results suggest that bacteriocins have a major role in intraspecific competition in nature, but also suggest that bacterocins are important in mediating interspecific interactions among coexisting species in natural communities.

Keywords: Bacteriocins; Xenorhabdus bacteria; entomopathogenic bacteria; interspecific interactions; intraspecific interactions; recognition mechanisms.

Figure 1

Left: A hillside at Indiana University Research and Teaching Preserve, Moore's Creek, Monroe County, Indiana that was systematically surveyed for Xenorhabdus bacteria. Right: Examples of inhibition assays between bacterial colonies. Top photo shows a standard inhibition; bottom photo shows no inhibition.

Figure 2

Schematic representation of seven Xenorhabdus bovienii (filled triangles) and five X. koppenhoeferi (open triangles) bacteria isolated at five sites across a hill. Reproduced from Hawlena et al. (2010a).

Figure 3

Bacteriocin-mediated competition between pairs of Xenorhabdus bovienii isolates (1–7, white background), pairs of X. koppenhoeferi isolates (a–e, white background), and between pairs of the two species (gray background). Hatched cells indicate inhibition of recipient (columns) growth by cell-free extracts of actor cultures (rows), whereas empty cells indicate interactions in which the recipient growth was not affected by cell-free extracts of actor cultures. “S” indicates self-tests, in which, in accordance with the theory, no inhibitions were detected.