Phenotypic and Physiological Characterization of the Epibiotic Interaction Between TM7x and Its Basibiont Actinomyces


Despite many examples of obligate epibiotic symbiosis (one organism living on the surface of another) in nature, such an interaction has rarely been observed between two bacteria. Here, we further characterize a newly reported interaction between a human oral obligate parasitic bacterium TM7x (cultivated member of Candidatus Saccharimonas formerly Candidate Phylum TM7), and its basibiont Actinomyces odontolyticus species (XH001), providing a model system to study epiparasitic symbiosis in the domain Bacteria. Detailed microscopic studies indicate that both partners display extensive morphological changes during symbiotic growth. XH001 cells manifested as short rods in monoculture, but displayed elongated and hyphal morphology when physically associated with TM7x. Interestingly, these dramatic morphological changes in XH001 were also induced in oxygen-depleted conditions, even in the absence of TM7x. Targeted quantitative real-time PCR (qRT-PCR) analyses revealed that both the physical association with TM7x as well as oxygen depletion triggered up-regulation of key stress response genes in XH001, and in combination, these conditions act in an additive manner. TM7x and XH001 co-exist with relatively uniform cell morphologies under nutrient-replete conditions. However, upon nutrient depletion, TM7x-associated XH001 displayed a variety of cell morphologies, including swollen cell body, clubbed-ends, and even cell lysis, and a large portion of TM7x cells transformed from ultrasmall cocci into elongated cells. Our study demonstrates a highly dynamic interaction between epibiont TM7x and its basibiont XH001 in response to physical association or environmental cues such as oxygen level and nutritional status, as reflected by their morphological and physiological changes during symbiotic growth.

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We thank the members of the Shi and Lux laboratories for their feedback and invaluable discussion. We also thank Melissa Agnello for providing extensive editing of the manuscript. We thank the Chemistry and Biochemistry instrumentation facility at UCLA for providing access to the confocal microscope. This work was supported in part by grants from the National Institutes of Health (1R01DE023810-01) and Oral Health-Research Postdoctoral Training Program (B.B., UCLA School of Dentistry T90 award).

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Correspondence to Xuesong He or Wenyuan Shi.

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Fig. S1

TM7x induces morphological changes in XH001. a XH001/TM7x co-culture grown under microaerophilic condition for 24 h showing clear micro-aggregation. Scale bar is 10 μm. b Monoculture of XH001. c, d Establishment of physical association between XH001 and TM7x via attachment assay (see supplementary methods). The re-attached XH001 cells were passaged two times (c) and four times (d) respectively in fresh medium. Scale bars are 5 μm (PDF 4683 kb)

Fig. S2

Morphology of XH001 under different oxygen conditions. Phase contrast image of XH001 alone (a) under high oxygen condition (19.7 % O2, 5 % CO2). Phase contrast images of XH001 alone (b) and with TM7x (c) under normal atmospheric condition (20.9 % O2, 0.04 % CO2) after 24 h. df XH001 alone cells grown in microaerophilic condition (d) were shifted to anaerobic condition (e) and then back to the microaerophilic condition (f) before taking the phase contrast images. All scale bars indicate 5 μm (PDF 8569 kb)

Fig. S3

FISH staining of XH001 alone and with TM7x. FISH probes specific to TM7x (white) and XH001 (red) were used to stain the fixed samples. Green represents syto9 staining of all bacteria. a XH001 monoculture grown under a microaerophilic condition for 24 h shows short rod morphology with XH001-specific probe (red) and universal DNA stain syto9 (green), but no staining with TM7x-specific probe, confirming our probe specificity. b, c XH001 alone (b) and with TM7x (c) grown under anaerobic condition for 24 h. Similar to a, we do not see any staining of TM7x probe in the XH001 alone cells, whereas in the co-culture, we saw elongated TM7x. Under anaerobic condition, XH001-specific probe stained the cells non-uniformly, suggesting that these cells were stressed and probably lost their cell content. All scale bars indicate 10 μm. (PDF 26435 kb)

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Bor, B., Poweleit, N., Bois, J.S. et al. Phenotypic and Physiological Characterization of the Epibiotic Interaction Between TM7x and Its Basibiont Actinomyces . Microb Ecol 71, 243–255 (2016).

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  • Obligate
  • Epibiont
  • Symbiosis
  • Bacterial interaction
  • TM7
  • Actinomyces