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

Abstract

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 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.

Ecology of the Oral Microbiome: Beyond Bacteria

“…IC). Genomic and transcriptomic analysis in combination with the phenotypic data supports the hypothesis that TM7x employs a parasitic rather than mutualistic/commensal epibiotic relationship with XH001 [89]. This epiparasitic relationship is likely a life style adopted by many CPR members, which share the characteristics of small cell size and a reduced genome.…”

“…TM7x was recently isolated from the human oral cavity as an obligate epibiont that lives on the surface of its bacterial host, XH001, representing thus far the only in vitro cultivated and stably maintained member of the CPR group [32, 89]. Isolation of pure TM7x is achieved by separating TM7x cells from host XH001 via the repeated passage of the co-culture through a 28-gauge needle, followed by filtering of the mixture through a 0.45-μm filter to collect TM7x.…”

“…Isolation of pure TM7x is achieved by separating TM7x cells from host XH001 via the repeated passage of the co-culture through a 28-gauge needle, followed by filtering of the mixture through a 0.45-μm filter to collect TM7x. Isolated TM7x fails to grow, but maintains its ability to re-establish parasitic association with naïve XH001 [32, 89]. During symbiotic growth under nutrient-replete conditions, TM7x mainly grows attached to the host surface as cocci with bud-like protrusions, while XH001 displays an elongated cell structure compared to the short rod-shaped naïve XH001.…”

“…However, under starvation or other stress conditions, TM7x induces additional morphological changes in the host, such as clubbed ends and swollen cell body, and eventually results in host cell lysis. TM7x displays multiple cell morphotypes, ranging from cocci to short rods to filaments [89] (Fig. IA).…”

Cultivation strategies for growth of uncultivated bacteria

“…They did so by plating saliva samples on a specialised oral culture medium (SHI medium) and targeting the enrichment of TM7 taxa by streptomycin selection. This and their subsequent ground-breaking work [68] have revealed that strain TM7x leads an epibiotic parasitic lifestyle associated with another bacterium, its basibiont Actinomyces odontolyticus XH001. Furthermore they confirmed, by transcriptomic and metabolomic analysis, a signalling interaction between the two.…”