Dynamic interaction between human oral epiparasitic TM7x and its bacterial host—A.odontolyticus XH001

Objectives: In our recent study (He et al., PNAS 2015), we discovered a unique and intimate association between two oral commensal bacterial species, a TM7 phylotype (TM7x) and XH001 (Actinomyces odontolyticus strain) which were co-isolated from the human oral cavity. TM7x was characterized as an obligate epibiotic parasite that lives on the surface of its basibiont, XH001, a novel relationship that has never been reported in bacterial kingdom before. In this study, we intended to further analyze the interaction between TM7x and XH001 by investigating their morphological and physiological changes during their symbiotic growth.
Methods: TM7x and its basibiont XH001 were cultivated under various gas and nutrient conditions to determine the optimal and stressed growth conditions. The morphological changes in both TM7x and XH001 were analyzed by FISH fluorescence microscope, phase contrast microscope and SEM; while stress response in XH001 induced by environmental changes and TM7x association was determined by targeted qRT-PCR.
Results: 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 could also be induced by depletion of oxygen even in the absence of TM7x. Interestingly, both the physical association with TM7x and oxygen depletion triggered up-regulation of key stress response genes in XH001. Furthermore, TM7x and XH001 co-exist well 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; while a large portion of TM7x cells transformed from ultra-small cocci to larger filamentous cells.
Conclusions: Our study demonstrates a highly dynamic interaction between epibiont TM7x and its basibiont XH001 in response to physical association as well as environmental cues such as oxygen level or nutritional status, as reflected by their morphological and physiological changes during symbiotic growth.