Abstract
The ultra-small, obligate parasitic epibiont, TM7x, the first and only current member of the long-elusive Saccharibacteria (formerly the TM7 phylum) phylum to be cultivated, was isolated in co-culture with its bacterial host, Actinomyces odontolyticus subspecies actinosynbacter, XH001. Initial phenotypic characterization of the TM7x-associated XH001 co-culture revealed enhanced biofilm formation in the presence of TM7x compared to XH001 as monoculture. Genomic analysis and previously published transcriptomic profiling of XH001 also revealed the presence of a putative AI-2 quorum sensing (QS) operon, which was highly upregulated upon association of TM7x with XH001. This analysis revealed that the most highly induced gene in XH001 was an lsrB ortholog, which encodes a putative periplasmic binding protein for the auto inducer (AI)-2 QS signaling molecule. Further genomic analyses suggested the lsrB operon in XH001 is a putative hybrid AI-2/ribose transport operon as well as the existence of a luxS ortholog, which encodes the AI-2 synthase. In this study, the potential role of AI-2 QS in the epibiotic-parasitic relationship between XH001 and TM7x in the context of biofilm formation was investigated. A genetic system for XH001 was developed to generate lsrB and luxS gene deletion mutants in XH001. Phenotypic characterization demonstrated that deletion mutations in either lsrB or luxS did not affect XH001’s growth dynamic, mono-species biofilm formation capability, nor its ability to associate with TM7x. TM7x association with XH001 induced lsrB gene expression in a luxS-dependent manner. Intriguingly, unlike wild type XH001, which displayed significantly increased biofilm formation upon establishing the epibiotic-parasitic relationship with TM7x, XH001ΔlsrB, and XH001ΔluxS mutants failed to achieve enhanced biofilm formation when associated with TM7x. In conclusion, we demonstrated a significant role for AI-2 QS in modulating dual-species biofilm formation when XH001 and TM7x establish their epibiotic-parasitic relationship.
Highlights
One of the greatest scientific revelations in recent history was the discovery of the enormous diversity and large abundance of microbes associated with the human body
Contrary to the PHYRE analysis, protein sequence alignment revealed that XH001 LsrB shared low sequence identity with known representative LsrB species in S. typhimurium 14028 (23%), E. coli K-12 (26%), and A. actinomycetecomitans HK1651 (25%)
Among the six proposed conserved amino acid residues (K35, D116, D166, Q167, P220, and A222) for identifying LsrB-like orthologs (Pereira et al, 2009), which are predicted to form hydrogen bonds with [(2R,4S)2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran (RTHMF)], the auto inducer (AI)-2 signaling molecule bound by the LsrB periplasmic binding protein in S. typhimurium (Miller et al, 2004), only Lysine at position 35 was identified in XH001 via protein sequence alignment (Figure 1A)
Summary
One of the greatest scientific revelations in recent history was the discovery of the enormous diversity and large abundance of microbes associated with the human body. Overlapping characteristics between the phyla comprising of the CPR organisms include reduced genomes, lack of the biosynthetic capacity for most amino acids, as well as ultra-small cell morphological sizes inferred from representatives of the WWE3, OP11, and OD1 phyla, which are capable of passing through 0.2-μm-sized filters (Luef et al, 2015). This suggests that a proportion of other CPR representatives may exist in epibiotic-parasitic relationships comparable to TM7x with XH001. It is imperative to characterize this interaction on a molecular level to uncover important knowledge for studying other “yet-to-be cultivated” CPR bacterial species
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