Domain structure and cross-linking in a giant adhesin from the Mobiluncus mulieris bacterium.

Cell-surface proteins known as adhesins enable bacteria to colonize particular environments, and in Gram-positive bacteria often contain autocatalytically formed covalent intramolecular cross-links. While investigating the prevalence of such cross-links, a remarkable example was discovered in Mobiluncus mulieris, a pathogen associated with bacterial vaginosis. This organism encodes a putative adhesin of 7651 residues. Crystallography and mass spectrometry of two selected domains, and AlphaFold structure prediction of the remainder of the protein, were used to show that this adhesin belongs to the family of thioester, isopeptide and ester-bond-containing proteins (TIE proteins). It has an N-terminal domain homologous to thioester adhesion domains, followed by 51 immunoglobulin (Ig)-like domains containing ester- or isopeptide-bond cross-links. The energetic cost to the M. mulieris bacterium in retaining such a large adhesin as a single gene or protein construct suggests a critical role in pathogenicity and/or persistence.

Structure determination of CAMP factor of Mobiluncus curtisii and insights into structural dynamics.

Bacterial vaginosis (BV) is a common type of vaginal inflammation caused by a proliferation of pathogenic bacteria, among which Mobiluncus curtisii. In our previous studies on M. curtisii genome, we identified the presence of a genomic fragment encoding a 25 kDa pore-forming toxin, the CAMP factor, which is known to be involved in the synergistic lysis of erythrocytes namely CAMP reaction. However, whether this hypothetical gene product has hemolytic activity is unknown. Moreover, its relative structure and function are not yet solved. Here we found that the M. curtisii CAMP factor is a monomer at pH 4.4 and oligomer at pH > 4.6. Hemolysis assays showed that M. curtisii CAMP factor could lyse sheep red blood cells efficiently in pH 5.4-7.4. Negative staining electron microscope analysis of the CAMP factor revealed ring-like structures at pH above 4.6. Additionally, the crystal structure of M. curtisii CAMP factor, determineded at 1.85 Å resolution, reveals a 5 + 3 helix motif. Further functional analysis suggested that the structural rearrangement of the N-terminal domain might be required for protein function. In conclusion, this structure-function relationship study of CAMP factor provides a new perspective of the M. curtisii role in BV development.

Transfer of members of the genus Falcivibrio to the genus Mobiluncus, and emended description of the genus Mobiluncus.

It has long been thought that the genera Mobiluncus and Falcivibrio contain the same organisms. Using a polyphasic approach, it was found that Mobiluncus curtisii and Mobiluncus mulieris were the same as Falcivibrio vaginalis and Falcivibrio grandis, respectively. As the genus name Mobiluncus takes precedence, it is proposed that F. vaginalis and F. grandis be transferred to the genus Mobiluncus. In agreement with previous studies, results from phenotypic tests did not support the separation of M. curtisii strains into its two subspecies, M. curtisii subsp. curtisii and M. curtisii subsp. holmesii. Phenotypic complexity within M. curtisii dictates that the species should be treated as a complex until more in-depth analyses of the species have been performed. Phylogenetic analyses, based on 16S rRNA gene sequences, demonstrated that the genus Mobiluncus was associated with Varibaculum cambriense and the two subspecies of Actinomyces neuii, and that A. neuii is only distantly related to Actinomyces sensu stricto.

Inhibition of chemotaxis by organic acids from anaerobes may prevent a purulent response in bacterial vaginosis.

It has been postulated that certain organic acids produced by the anaerobes associated with bacterial vaginosis (BV) could prevent a purulent response in this infection. Varying concentrations of pure succinic, acetic and lactic acids were incubated in vitro with a monocytic cell line (MonoMac 6). High inhibition of chemotaxis was produced by succinic acid; lower inhibition and no inhibition was shown by acetic acid and lactic acid respectively. Succinic and acetic acids were detected in high concentrations in the vaginal fluid of women with BV and in culture supernates of Prevotella and Mobiluncus spp.; these acids impaired chemotaxis of MonoMac 6 cells in vitro. The vaginal fluids of normal women and the culture supernates of Lactobacillus spp. had no effect on chemotaxis.

Isolation of Mobiluncus species from the human vagina.

We report the results of a study concerning the characteristics of 52 strains of Mobiluncus spp. isolated from 982 vaginal secretions from patients with suspected bacterial vaginosis. 158 of these women presented the features of this bacterial infection. Of the strains isolated, 39 belonged to the species Mobiluncus curtisii, (25 of these which to M. curtisii subsp. curtisii and 14, to M. curtisii subsp. holmesii), and 13, to Mobiluncus mulieris. The vaginal isolates of Mobiluncus spp. were identified by comparing their biochemical profiles with those of the type strains M. curtisii subsp. holmesii (ATCC 35,242), M. curtisii subsp. curtisii (ATCC 35,241) and M. mulieris (ATCC 35,243). All strains of M. mulieris proved to be sensitive to the antimicrobial agents assayed, while strains of M. curtisii were seen to be resistant only to metronidazole.