Bacteria take up and metabolize sugar as a carbohydrate source for survival. Most bacteria can utilize many sugars, including glucose, sucrose, and galactose, as well as amino sugars, such as glucosamine and N-acetylglucosamine. After entering the cytoplasm, the sugars are mainly allocated to the glycolysis pathway (energy production) and to various bacterial component biosynthesis pathways, including the cell wall, nucleic acids and amino acids. Sugars are also utilized to produce several virulence factors, such as capsule and lipoteichoic acid. Glutamine-fructose-6-phosphate aminotransferase (GlmS) and glucosamine-6-phosphate deaminase (NagB) have crucial roles in sugar distribution to the glycolysis pathway and to cell wall biosynthesis. In Streptococcus mutans, a cariogenic pathogen, the expression levels of glmS and nagB are coordinately regulated in response to the presence or absence of amino sugars. In addition, the disruption of this regulation affects the virulence of S. mutans. The expression of nagB and glmS is regulated by NagR in S. mutans, but the precise mechanism underlying glmS regulation is not clear. In Staphylococcus aureus and Bacillus subtilis, the mRNA of glmS has ribozyme activity and undergoes self-degradation at the mRNA level. However, there is no ribozyme activity region on glmS mRNA in S. mutans. In this review article, we summarize the sugar distribution, particularly the coordinated regulation of GlmS and NagB expression, and its relationship with the virulence of S. mutans.

Periodontitis is a chronic inflammatory disease of the periodontal tissues primarily caused by dysbiotic bacterial communities. Accumulating evidence suggests that periodontal pathogens not only drive the initiation and progression of periodontitis but also significantly contribute to systemic disorders, including diabetes mellitus, cardiovascular disease, cancer, and adverse pregnancy outcomes, such as preterm birth. The key periodontal pathogens implicated in disease pathogenesis include Porphyromonas gingivalis, Prevotella intermedia, Treponema denticola, Tannerella forsythia, Aggregatibacter actinomycetemcomitans, and Fusobacterium nucleatum. Among the diverse factors governing bacterial colonization and biofilm formation, interspecies interactions, particularly coaggregation, play a critical role in dental plaque maturation and the establishment of pathogenic microbial communities. Coaggregation facilitates the spatial organization of bacteria within biofilms, enhances bacterial survival, and modulates virulence factor expression. This review summarizes current knowledge regarding bacterial interactions involving major periodontal pathogens, with particular emphasis on coaggregation mechanisms, and discusses the implications of this coaggregation for periodontitis pathogenesis and associated systemic diseases.

Periodontal disease is a polymicrobial infection in which Fusobacterium nucleatum and Aggregatibacter actinomycetemcomitans have been identified as key pathogens associated with disease progression. F. nucleatum is capable of coaggregating with several oral bacterial species, including A. actinomycetemcomitans , and these coaggregations play crucial roles in dental plaque formation. In this study, we investigated the coaggregation properties of both bacteria and identified the molecules that mediate their interaction. Among A. actinomycetemcomitans serotypes a, b, c, d, and g, F. nucleatum strains coaggregated specifically with serotypes b and d. The coaggregation between F. nucleatum and A. actinomycetemcomitans serotype b was significantly suppressed by the addition of lipopolysaccharide (LPS) extracted from A. actinomycetemcomitans serotype b. Among the sugars comprising the O-polysaccharide (O-PS) of serotype b LPS, the addition of N -acetyl- _D -galactosamine markedly inhibited their coaggregation. Similarly, the coaggregation between F. nucleatum and A. actinomycetemcomitans serotype d was strongly inhibited by the addition of either the LPS extracted from A. actinomycetemcomitans serotype d or the _L -rhamnose present in serotype d O-PS. Furthermore, analysis of outer membrane autotransporter proteins as potential coaggregation factors of F. nucleatum revealed that mutants lacking functional fap2 or cmpA exhibited significantly reduced coaggregation with A. actinomycetemcomitans serotype b or d, respectively. Additionally, we found that these coaggregations significantly promoted biofilm formation during coculture. These findings suggest that the coaggregation between F. nucleatum and A. actinomycetemcomitans serotypes b and d is mediated through interactions between autotransporter proteins and LPSs, thereby contributing to the development of mature biofilms. IMPORTANCE: Fusobacterium nucleatum is a periodontal pathogen that acts as a bridging organism by coaggregating with various oral microbes, including other periodontal pathogens. Recently, this bacterium has been reported to be associated with systemic diseases, including cardiovascular diseases, adverse pregnancy outcomes, and cancer. F. nucleatum possesses numerous adhesins, including autotransporter proteins that are considered to mediate colonization in human tissues. Our study demonstrated that F. nucleatum utilizes two autotransporter proteins (Fap2 and CmpA) to coaggregate with another periodontal pathogen, Aggregatibacter actinomycetemcomitans , through specific interactions with bacterial surface polysaccharides. In addition to the roles of these autotransporters in periodontitis through bacterial coaggregation with other oral bacterial species, our findings potentially provide new insights into the mechanism underlying systemic diseases caused by F. nucleatum binding to host cell carbohydrates and glycoproteins.

Mitis group streptococci (MGS) are the predominant oral bacteria that cause bacteremia and infective endocarditis. Although membrane vesicle (MV) secretion has been reported in Streptococcus pneumoniae among MGSs, comprehensive studies using various streptococcal species are limited. We aimed to determine whether MGS species produce MVs and to examine their biological functions. MVs were isolated from MGS cultures using density gradient ultracentrifugation. The particle sizes of MVs were measured, and proteins in MVs were identified by liquid-chromatography tandem mass spectrometry analysis. Effects of MVs on host cells and oral pathogenic bacteria were investigated. MV production was confirmed in Streptococcus mitis strains NCTC12261, Nm-65, and Nm-76, with particle diameters ranging from 100 to 120 nm. The MVs contained numerous cytoplasmic proteins. The MVs showed internalization into alveolar epithelial cells and induced the production of multiple cytokines, including TNF-α, IL-8, IL-6, IL-1β, and IL-10, in macrophages while suppressing phagocytic activity. In neutrophil-differentiated cells, MVs induced IL-8 but not TNF-α production. MVs from S. pneumoniae TIGR4 and S. mitis Nm-65 inhibited biofilm formation of Aggregatibacter actinomycetemcomitans. MVs play crucial roles in MGS survival strategies through immune modulation and interspecies competition, contributing to their pathogenicity and host-pathogen interactions.

Aggregatibacter actinomycetemcomitans is a Gram-negative facultative anaerobe and is associated with periodontal disease. This bacterium is exposed to environmental stresses, such as osmotic pressure, temperature shifts, pH shifts, and antimicrobial substances, including reactive oxygen species (ROS), in the human oral cavity. The bacterial two-component system ArcAB modulates gene expression in response to environmental changes, primarily by sensing oxygen pressure in several pathogens belonging to the γ-proteobacteria. It is also known to provide adaptation to ROS stress; however, its function in A. actinomycetemcomitans remains unclear. In this study, we found that the expression of sod , which encodes superoxide dismutase, was increased in the inactivated mutant of arcA , which encodes a response regulator. The mutant exhibited reduced susceptibility to superoxide and hydrogen peroxide (H ₂ O ₂ ). Additionally, this strain showed reduced susceptibility to H ₂ O ₂ from Streptococcus sanguinis and increased survival in macrophages. Since ArcB is the cognate histidine kinase of ArcA, the inactivated mutant of arcB was analyzed for its phenotypes. The arcB mutant exhibited reduced susceptibility to superoxide and H ₂ O ₂ . Compared to wild type, the phosphorylation level of ArcA in the arcB mutant was decreased. These results suggest that the ArcA response regulator receives phosphate groups from ArcB histidine kinase and negatively regulates the expression of sod , thereby affecting bacterial survival in response to ROS produced by oral commensals and host immune cells. IMPORTANCE: Aggregatibacter actinomycetemcomitans is an oral pathogen that is known to be a highly virulent periodontal pathogen, showing strong adherence to periodontal tissue and toxin production, which leads to aggressive periodontitis. This bacterium is associated not only with oral infections but also with systemic infections, such as infective endocarditis and brain abscesses. Therefore, elucidating the adaptation mechanisms of this bacterium is important for human health. Bacterial two-component systems (TCSs) have been studied as attractive targets for elucidating bacterial fitness and pathogenicity in the host. This study characterized a TCS in A. actinomycetemcomitans , ArcAB, which is associated with susceptibility to ROS produced by host cells or oral commensals. Our findings provide insights into the bacterial adaptation mechanism against oxidative stress, which is crucial for understanding the survival strategies of the periodontal pathogen.