Introduction

Candidatus Saccharibacteria, historically referred to as TM7, represents one of the most enigmatic members of the human microbiome. As part of the Candidate Phyla Radiation (CPR), these bacteria are characterized by their ultrasmall cell size (200 to 500 nm) and highly reduced genomes, often smaller than 1 megabase. Because they lack the genetic capacity to synthesize many essential molecules, including vitamins and amino acids, they lead an obligate episymbiotic lifestyle, attaching to the surface of host bacteria to survive and propagate.

Location of Microbe

Oral Ecosystem

Saccharibacteria are ubiquitous in the oral cavity, frequently detected in saliva, dental plaque, and tongue coatings. They are known to associate with specific host bacteria, particularly within the phylum Actinobacteria, and can be found in various niches including subgingival plaque and on the surface of dental instruments in clinical settings.

Gut Ecosystem

While more prominent in the oral cavity, Saccharibacteria are also present in the human gastrointestinal tract. They are identified through metagenomic sequencing of fecal samples and are associated with the broader gut microbial community, though their specific episymbiotic partners in the gut are less characterized than those in the mouth.

Behavior During Dysbiosis

Oral Dysbiosis

In the oral ecosystem, Saccharibacteria often act as opportunistic pathogens. They are frequently enriched in inflammatory environments, such as periodontal pockets, where they may overgrow their host bacteria, potentially shifting the microbial balance and contributing to the persistence of biofilms that are resistant to disinfection.

Gut Dysbiosis

Within the gut ecosystem, increased microbial abundance of Saccharibacteria is associated with states of dysbiosis. This enrichment is observed in conditions involving systemic inflammation or specific infections, where the typical balance of the gut microbiome is disrupted, favoring the expansion of these reduced-genome bacteria.

Disease Associations

Oral Ecosystem

Saccharibacteria are strongly associated with several oral pathologies. In periodontitis, they are regarded as 'inflammophilic' bacteria that thrive in nutrition-rich inflammatory conditions. Specifically, in patients with type 2 diabetes mellitus (T2DM), there is a significant enrichment of Saccharibacteria (such as Nanosynbacter lyticus) in subgingival plaque, which correlates positively with HbA1c and fasting blood glucose levels. They are also identified as biomarkers for halitosis in children, where shifts in their abundance on tongue coatings can predict the onset of the condition. Additionally, their presence is associated with childhood caries and has been detected in biofilms on dental elevators, highlighting their role in clinical infection risks.

Gut Ecosystem

In the gut, Saccharibacteria abundance is associated with systemic and infectious diseases. A significant increase in their relative abundance has been observed in patients with SARS-CoV-2 (COVID-19) infection, both in symptomatic and asymptomatic cases, suggesting they may serve as metagenomic markers for the infection. Furthermore, an increase in the TM7 phylum is associated with acute Schistosoma japonicum infection and has been observed in the gut microbiota of patients with sickle cell disease, where they contribute to a pro-inflammatory microbial profile. They are also enriched in the gut of primiparous mothers during postpartum recovery.

Foods Supporting Healthy Balance

Maintaining a balanced microbiome involves managing the nutrients that favor opportunistic overgrowth. Research suggests that high-glycemic diets may influence the abundance of Saccharibacteria. In the oral cavity, a higher intake of sucrose and foods with a high glycemic load (GL) is inversely associated with overall microbial alpha-diversity and may favor the expansion of certain inflammatory-associated taxa. Conversely, reducing the intake of refined sugars may help prevent the nutrient-rich environments that 'inflammophilic' bacteria like Saccharibacteria prefer.

Additionally, the use of probiotics has shown promise in modulating these populations. For instance, certain probiotic therapies (such as koumiss or specific multi-strain formulations) have been associated with an increase in beneficial Gram-positive bacteria and a rebalancing of the microbiome, which may help manage the relative abundance of opportunistic episymbionts. Emphasizing a diet rich in fiber and complex carbohydrates supports a diverse gut ecosystem, which generally resists the dominance of single opportunistic phyla.

Actionable Insights

• Glycemic Control: For individuals with diabetes, maintaining stable blood glucose levels is critical, as hyperglycemia in the gingival crevicular fluid can selectively favor the expansion of Saccharibacteria.
• Oral Hygiene: Regular professional scaling and the use of targeted oral hygiene practices can reduce the subgingival reservoirs where Saccharibacteria thrive.
• Instrument Sterilization: In dental settings, enhanced sterilization protocols are necessary to disrupt the stable biofilms that Saccharibacteria help maintain on stainless-steel instruments.
• Probiotic Support: Consider the use of evidence-based probiotics to promote a diverse microbial community, which can help maintain a healthy balance and reduce the impact of opportunistic episymbionts.
• Dietary Adjustments: Reducing the intake of high-sucrose foods may limit the metabolic precursors that support the overgrowth of inflammophilic bacteria in the oral cavity.

Conclusion

Candidatus Saccharibacteria is a complex organism whose impact on human health depends heavily on its strain and the niche context. In the oral ecosystem, they act primarily as episymbionts that can exacerbate periodontal inflammation, particularly when metabolic triggers like hyperglycemia are present. In the gut, their abundance often mirrors systemic inflammatory states or the presence of specific parasitic and viral infections. Because they rely entirely on host bacteria for survival, managing the primary host populations—such as Actinobacteria in the mouth—is key to managing Saccharibacteria. Understanding these tripartite interactions between the episymbiont, the host bacterium, and the human host is essential for developing future preventive health strategies.