Cetobacterium spp.

Cetobacterium spp. are anaerobic, Gram-negative bacteria primarily found in the gastrointestinal tracts of freshwater fish, but they have also been identified in the human gut microbiome. Notably, Cetobacterium somerae plays a role in vitamin B12 production, immune modulation, and gut health, though its significance in human health remains an area of ongoing research.

What is Cetobacterium spp.?

​Cetobacterium spp. are anaerobic, Gram-negative bacteria predominantly found in the gastrointestinal tracts of freshwater fish. While they were initially identified as part of the microbiome of fish, they have since been identified in the human microbiome, suggesting human exposure through diet or the environment.

Cetobacterium somerae is a notable member of this genus, as it has been identified as a significant producer of vitamin B12 (cobalamin), an essential nutrient for various physiological functions.

It has been identified in human feces, which confirmed its presence as a bile acid-resistant, acetic acid-producing bacterium. ​

Vitamin B12 Production

Research indicates that C. somerae synthesizes vitamin B12 de novo. 

In a study involving zebrafish, the presence of C. somerae was associated with enhanced vitamin B12 production, which in turn improved the host's resistance to pathogenic infections by strengthening gut microbiota interactions and enhancing gut barrier functions. ​

Implications for Fish Health

The dominance of Cetobacterium in the gut microbiota of various freshwater fish species suggests its beneficial role in gut health and energy metabolism. 

For example, one study found that Cetobacterium somerae enhances antiviral immunity in zebrafish by stimulating TLR2-type I IFN signaling, primarily through its exopolysaccharides (CsEPS), which activate neutrophils and restrict viral infection. 

Additional research has shown that Cetobacterium and its metabolite acetate can improve glucose homeostasis in fish, indicating a positive impact on metabolic processes. ​It may also support liver health in fish.

Relevance to Human Health

While Cetobacterium is primarily associated with fish, its presence in the human gut has been documented. Notably, C. somerae has been isolated from human feces, suggesting potential implications for human health. 

For example, C. somerae may contribute positively to human host health through vitamin B12 production, enhancing immune responses, and supporting gut integrity via acetic acid production.

However, the extent of its role in vitamin B12 synthesis within the human gastrointestinal tract remains under investigation. The human colon, where Cetobacterium resides, is located downstream from the small intestine—the primary site of nutrient absorption—raising questions about the bioavailability of bacterially synthesized vitamin B12 for human hosts. ​

Clinical Considerations

Cetobacterium spp. has been identified in the feces of autistic children; additionally, there has been a documented case of C. somerae bacteremia following necrotizing cholecystitis, marking the first such instance of human infection by this bacterium. 

This case underscores the need for further research into the pathogenic potential of Cetobacterium species in humans. ​

Who Should Get Cetobacterium spp. Levels Assessed?

Currently, the assessment of Cetobacterium spp. levels is mainly relevant in research contexts. It may be used in studies that examine:

• The role of the gut microbiome in human health and disease.
• How the microbiome contributes to vitamin B12 production and availability.
• The effects of diet and other factors on gut microbiome composition.

Cetobacterium spp. may also be studied in individuals with suspected vitamin B12 deficiency, although it’s important to note that it cannot replace traditional clinical tests for B12 levels. 

New research may also assess Cetobacterium spp. abundance in individuals with gastrointestinal disorders as part of broader microbiome analyses.

Test Procedure and Interpretation

The following section outlines important testing and interpretation information: 

Testing Procedure and Preparation Requirements

Cetobacterium spp. is typically tested using stool samples, which can be conveniently collected at home. Patients are advised to avoid medications and supplements, including probiotics, before collection. Always follow the lab company's recommendations regarding preparation and sample collection.

The following information is generally accepted regarding microbiome analysis and testing; always consult the laboratory company used for their recommendations.

Normal Reference Ranges

Currently, there are no established clinical reference ranges for Cetobacterium spp. levels. The abundance of Cetobacterium spp. can vary widely between individuals, and interpretation depends on the specific research context. 

Since Cetobacterium is still primarily studied in research, it is not considered an appropriate biomarker of vitamin B12 status for diagnostic purposes.

Generally, normal reference ranges for microbiome species are reported as a balanced abundance, with excessively high or low levels being out of range. The testing laboratory used should provide specific reference ranges.

Clinical Implications of Elevated Levels

Higher levels of Cetobacterium spp., particularly those species known to produce vitamin B12, may suggest increased B12 production in the gut, although they should not be used as a direct indicator of vitamin B12 status. 

Elevated levels of C. somerae have been seen in at least one case of necrotizing cholecystitis, suggesting that elevated levels of Cetobacterium spp. In the setting of illness may indicate an infection requiring treatment.

Clinical Implications of Decreased Levels

Low levels of Cetobacterium spp. could be associated with a reduction in vitamin B12 production in the gut. However, it is important to recognize that many other bacteria in the gut also contribute to B12 synthesis, and dietary intake is the primary source of B12 for most individuals. 

Low Cetobacterium spp. abundance may also reflect broader gut dysbiosis, but more research is needed to establish this connection. Gut dysbiosis must be understood within the broader balance of gut bacteria present in an individual.

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