The notion that blood was a sterile environment in the absence of specific diseases has been widely accepted for many years. However, advancements in analytical techniques such as real-time PCR have challenged this notion by detecting bacterial ribosomal DNA in healthy individuals. Additionally, methods like dark field microscopy, fluorescent in situ hybridization, and flow cytometry have revealed the presence of pleiomorphic bacteria in the blood, which are now collectively referred to as the blood microbiota or microbiome. The microbiota comprises bacteria, viruses, and fungi, while the microbiome refers to the collection of genomes or genomic fragments, including the DNA and/or RNA of these microorganisms.
Studies have shown that the blood microbiota undergoes several transformations within peripheral blood mononuclear cells (PBMCs), including vesiculation, tubulation, budding, and the protrusion of progeny cells from large electron-dense bodies. Current research indicates that the blood microbiota in healthy individuals is predominantly composed of the Bacillota, Actinomycetota, Pseudomonadota, and Bacteroidota phyla. These microbial DNA signatures coexist harmoniously with the host and exhibit immunomodulatory phenotypes. Their presence or absence may influence health conditions or contribute to disease states such as sepsis.
However, recent research involving DNA characterization of blood from 9,770 healthy individuals found no evidence of a common blood microbiota. This suggests that commensal microbes’ translocation into the bloodstream from other sites may be a transient process. Some researchers argue that blood microbiota may result from microbial contamination in low-biomass samples and that their viability is undetermined due to culture-independent procedures. The origin of blood microbiota is debated, with some suggesting a direct derivation from the gut microbiota or the skin–oral–gut axis.
Maternal origin of the blood microbiota is also considered, given its presence in prenatal tissues such as umbilical cord blood, meconium, amnion, and the placenta. Some research suggests that a combination of bacteria in the placenta may play a significant role in premature births and that periodontal disease in pregnant women may increase the probability of premature births. These hypotheses highlight the importance of oral hygiene during pregnancy as a preventive measure. However, more recent studies have denied the presence of a placental microbiota, confirming placental asepticity and reinforcing the idea that the fetus lives in a sterile environment, with first contact with bacteria occurring during childbirth through the birth canal.
The mechanisms behind the maternal transfer of blood microbiota remain unknown, despite proposals of oral and gut fetal compartment colonization or the ingestion of amniotic fluid during gestation. Interestingly, blood microbiota sequencing studies have revealed geographical differences. For instance, Germany and Poland exhibit higher blood microbiota values, with intermediate levels in Italy and Finland, and lower levels in Belgium and Austria. These differences may depend on genetic and immune factors, diet, hygiene, and parasitic loads. The environment appears to have a greater impact than age, although an association between blood microbiota and aging cannot be excluded.
Persistent blood microbes can lead to various diseases affecting the cardiovascular system, liver, and kidneys. Unlike the gut, the blood microbiota interacts differently with the host, particularly with leukocytes, whose responses may determine disease status. In healthy humans, blood microbiota interacts with host cells through an array of products, such as metabolites, lipoglycans, quorum-sensing peptides, proteins, and bacterial extracellular vesicles (EVs). Outer membrane vesicles (OMVs) produced by Gram-negative bacteria are rich in lipopolysaccharides (LPSs) and membrane proteins. LPSs are key molecules that interact with Toll-like receptors (TLR)-4 on monocytes and endothelial cells, activating the NF-kB pathway and releasing proinflammatory cytokines. However, LPS micelles can also be tolerated by the immune system, becoming innocuous to the host.
Disturbance of a core healthy microbiota in the blood can contribute to disease outcomes. For example, microbial diversity in patients with myocardial infarction (MI) and chronic coronary syndrome is higher than in healthy individuals. Liver fibrosis and cirrhosis also feature diverse blood microbiota, with certain bacteria provoking the release of proinflammatory cytokines and nitric oxide. In kidney disease, circulating bacteria are not typical commensals of the urinary tract, suggesting their origin from other sources, such as the gut. In cancer, blood microbiota profiles can help distinguish different cancer types and predict responses to treatments, such as advanced colon cancer therapies. Blood bacteria have also been detected in autoimmune diseases, immunosuppression, HIV, and inflammatory bowel disease (IBD), though it remains unclear whether bacteria cause these diseases or are a consequence of disease outcomes.
The concept of blood microbiota, though controversial, is gaining acceptance. Some researchers believe that the blood microbiota naturally exists from birth throughout life, consisting of harmless organisms living in equilibrium with the host. Others have identified circulating microbial cell-free DNA or microbial vesicles containing metabolites and fragmented DNA or RNA in the blood. Conversely, some groups deny the existence of blood microbiota. Among those who support its existence, there is no consensus on its composition. Staphylococcus spp. is a commonly detected genus in the blood, but detailed species-level information is lacking. The Pseudomonadota phylum and Cutibacterium acnes are also found in the blood. Bacterial diversity varies between the buffy coat, red blood cells, and plasma, with 117 blood microbial species, including 110 bacteria, five viruses, and two fungi. These data suggest the absence of a core healthy blood microbiota, with transient and sporadic translocation of commensals into the circulation, which are rapidly cleared out and do not colonize.
Electron microscopy has recently contributed to blood microbiota research by demonstrating that circulating microbiota in PBMCs from healthy donors undergo complex life cycles involving various morphological transformations. Blood microbiota can reproduce through irregular binary fission, budding, protrusion-extrusion of progeny cells from large electron-dense bodies, vesiculation, tubulation, or a combination of these processes. The morphology of blood microbiota supports the existence of microorganisms in the blood of healthy individuals.
Blood microbiota profiles are assessed in healthy individuals or patients to identify genetic signatures for risk stratification, diagnosis, disease surveillance, and drug development. However, the existence of human blood microbiota remains debatable due to high-risk contamination in low-biomass samples and the undetermined viability of blood microbiota via culture-independent profiling methods.
Many factors influence blood microbiota composition, such as leaking epithelial junctions, mucosal disruption, periodontal disease, chewing, and tooth brushing. Alterations in blood microbiota equilibrium can result in dysbiosis, leading to various diseases and low-grade inflammation. Chronic low-intensity inflammation can develop pathophysiologically over a long period without symptoms, eventually triggering serious diseases.
In cardiovascular diseases, bacteria, fragments, and their DNA have been identified in patients. Importantly, LPS from blood bacteria participates in atherosclerosis via the formation of macrophage-derived foam cells. Pseudomonadota, Actinomycetota, Cyanobacteria, and Verrucomicrobia from circulating microbiota play a role in cardiovascular disease outcomes, with the Proteobacteria phylum predominant in acute coronary patients. Dysbiosis of human blood microbiota has been proposed as a marker for cardiovascular disease prediction. Blood Desulfobacterota increases in acute coronary syndrome but decreases in chronic coronary syndrome, contributing to butyrate breakdown via the butyrate beta-oxidation pathway and promoting atherosclerotic progression.
In respiratory diseases, studies on blood microbiota are scarce. Bacteroides, Alistipes, Parabacteroides, and Prevotella predominate in the blood, with decreases in Actinobacter, Verrucomicrobia, and Cyanobacteria. Differential blood bacterial profiles allow accurate asthma diagnosis with elevated sensitivity and specificity. Blood bacteria such as Acinetobacter, Serratia, Streptococcus, and Bacillus are associated with severe dyspnea in smokers. In severe COVID-19, blood bacteria such as Escherichia coli, Bacillus spp., Campylobacter hominis, Pseudomonas spp., Thermoanaerobacter pseudethanolicus, Thermoanaerobacterium thermosaccharolyticum, and Staphylococcus epidermis are correlated with disease severity.
In liver diseases, liver fibrosis and cirrhosis feature diverse gut-derived circulating bacteria, with elevated Bacteroides and Enterobateriaceae in cirrhotic patients. Certain circulating bacteria, such as Corynebacteriales, predict the reversal of portal hypertension in HCV-induced cirrhosis upon antiviral treatment termination. Circulating LPSs in HCV patients contribute to inflammatory damage via the release of proinflammatory cytokines.
In kidney diseases, blood microbiota dysbiosis plays a role in chronic kidney disease (CKD). An inverse correlation between glomerular filtration rate and increased circulating Pseudomonadota has been documented. The genus Devosia in the blood predicts increased mortality risk in CKD patients on peritoneal dialysis, while elevated blood Legionella is implicated in kidney impairment and mortality in IgA nephropathy patients.
In neoplastic diseases, microbiome analysis of blood and tissue reveals sequence reads belonging to microorganisms such as bacteria, archaea, and viruses. Blood microbiota profiles help distinguish different cancer types at early stages, predict cancer therapy responses, and identify specific bacteria associated with estrogen receptor-positive breast cancer in post-menopausal women. Beta-glucuronidase-producing bacteria predominate in the blood of breast cancer patients, while beta-galactosidase bacteria are more common in healthy subjects.
In immune and inflammatory diseases, blood microbiota dysbiosis plays a pathogenic role. Elevated blood levels of certain genera and phyla are associated with autoimmune diseases such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). In immunosuppressed patients, opportunistic bacteria growth may be a side effect of immunosuppressant therapy. In HIV patients, increased genera such as Veillonella and Massilia are observed, while blood bacteria in inflammatory bowel disease patients aggravate inflammatory conditions.
The interaction between blood microbiota and host cells involves various mechanisms. Blood microbes interact with red blood cells and leukocytes, with bacteria engulfed by macrophages and polymorphonuclear cells. Bacterial products such as LPS, lipoteichoic acid, peptidoglycans, and mycolic acids activate immune cells via the production of proinflammatory cytokines and free radicals. Microbiota-derived EVs promote intercellular communication by carrying proteins, lipids, sugars, nucleic acids, and metabolites. OMVs from Gram-negative bacteria induce potent innate immune responses via LPS and protein components. EVs from bacteria such as E. coli interact with monocytes, activating the NF-kB pathway and releasing proinflammatory cytokines.
In summary, systemic dysbiosis accounts for many pathological conditions mediated by blood microbes. Conversely, commensal microbiota and restored eubiosis play a crucial role in host recovery. Understanding the composition and interaction mechanisms of blood microbiota with host cells is essential for developing therapeutic strategies and improving disease outcomes.
APPENDIX 1 – Blood Microbiota
| Concept | Explanation |
|---|---|
| Blood Microbiota | Tiny living organisms like bacteria, viruses, and fungi found in the blood. Scientists used to think blood was sterile (free of these organisms) unless there was a specific disease. |
| Blood Microbiome | The collection of genetic material (DNA and RNA) from the microorganisms present in the blood. |
| Analytical Techniques | Advanced methods like real-time PCR, microscopy, and flow cytometry that help detect and study these microorganisms in the blood. |
| Pleiomorphic Bacteria | Bacteria that can change their shape and form found in the blood. |
| Peripheral Blood Mononuclear Cells (PBMCs) | A type of blood cell that includes lymphocytes (like T cells and B cells) and monocytes. These cells interact with blood microbiota. |
| Blood Microbiota in Healthy Individuals | In healthy people, certain types of bacteria and other microorganisms can live in the blood without causing harm. They might even help the immune system. |
| Microbial Translocation | The process where microbes from other parts of the body, like the gut or skin, temporarily enter the bloodstream. |
| Maternal Transfer of Blood Microbiota | The possibility that a baby’s blood microbiota could come from the mother during pregnancy or childbirth. |
| Geographical Variations in Blood Microbiota | Differences in the types and amounts of blood microbiota found in people from different countries. This can be influenced by diet, hygiene, and genetics. |
| Dysbiosis | An imbalance in the blood microbiota that can lead to diseases and inflammation. |
| Cardiovascular Diseases and Blood Microbiota | Certain bacteria in the blood are linked to heart diseases. For example, bacteria can help form blockages in arteries. |
| Respiratory Diseases and Blood Microbiota | Specific bacteria in the blood are connected to lung diseases and conditions like asthma and severe COVID-19. |
| Liver Diseases and Blood Microbiota | In liver diseases, bacteria from the gut can end up in the blood, causing inflammation and liver damage. |
| Kidney Diseases and Blood Microbiota | Changes in blood microbiota are associated with chronic kidney disease and other kidney problems. |
| Neoplastic Diseases (Cancers) and Blood Microbiota | The types of bacteria in the blood can help identify different cancers and predict how well a patient might respond to treatments. |
| Immune and Inflammatory Diseases | Blood microbiota imbalances can play a role in autoimmune diseases like lupus and rheumatoid arthritis, as well as in conditions affecting the immune system. |
| Extracellular Vesicles (EVs) and Outer Membrane Vesicles (OMVs) | Small particles released by bacteria that carry proteins, DNA, and other molecules. These can interact with immune cells and influence health. |
| Lipopolysaccharides (LPS) | Molecules found on the outer membrane of certain bacteria. They can trigger strong immune responses, leading to inflammation. |
| Toll-like Receptors (TLR) | Proteins on immune cells that recognize harmful bacteria and activate the body’s defense mechanisms. |
| NF-kB Pathway | A key signaling pathway in cells that controls inflammation and immune responses when activated by bacterial products like LPS. |
| Septicemia and Endotoxemia | Severe conditions caused by the presence of bacteria or their toxins in the blood, leading to widespread inflammation and organ damage. |
This table provides a clear and accessible overview of the key concepts related to blood microbiota, making it easier for non-medical readers to understand the document’s content.
APPENDIX 2 – The Role and Dynamics of Blood Microbiota in Peripheral Blood Mononuclear Cells
For many years, the prevailing belief in medical science was that blood is a sterile environment, free from microbial presence unless there is an infection or disease. However, recent advancements in analytical techniques have challenged this notion. Real-time PCR and other sophisticated methods have revealed the presence of bacterial ribosomal DNA in the blood of healthy individuals. This discovery has led to the concept of blood microbiota or microbiome, which includes bacteria, viruses, and fungi coexisting within the human bloodstream.
Blood Microbiota Transformations in PBMCs
Peripheral blood mononuclear cells (PBMCs) play a crucial role in the immune system, comprising lymphocytes (T cells, B cells, and NK cells) and monocytes. Recent studies have shown that blood microbiota undergo several transformations within PBMCs, including vesiculation, tubulation, budding, and the protrusion of progeny cells from large electron-dense bodies. These transformations indicate a dynamic interaction between blood microbiota and host cells, which may have significant implications for health and disease.
Composition of Blood Microbiota
The blood microbiota in healthy individuals is predominantly composed of the Bacillota, Actinomycetota, Pseudomonadota, and Bacteroidota phyla. These microbial DNA signatures coexist harmoniously with the host and exhibit immunomodulatory phenotypes, meaning they can modulate the immune response. Their presence or absence may influence health conditions or contribute to disease states such as sepsis.
Bacillota
Bacillota, also known as Firmicutes, are a major phylum of bacteria that include various pathogenic and non-pathogenic species. In the context of blood microbiota, Bacillota are known to have immunomodulatory effects, which can help in maintaining a balanced immune response.
Actinomycetota
Actinomycetota, or Actinobacteria, are another major group found in the blood microbiota. These bacteria are known for their role in decomposing organic materials and have a significant presence in the human microbiome. In blood, their role may extend to influencing immune responses and maintaining homeostasis.
Pseudomonadota
Pseudomonadota, also known as Proteobacteria, include a wide variety of pathogens such as Escherichia, Salmonella, and Helicobacter. However, in the blood microbiota, the presence of non-pathogenic species from this phylum suggests a complex interaction with the host immune system, potentially aiding in immune regulation.
Bacteroidota
Bacteroidota, also referred to as Bacteroidetes, are a group of bacteria predominantly found in the gut. Their presence in the blood microbiota indicates a possible translocation from the gut to the bloodstream, which could play a role in systemic immune modulation.
Immunomodulatory Phenotypes
Microbial DNA detected in the blood of healthy individuals coexists with the host immune system and exhibits immunomodulatory phenotypes. This means that these microbes can influence the immune system in various ways, such as by regulating inflammation, enhancing immune tolerance, and preventing overactive immune responses that could lead to autoimmune diseases.
Health and Disease Implications
Health Conditions
The presence of a balanced blood microbiota is essential for maintaining health. These microorganisms can enhance immune tolerance and prevent chronic inflammation. For example, some bacteria in the blood can help regulate the production of cytokines, proteins that are crucial for cell signaling in immune responses.
Disease States
Conversely, an imbalance in blood microbiota, known as dysbiosis, can contribute to various disease states. For instance, an overabundance of certain bacteria can lead to sepsis, a severe and life-threatening response to infection. Dysbiosis has also been linked to chronic diseases such as cardiovascular diseases, liver diseases, and even certain cancers.
Current Research and Findings
Microbial DNA in Blood
Recent studies involving the sequencing of microbial DNA in blood samples from healthy individuals have provided significant insights into the composition and function of blood microbiota. For example, one study found no evidence of a common blood microbiota in a cohort of 9,770 healthy individuals, suggesting that microbial presence in the blood may be transient and influenced by external factors such as diet and environment.
Geographic Variations
Research has also shown that the composition of blood microbiota can vary significantly based on geographic location. For instance, higher levels of blood microbiota have been observed in individuals from Germany and Poland compared to those from Belgium and Austria. These variations may be due to differences in diet, hygiene practices, genetic factors, and exposure to environmental pathogens.
Maternal Transfer
The potential maternal transfer of blood microbiota is another area of active research. Microbes have been detected in prenatal tissues such as umbilical cord blood, meconium, amnion, and the placenta, suggesting that maternal microbiota could influence the development of the fetal immune system. This finding has important implications for understanding the origins of the human microbiome and its impact on health from early life stages.
Mechanisms of Microbial Interaction with PBMCs
The interaction between blood microbiota and PBMCs is a complex process that involves several mechanisms. These interactions can influence both innate and adaptive immune responses.
Vesiculation
Vesiculation involves the formation of small vesicles or sacs that can transport microbial DNA and other molecules within PBMCs. These vesicles can facilitate communication between microbes and host cells, potentially influencing immune responses.
Tubulation
Tubulation refers to the formation of tube-like structures that can help transport microbial components within PBMCs. This process may play a role in the dissemination of microbial signals within the immune system.
Budding
Budding is a process by which new microbial cells are formed and released from existing ones. In the context of blood microbiota, budding can contribute to the propagation and maintenance of microbial populations within the bloodstream.
Protrusion of Progeny Cells
The protrusion of progeny cells from large electron-dense bodies involves the release of new microbial cells from larger parent cells. This process can enhance the diversity and functionality of blood microbiota, potentially influencing immune regulation and homeostasis.
Analytical Techniques for Studying Blood Microbiota
Real-Time PCR
Real-time PCR is a highly sensitive technique used to detect and quantify microbial DNA in blood samples. This method allows researchers to identify specific microbial species and assess their abundance in the bloodstream.
Dark Field Microscopy
Dark field microscopy is used to visualize live bacteria in blood samples. This technique enhances contrast and allows for the detection of bacteria that are otherwise difficult to see under a standard light microscope.
Fluorescent In Situ Hybridization (FISH)
FISH is a technique that uses fluorescent probes to bind to specific DNA sequences within microbial cells. This method enables the visualization and identification of specific microbes in blood samples, providing insights into their distribution and interactions with host cells.
Flow Cytometry
Flow cytometry is a powerful tool for analyzing the physical and chemical characteristics of cells in a blood sample. This technique can be used to identify and quantify different types of PBMCs and assess their interactions with blood microbiota.
Potential Therapeutic Applications
Understanding the composition and function of blood microbiota has significant implications for developing new therapeutic strategies. For example, probiotics and prebiotics could be used to modulate blood microbiota and enhance immune function. Additionally, targeted antibiotics or antimicrobial peptides could be developed to selectively eliminate harmful microbes while preserving beneficial ones.
REFERENCE : https://www.mdpi.com/2038-8330/16/3/43
