Medical implications of gas producing probiotics: anti-inflammation and immunity enhancement for ameliorating disease severity

Inflammation is a typical immune response expressed during injury or infection that plays a pivotal role in maintaining the health and integrity of the human body. It is a complex and highly orchestrated process that works as a defense mechanism to eliminate microbial pathogens, promote tissue repair, and restore equilibrium within the body. Inflammation is characterized by hallmarks, such as vasodilation and increased permeability, immune cell infiltration, and cytokine signaling. While acute inflammation is a transient response during trauma/infection, chronic inflammation could involve disorders that result in aberrant and persistent activation of the inflammatory cascade.

Recently, coronaviruses have emerged periodically in different areas across the world, causing high mortalities. Severe acute respiratory syndrome coronavirus (SARS-CoV) outbreak occurred for the first time in 2002, which reportedly infected 8422 people and caused 916 deaths worldwide.1 SARS-CoV can induce inflammation through infection and multiorgan impairment, including lungs, brain, heart, and liver. The recent reemergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the form of global pandemic has significantly delineated the interplay between viral infections and the immune system. One of the hallmark features of severe SARS-CoV-2 cases is dysregulated and exaggerated inflammatory response, often referred to as the “cytokine storm.”2 Such dysregulated immune response is responsible for exacerbating the severity of SARS-CoV-2, leading to high mortality. SARS-CoV-2 enters the human body via respiratory droplets, binds to angiotensin-converting enzyme 2 receptors in the lungs, triggers viral replication, and prompts an immune response. However, in some cases, the immune system becomes dysregulated, leading to a cytokine storm, characterized by excessive release of pro-inflammatory cytokines (interleukin (IL)-6, IL-Ιβ, tumor necrosis factor-alpha, etc.). Such a response results in widespread inflammation and endothelial dysfunction, increasing vascular permeability and blood clot formation.2

Another recently re-emerging pathogen is Nipah virus that causes severe respiratory and neurological infections in humans. In contrast to SARS-CoV-2 known for its systemic and chronic inflammation affecting multiple organs, Nipah virus infection primarily leads to acute encephalitis, manifested by localized inflammation in the brain.3 Nipah virus is primarily transmitted to humans from animals, particularly fruit bats, and can also spread through human-to-human contact. Upon entry, Nipah virus initially infects the respiratory tract, and then spreads to various organs, including the central nervous system. The immune system recognizes the virus as a threat and induces a defense response. This response involves the activation of immune cells, particularly macrophages and dendritic cells, which release pro-inflammatory cytokines, such as IL-6 and tumor necrosis factor-alpha. Once inside the host cell, Nipah virus triggers a complex immunological response, leading to inflammation.3 The inflammatory response can become dysregulated in certain cases, leading to “cytokine storm” and high mortality, similar to that in severe SARS-CoV-2 infection. The excessive release of cytokines results in widespread inflammation throughout the body, contributing to disease severity.4 To mitigate such viral induced inflammation, the host cell stimulates the production of anti-inflammatory cytokines (e.g., IL-10, transforming growth factor-beta), regulatory T cells, and specialized pro-resolving mediators along with clearing dead cells, debris mitigating inflammation and promoting recovery.5 Although, in some cases, the inflammatory response can exacerbate and lead to serious complications and require medical treatment.

Hospital management of inflammation in viral infections, involving SARS-CoV-2 and Nipah virus utilizes corticosteroids, such as dexamethasone to reduce inflammation, through alleviating “cytokine storm.” However, prolonged or high-dose corticosteroid use can weaken the immune system and lead to adverse effects, such as secondary infections, including black fungus, elevated blood sugar levels and mood disturbances.6 Similarly, non-steroidal anti-inflammatory drugs like ibuprofen alleviate the inflammatory symptoms; however, they can induce gastrointestinal side effects and increase the risk of bleeding and ulcers, and so, careful monitoring and balanced use are essential in treatment.6 Besides the conventional approaches, various complementary and alternative medicine strategies are being explored for mitigating inflammation induced by SARS-CoV-2, Nipah virus and similar other viruses such as dengue and encephalitis. Such compounds include omega-3 fatty acids, curcumin, and probiotics to name a few.4,7,8 Notably, probiotics have demonstrated significant potential in attenuating inflammation and enhancing clinical outcomes; and being natural, confer least adverse effects.7 Probiotics, live microorganisms akin to beneficial gut bacteria, are often termed “good” bacteria because of their role in gut health maintenance. They have the potential to mitigate viral-induced inflammation by stimulating the production of anti-inflammatory cytokines, such as IL-10 and transforming growth factor-beta, thereby aiding in immune response balance.7 Moreover, probiotics support healthy gut microbiota, crucial for a robust immune system and alleviating inflammatory disease severity through the production of unique health-promoting compounds. Studies have shown that probiotics can reduce inflammation and enhance outcomes in SARS-CoV-2 patients, such as improved lung function.8,9,10 Probiotics derived from bacterial strains like Lactobacillus and Bifidobacterium are beneficial inhabitants of the gut and stimulate mucosal immunity. When introduced into the body, they promote balanced microbiota by displacing harmful bacteria and maintaining the gut equilibrium.

Probiotic producing anti-inflammatory medically relevant gaseous mediators: While the concept of probiotics as gut guardians is widely accepted, the discovery of their ability to produce gaseous mediators adds a new dimension to their multifaceted role. Such gaseous mediators, including nitric oxide (NO), hydrogen sulfide (H₂S), and carbon monoxide, possess unique properties and biological activities that extend beyond the confinement of the gut.

NO-producing probiotics and mechanism of action: NO produced by probiotics, plays vital roles in regulating blood flow, mucosal defense, and immune responses in the gut. Lactobacillus fermentum (L. fermentum) can produce NO through two distinct pathways. Firstly, L. fermentum can metabolize L-arginine, an amino acid, to generate NO, possibly as part of its metabolic processes. Secondly, under aerobic conditions, L. fermentum can reduce nitrate (NO₃⁻) to nitrite (NO₂⁻), and then convert nitrite to NO, contributing to NO production through the nitrate reduction pathway.9 NO serves as a crucial signaling molecule in immune responses, with potential antiviral effects. Studies have reported the antiviral effect of NO that inhibit viral replication in Epstein–Barr virus,8 and mitigates excessive inflammation by the enhanced production of various anti-inflammatory cytokines, including IL-10, IL-13, transforming growth factor-beta through influencing both nuclear factor kappa-B and Janus kinase-signal transducer and activator of transcription signaling.8 These immune-signal molecules downregulate pro-inflammatory cytokines that makes it beneficial for treating chronic inflammatory conditions and protecting against viral infections.

H₂S-producing probiotics and mechanism of action: H₂S is synthesized by specific gut microbiota, especially sulfate-reducing bacteria. H₂S is primarily generated through two main processes: cysteine catabolism and the activity of sulfate-reducing bacteria. These include microrganisms such as Desulfovibrio sulfuricans, Fusobacterium sp., and Salmonella sp., which employ the enzyme cysteine desulfhydrase to transform cysteine into H₂S, pyruvate, and ammonia.9,10 It has been found that the administration of H₂S significantly reduces pro-inflammatory cytokines IL-6 and IL-8, and increases anti-inflammatory cytokine IL-10 in the lungs.10 H₂S at low concentration assists in supporting mucosal health and anti-inflammatory functions by inhibiting the proinflammatory signaling pathways. However, at elevated levels, H₂S can become toxic and exacerbate inflammation, highlighting the importance of balance and precise regulation.11 In the context of SARS-CoV-2 and Nipah virus infection, H₂S and NO have the potential to mitigate inflammation by inhibiting the generation of pro-inflammatory cytokines, encouraging the production of anti-inflammatory cytokines, ameliorating oxidative stress, and enhancing vascular function.

Future direction: Utilization of external clinically relevant gaseous mediators has demonstrated the capacity to reduce microbial infections, dampen inflammation, and enhance antiviral immune responses. Interestingly, studies have explored the development of an innovative NO-producing probiotic patch to treat ischemic and infected full-thickness dermal wounds, leading to enhanced wound healing as indicated by reduced surface depression, diminished crusting and exudation, and decreased inflammation and infiltration.12 Such novel probiotic patches producing NO and H₂S can be designed for reducing inflammation caused by pathogenic virus by using genetically engineered probiotics, rendering them to produce high amounts of gaseous mediators. This can be done by “knocking in” genes that encode enzymes involved in the production of NO and H₂S, or by “knocking out” genes that encode enzymes that degrade these gaseous mediators. Once the probiotics have been engineered, they can be embedded in a patch material that is compatible with the human skin and allows sustained release of NO and H₂S. The patch can then be applied to the skin of patients affected by a particular viral infection, thereby ameliorating inflammation by diffusing into the surrounding tissues and suppressing the production of pro-inflammatory cytokines, and promoting the production of anti-inflammatory cytokines, which in turn will alleviate oxidative stress (Figure 1). Both H₂S and NO are important signaling molecules in the body and play crucial roles in various physiological processes. However, an overproduction of these gases can have some degree of adverse health effects such as toxicity, excessive vasodilation, and neurotoxicity. Therefore, it is crucial to build a balance in the application of such gases, and medical conditions associated with their overproduction should be properly managed by healthcare professionals. Complementation of such probiotics producing medically relevant gaseous mediators along with conventional antibiotics may lead to more effective treatment strategies and ameliorate inflammation during the treatment regime.

Figure 1.

Mechanistic diagram of the development and application of NO and H₂S producing probiotics to address severe inflammation.

Note: Created with BioRender.com. D. sulfuricans: Desulfovibrio sulfuricans; gH₂S: hydrogen sulfide gas; gNO: nitric oxide gas; H₂S: hydrogen sulfide; L. fermentum: Lactobacillus fermentum; NO: nitric oxide.