Introduction
Colon cancer, often called colorectal cancer, is a significant killer all over the world. There is a complicated interaction between genetic, behavioral, and environmental variables in the development of colon cancer. Recent years have seen an uptick in studies demonstrating probiotics and other gut microbiota's ability to inhibit the growth and spread of colon cancer. Probiotics are defined as "live microorganisms that, when administered to the host, have a beneficial effect on the host's biology and health." These good bacteria are essential for a healthy gut microbiome and for keeping things like immune responses and homeostasis stable. Probiotics have been proven in multiple studies to mitigate the effects of colon cancer in a number of ways [
1,
2,
3,
4]:
1. Inflammation Modification:
Cancer is characterized by inflammation, and colon precancerous lesions can grow into malignant tumors due to prolonged inflammation. Anti-inflammatory effects of probiotics like Lactobacillus and Bifidobacterium strains have been studied and proven to be significant. Inhibiting pro-inflammatory cytokines and adjusting immune cell activity, they reduce the likelihood of cancer developing from chronic inflammation.
2. Production of Regulatory Metabolites:
Short-chain fatty acids (SCFAs) like butyrate are produced through the fermentation of dietary fibers and complex carbohydrates by the gut microbiota. Because they provide fuel for colonic cells and aid in keeping the colon's lining in good condition, SCFAs are crucial to good colon health. Particularly butyrate promotes apoptosis (programmed cell death) in cancer cells and suppresses their growth, suggesting it has anti-cancer effects. By increasing the production of these helpful SCFAs, probiotics can foster a tumor-free colonic environment.
3. Tumor Microenvironment Modulation:
Tumor growth and metastasis may be affected by changes in the gut microbiota. Researchers have found that probiotics improve the tumor microenvironment by increasing the number of anti-tumor immune cells such T cells and natural killer cells. Tumor growth and metastasis can be slowed through the immune system's ability to better recognize and target cancer cells through immune modulation.
4. Reducing Cell Division
It has been discovered that certain probiotics can prevent the growth of colon cancer cells. Bifidobacterium and Lactobacillus strains, for example, have been found to inhibit colon cancer cell growth and metastasis by inducing cell cycle arrest and apoptosis.
5. Eliminating Toxins:
Toxins produced by certain cancer-promoting gut bacteria can lead to DNA damage. By outcompeting harmful bacteria for resources and adhesion sites in the gut, probiotics can help mitigate the negative effects of these toxins on the colon.
6. Using Probiotics to Fight Colon Cancer:
There is a growing interest in employing probiotics as adjuvant therapy with standard cancer therapies due to the mounting evidence of their favorable benefits in lowering the severity of colon cancer. However, caution should be exercised when using probiotics in cancer patients, and it is especially vital for those receiving cancer therapies like chemotherapy and radiation to check with medical authorities. Research on the impact of gut microbiota, especially probiotics, on lowering colon cancer risk is encouraging. Modulating the tumor microenvironment, limiting cell proliferation, and lowering toxin production are just some of the ways in which probiotics can aid in the management of colon cancer. There is promising research suggesting that probiotics may improve patient outcomes and the efficiency of colon cancer care when incorporated into preventative and therapy plans. However, more study is needed to determine which probiotic strains, doses, and therapy protocols are most effective in combating colon cancer.
Immune Response Modulation in the Tumor Microenvironment via Microbiota
The immune system's ability to identify and destroy cancer cells is crucial. Tumor cells can avoid immune monitoring when the TME provides an immunosuppressive environment. New evidence reveals that the tumor-associated microbiome helps regulate immune responses in the tumor microenvironment (TME). Cancer development, therapy responses, and future therapeutic strategies are all profoundly affected by the intricate interplay between microbial populations and the immune system [
5].
Modulation of Immune Responses by Microbiota [
6,
7,
8,
9,
10]:
Certain communities of microbes in the TME can have an effect on different facets of the immune response.
1. Microbiota can affect immune cell recruitment, activation, and function in the tumor microenvironment (TME). Some bacteria, for instance, have been demonstrated to draw in immune cells including tumor-associated macrophages and myeloid-derived suppressor cells, which can create a suppressive microenvironment for immune function.
2. Immune checkpoints are molecules that regulate immune responses to prevent excessive tissue damage; microbiota can influence their expression. The overexpression of immunological checkpoints such as PD-L1 by some bacteria has been shown to reduce the effectiveness of cytotoxic T cells and encourage cancer cells to evade the immune system.
3. Tregs, or regulatory T cells, are immune cells that work to keep the body's immune system from attacking itself. Immune dysfunction may result from a shortage or surplus of Tregs. The immune response against malignancies can be affected by the microbiota composition.
4. Microbiota have been shown to affect the polarization of immune responses by regulating cytokine production. Pro-tumorigenic inflammation can be triggered by a TME with a dysregulated cytokine profile.
Implications for the Body's Natural Ability to Fight Cancer:
Significant consequences for cancer progression and treatment may result from the interplay between tumor-associated bacteria and immune responses.
1. Progression of Tumors: Microbiota-mediated immunosuppression can foster conditions favorable to tumor expansion and dissemination. Cancer cells may be able to evade identification and killing by the immune system in a manner controlled by particular bacteria.
2. Immunotherapy Response: Tumor-associated microbiota composition is correlated with therapeutic responses. By creating an immunostimulatory TME, certain bacteria can improve the efficacy of immunotherapies, whereas other bacteria may reduce the effectiveness of treatments.
3. Novel therapeutic approaches may be possible by manipulation of the microbiota-immune interaction. Anti-tumor immunity could be strengthened and treatment outcomes improved by the use of techniques such as fecal microbiota transplantation (FMT) or microbiota-targeted treatments.
Problems and Possible Solutions:
There are a number of problems that require fixing in this area:
1. Complexity: The interplay between bacteria and the immune system is nuanced and situational. Immunomodulation within the TME is a complex process that requires in-depth research to identify the specific microorganisms and processes involved.
2. Microbiological Diversity: The make-up of microorganisms differs from person to person and even from one part of a tumor to another. It's difficult work to decipher the practical significance of all this variety.
3. The individualized nature of microbial profiles necessitates stringent confirmation when translating microbiota-based immunomodulation into clinical applications. Tumor-associated microbiota's control of immune responses increases the complexity of cancer biology. Consideration of the microbiota as a key role in cancer progression is warranted by evidence showing that microbial communities within the TME can affect immune cells, checkpoints, and cytokine profiles. Microbiota-mediated immunomodulation has great therapeutic potential, and understanding how it works could lead to new approaches that better direct the immune system against cancer.
Cancer's Metabolic Nexus Revealed via Analysis of Metabolic Products
Cancer cells interact with stromal cells and invading immune cells in the tumor microenvironment (TME), creating a dynamic ecology. Recent studies have shown that the microbial communities found in and around tumors, collectively referred to as the tumor-associated microbiota, are also involved in this intricate relationship. The metabolic cross-talk between these microorganisms and tumor cells is one fascinating feature of this connection. Metabolites produced by microbial communities provide a new layer of complexity to our understanding of cancer biology by having a significant impact on tumor development, angiogenesis, and immune responses [
10,
11,
12,
13,
14,
15].
Tumor Microenvironmental Bacterial Metabolites:
Through their metabolic processes, microbial communities can generate a wide variety of metabolites. Direct and indirect impacts of several of these metabolites on tumor growth have been observed:
1. Short-chain fatty acids (SCFAs) are the byproduct of microbial fermentation of dietary fibers and include acetate, propionate, and butyrate. The energy contained in these compounds is used by both the bacteria and the host cells. Examples of butyrate's anti-cancer activities include its ability to stimulate tumor cell death, block angiogenesis, and tinker with the immune system.
2. Tumor growth can be affected by metabolites produced by microbes from amino acids. Some bacteria, for instance, can produce kynurenine, an immunosuppressant, through the metabolism of tryptophan.
3. Bile acids, formed through the microbial alteration of bile salts, have been shown to affect tumor development. It has been found that several of the secondary bile acids contribute to an inflammatory microenvironment.
4. Neuromodulators: The connection between tumors and the neurological system can be altered by microbes that produce neurotransmitters and neuromodulators.
Immunological Responses, Angiogenesis, and Tumor Growth:
1. Cancer Progression: SCFAs and amino acid derivatives are two examples of metabolites that might affect tumor cell growth and apoptosis. As an illustration, butyrate has been demonstrated to cause cancer cells to stop dividing and eventually die off.
2. The process of new blood vessel formation to supply malignancies with nutrients and oxygen is called angiogenesis. Tumor growth and metastasis are both affected by the angiogenic potential of certain metabolites.
3. Metabolites produced by bacteria have the ability to regulate immunological responses in the TME. These metabolites modulate immune cell activity, shifting the scales between pro- and anti-tumor responses.
Implications for Therapy and Potential Obstacles:
It is important for cancer therapy to get insight into the role that microbial metabolites play in tumor development.
1. Therapeutic Targeting: Interrupting pro-tumorigenic processes inside the TME by focusing on particular microbial metabolites or pathways is a possibility.
2. As a potential treatment technique, modulating microbial metabolism involves altering the make-up of the microbiota found in tumors in order to increase the synthesis of beneficial compounds.
3. Interventions could be tailored to a person's unique microbiome according to information gleaned via microbial metabolite analysis.
Complex microbial metabolism, wide variations in metabolite production, and the requirement to isolate and characterize important metabolites with therapeutic potential all present difficulties. It is crucial to see the microbiota as a dynamic contribution to cancer progression due to the complex metabolic cross-talk between microbial communities and tumor cells in the TME. Microbes' metabolite production can affect anything from tumor development and angiogenesis to immune system responses. By better understanding the functions of individual microbial metabolites in the TME, novel therapeutic approaches can be developed to take advantage of these interactions and enhance cancer therapy outcomes.
Role of Microbiota in Tumor Inflammation
Cancer is characterized by persistent inflammation throughout its life cycle. While the immune response to tissue damage is critical for homeostasis, chronic and poorly controlled inflammation can promote tumor development, progression, and spread. Intriguingly, the tumor-associated microbiota has recently been found to have a key role in generating chronic inflammation inside the tumor microenvironment (TME). Insights into the complexity of cancer biology and prospective therapeutic approaches are provided by the link between microbial populations and pro-tumorigenic inflammation [
14,
15,
16,
17].
Inflammation Caused by Microbes and Cancer:
There are a number of ways in which the microbial communities that live in and around tumors might foster an inflammatory milieu.
1. Pathogen-associated molecular patterns (PAMPs) are chemicals given off by microbes and recognized by the immune system as indicators of infection or threat. Chronic inflammation can result from an immunological response triggered by PAMPs in the TME.
2. Toll-like receptors (TLRs) are immunological receptors that play a crucial function in identifying microbial components and sending out a signal when they do. The activation of toll-like receptors (TLRs) by microbial compounds can promote chronic inflammation by setting off inflammatory signaling cascades.
3. Some microorganisms release cytokines, which can cause inflammation. A pro-inflammatory milieu can be fostered by the presence of these cytokines in the TME, which can then attract immune cells.
4. The integrity of the gut barrier can be compromised by dysbiosis, an imbalance in microbial communities. As a result, microbes can enter the bloodstream and cause systemic inflammation, which in turn promotes tumor growth.
Therapeutic Implications of Microbiota in Cancer:
The paradigm in cancer research has shifted with the identification of the tumor microenvironment (TME) as a dynamic ecosystem containing not only cancer cells but also different microbial communities. This finding has prompted the investigation of cutting-edge therapeutic techniques that harness the strength of the tumor-associated microbiome to improve the success rate of cancer treatments. A variety of therapeutic interventions, from microbial-based techniques to tailored tactics that modify the microbiota, can be implemented once it is understood how microbial populations interact with the TME [
18,
19,
20,
21].
Antimicrobial Strategies:
Therapeutic approaches based on microbes take advantage of the characteristics of the microbiota seen in tumors.
1. Probiotics are live microorganisms that have been shown to have health benefits, including influencing the immune system, lowering inflammation, and slowing tumor growth. Potentially improving the TME for cancer therapy by including specific probiotics in treatment plans.
2. Compounds Derived from Bacteria: Toxins and metabolites produced by bacteria have the potential to be modified for medical use. To kill cancer cells selectively or to boost immune responses to tumors, these chemicals may be engineered to do either.
3. Synthetic microbes are possible thanks to synthetic biology, which allows for the creation of organisms with specific, desired properties. Therapeutic drugs might be delivered to the TME by engineered microorganisms, or specific immune responses could be modulated.
Enhancing Cancer Treatments through Microbiome Manipulation:
Potential adjuvants to standard cancer treatments include personalized techniques that alter the microbiome found in tumors.
1. Increased immunotherapy efficacy has been linked to the presence of specific microbial populations. The efficiency of checkpoint inhibitors and other immuntherapies may be enhanced by microbiota manipulation to produce a more immunostimulatory TME.
2. Chemotherapy efficacy may be affected by microbes' ability to digest chemotherapy medicines. Strategic microbiome manipulation has the potential to improve medication metabolism and boost the efficacy of chemotherapy.
3. Radiosensitivity: Microbiota may modify radiation therapy's results. Sensitizing tumors to radiation may require first learning how different microbial populations affect radiation responses.
Disadvantages:
There are obstacles that must be overcome notwithstanding the potential of microbial-based and microbiota-manipulated therapeutics.
1. Microbiota Variability: The make-up of microorganisms differs from person to person and even from tumor location to tumor region. Taking this variation into account is necessary for developing solutions that can be applied universally.
2. To ensure the safety and specificity of microbial-based medicines, it is essential to test them thoroughly. Treatments for cancer should be highly selective, affecting only the cells of interest (the cancerous ones) and avoiding the surrounding healthy tissue.
3. Clinical Translation: Validation, standardization, and regulatory approval are essential steps in taking microbial-based medicines from the laboratory to the clinic. Cancer treatment has been revolutionized by the discovery of novel approaches that tap into the strength of microbial communities, thanks to research into the role of microbiota in the TME. These methods, which range from microbial-based interventions to individualized manipulation of the microbiota, have the potential to both improve the efficacy of current cancer treatments and pave the way for wholly new therapeutic innovations. Although there are still obstacles to overcome, there is hope that microbiota-driven solutions can improve patient outcomes and push oncology forward.
Microbiome profiling for cancer treatment:
Cancer cells aren't the only ones shaping the tumor microenvironment (TME); the many microbial populations therein have a hand in it as well. The microbiota found in tumors differs from person to person and plays a significant influence in carcinogenesis and metastasis. Using microbiota profiling allows for a more individualized approach to cancer care, one that takes into account each patient's unique microbiome in terms of diagnosis, prognosis, and treatment [
14,
17,
21].
Prognostic and Diagnostic Value of Microbiome Sequencing
The ability to profile a patient's microbiome has diagnostic and prognostic implications across the cancer spectrum.
1. Changes in the microbiome of the tumor microenvironment (TME) may be used as a biomarker for early detection of cancer. There is hope that earlier intervention and better results are possible if certain microbial signatures linked with early-stage cancers can be detected.
2. Microbiota profiling may be useful for estimating the potential for tumor growth and metastasis. Aggressive tumor characteristics and worse prognoses have been associated to specific microbial ecosystems.
3. The risk of developing a certain kind of cancer may vary depending on the exact makeup of one's microbiome. Screening procedures and personalized preventative interventions may be aided by microbiota-based risk stratification.
4. Response to Therapy: Microbiota profiling may reveal information about an individual's response to various cancer treatments. Better responses to immunotherapy or chemotherapy have been linked to specific microbial compositions.
Individualizing Care Plans:
Microbiome profiling can help doctors create more effective, patient-specific therapies.
1. Therapies based on the microbiome may be developed if researchers knew more about a patient's unique microbiome. To improve patient outcomes, it may be necessary to introduce new bacteria or microbial products.
2. Combinatorial Approaches: Microbiota profiling may help determine which combinations of medications are most likely to be successful for a certain patient, taking into account their unique microbiome.
3. Treating patients in a way that takes into account their unique microbiome may help lessen the likelihood of unpleasant side effects. Negative reactions to treatments like chemotherapy may be lessened through microbiome manipulation.
Disadvantages:
Despite its potential, microbiota profiling-based tailored treatments must overcome several obstacles.
1. Microbiota Variability: Genetics, lifestyle, and the environment all play a role in the great range of microbial composition observed in humans. Building consistent profiles and data repositories is crucial.
2. Differentiating between causation and correlation in regards to the relationship between a person's microbiome and their risk of developing cancer is difficult. It's possible that not all observed correlations have a causal relationship.
3. Clinical Translation: Validation in broad and diverse patient populations, as well as regulatory permissions, are necessary for incorporating microbiome profiling into clinical practice. Microbiome profiling has the potential to dramatically alter the way cancer is managed by providing unique information for each patient regarding their diagnosis, prognosis, and therapy options. One step toward precision oncology, in which treatments are individualized for each patient, is the capacity to personalize therapy based on a person's microbiome. While there are still obstacles to overcome, advances in technology and research into microbiome profiling hold the possibility of improving patient outcomes and fundamentally altering the field of oncology.
Conclusion
Incorporating microbiome profiling into cancer care has the potential to radically alter how we treat cancer. New opportunities for diagnosis, prognosis, and therapy optimization have arisen from the tumor microenvironment's classification as a dynamic ecosystem shaped not just by cancer cells but also by complex microbial populations. Cancer management is changing towards more proactive and individualized approaches, and microbiome profiling has the potential to function as a diagnostic tool, allowing for earlier cancer identification and enhanced risk stratification.
The use of microbiota profiling to individualize treatment plans also represents a significant step forward in the development of precision oncology. The promise of microbiota-based interventions is exemplified by the possibility of using a patient's unique microbial composition to develop specific therapeutics and fine-tune treatment plans. These individualized methods have the potential to improve treatment outcomes while reducing unwanted side effects for cancer patients by capitalizing on interactions between microorganisms and malignancies. Despite its promise, microbiome profiling faces obstacles to widespread adoption. Research and validation are needed to establish causal links between distinct microbial communities and cancer outcomes, given the intrinsic heterogeneity of microbial composition between individuals. Further, extensive efforts are required to standardize profiling methods, create reference databases, and find one's way through regulatory routes so that these tailored approaches can be implemented in clinical practice. In conclusion, microbiota profiling has enormous potential in the treatment of cancer. Early detection, enhanced prognostication, and individualized therapy options have a greater chance of being realized as technology continues to shed light on the complexity of microbial contributions to the tumor microenvironment. By adopting this cutting-edge strategy, we take the first step on a life-changing path toward improved cancer diagnosis, treatment, and care for each individual patient.
References
- Kilian, M.; Chapple, I.L.; Hannig, M.; Marsh, P.D.; Meuric, V.; Pedersen, A.M.; et al. The oral microbiome – An update for oral healthcare professionals. British Dental Journal 2016, 221, 657–666. [Google Scholar] [CrossRef] [PubMed]
- Scotti, E.; Boue, S.; Sasso, G.L.; Zanetti, F.; Belcastro, V.; Poussin, C.; et al. Exploring the microbiome in health and disease: Implications for toxicology. Toxicology Research and Application 2017, 1, 1–37. [Google Scholar] [CrossRef]
- Gao, L.; Xu, T.; Huang, G.; Jiang, S.; Gu, Y.; Chen, F.; et al. Oral microbiomes: More and more importance in oral cavity and whole body. Protein & Cell 2018, 9, 488–500. [Google Scholar]
- Yamashita, Y.; Takeshita, T.; Takeshita, T. The oral microbiome and human health. Journal of Oral Science 2017, 59, 201–206. [Google Scholar] [CrossRef] [PubMed]
- Lane, N. The unseen world: Reflections on Leeuwenhoek (1677) ‘concerning little animals’. Philosophical Transactions of the Royal Society B: Biological Sciences 2015, 370, 20140344. [Google Scholar] [CrossRef] [PubMed]
- Malla, M.A.; Dubey, A.; Kumar, A.; Patil, A.; Ahmad, S.; Kothari, R.; Yadav, S. Using RSM and GC-MS-based metabolomics, the organophosphorus and pyrethroid degradation pathways of a new bacterial group called C3 were figured out and made better. The Taiwan Institute of Chemical Engineers publishes this journal 2021, 144, 104744. [Google Scholar] [CrossRef]
- Munot, N.; Kandekar, U.; Rikame, C.; Patil, A.; Sengupta, P.; Urooj, S.; Bilal, A. A systematic study called Improved Mucoadhesion, Permeation, and In Vitro Anticancer Potential of Synthesised Thiolated Acacia and Karaya Gum Combination. Molecules 2022, 27, 6829. [Google Scholar] [CrossRef] [PubMed]
- Munot, N.; Kandekar, U.; Giram, P.S.; Khot, K.; Patil, A.; Cavalu, S. A Study of Quercetin-Loaded Nanocochleates and Liposomes: Formulation, Characterization, Assessment of Degradation, and In Vitro Anticancer Potential. Pharmaceutics 2022, 14, 1601. [Google Scholar] [CrossRef] [PubMed]
- Manikyam, H.K.; Tripathi, P.; Patil, S.B.; Lamichhane, J.; Chaitanya, M.; et al. Hesperidin from the unripe Citrus grandis/maxima fruit Nepal variety was taken out, cleaned up, and measured. Asian Journal of Biochemistry of Natural Products 2020, 20. [Google Scholar]
- Patil, A.; Munot, N.; Patwekar, M.; Patwekar, F.; Ahmad, I.; Alraey, Y.; Alghamdi, S.; et al. Encapsulation of lactic acid bacteria by lyophilization and its effects on the bacteria's ability to live and stick together. Complementary and alternative medicine based on evidence 2022, 2022. [Google Scholar] [CrossRef] [PubMed]
- Nalawade, A.S.; Gurav, R.V.; Patil, A.R.; Patwekar, M.; Patwekar, F. A full review of the studies of the genus Chlorophytum Ker Gawl's morphological, genetic, and phytochemical variety, breeding, and bioprospecting. made in India. Trends in the study of phytochemicals 2022, 6, 19–45. [Google Scholar]
- Patil, K.G.; Balkundhi, S.; Joshi, H.; Ghewade, G. (2011). International Journal of Pharmacy and Pharmaceutical Research: MEHSANA BUFFALO MILK AS PREBIOTICS FOR GROWTH OF LACTOBACILLUS, 1(1), 114-117.
- Das, N.; Ray, A.R.; Patil, S.S.; Saini, B.; Waghmode, C.; Ghosh, P.B. Patil, and others. By causing apoptosis and going after the -catenin/Wnt pathway, chosen Indian honey stops the growth of colon cancer cells. Food and work, 13, 8283-8303.
- Patil, M.J.; Mali, V. Different Tests of the Cytotoxicity of Lactobacillus Found in Sheep Milk. Acta Scientific Pharmaceutical Sciences 2021, 5, 69–70. [Google Scholar] [CrossRef]
- Abhinandan, P.S.P.; John, D. With its ability to stick to cells, Lactobacillus plantarum has probiotic promise. Global Pharma Technology Journal 2020, 10, 1–6. [Google Scholar]
- Patil, A.; Mali, V.; Patil, R. A 6-month-old baby's banana fibres looked like a gut worm. Iberoamerican Medical Journal 2019, 2, 245–247. [Google Scholar] [CrossRef]
- Munot, N.M.; Shinde, Y.D.; Shah, P.; Patil, A.; Patil, S.B.; and Bhinge, S.D. Formulation and Evaluation of Chitosan-PLGA Biocomposite Scaffolds Incorporated with Quercetin Liposomes Made by QbD Approach for Better Healing of Oral Lesions. PharmSciTech, AAPS 2021, 24, 147. [Google Scholar] [CrossRef] [PubMed]
- Patil, A. Psychology in a time when people are dependent on technology and the Mobile Dilemma. Preprints.org 2023, 2023070101. [Google Scholar]
- Patil, A.; Kotekar, D.; Chavan, G. How probiotics affect the development and spread of cancer and how to figure out how they work. Preprints.org 2023, 2023070243. [Google Scholar]
- Vijaykumar Pawar, Abhinandan Patil, Firoj Tamboli, Dinanath Gaikwad, Dipak Mali, Anilkumar Shinde. A complete look at how AI can be used in pharmacokinetics and pharmacodynamics. PharmSciTech, AAPS 2021, 14, 426–439.
- Patil, Kulkarni, Upadhye, Jangme, Dhane. (2023). The Role of Epigenetic Modifications in Drug Response and Resistance. Journal of Pharmacological Research and Developments (e-ISSN: 2582-0117), 25-35.
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