Probiotic Lactobacillus and Promotion of Tumor Growth

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Logan Gusmano

Introduction

Lactobacillus, a Gram-Positive, non-spore forming firmicute, rod-shaped Bacilli bacteria, inhabits the gastrointestinal tract of mammalian organisms gut microbiota. Also known as Acidophilus, lactobacillus is a common probiotic that promotes the digestion and breakdown of lactose in a stable gut microbiome. https://www.mayoclinic.org/drugs-supplements-acidophilus/art-20361967

Lactobacillus is divided into two distinct categories based on metabolic pathways: Obligately Facultatively homofermentative, and Obligately heterofermentative. Both bacterium groups are classified by the unique type of products produced. Obligately Facultatively homofermentative lactobacilli refers to a group of microbes that ferment a majority of hexoses into the metabolite lactic acid. Heterofermentative Lactobacilli, on the other hand, ferments glucose producing not only lactic acid, but also ethanol and carbon dioxide.

Classified as a lactic acid bacterium (LAB, Leuconostocaceae) https://www.mayoclinic.org/drugs-supplements-acidophilus/art-20361967, Lactobacilli are capable of proliferating in environments of low pH, tolerating the pH level in the mammalian gut microbiome, and breaking down lactose into the metabolite lactic acid. Although lactobacillus is considered to be a common microbe consumed by mammals through fermented products, in novel research, it has been discovered that lactobacillus could promote detrimental effects to the host's immune defences targeting tumour proliferation. As stated by a published research paper from Vrije Universiteit Brussel, one of the byproducts of lactobacillus metabolism, lactic acid, has the biological capability of weakening the antitumor defences of mammals. https://www.sciencedaily.com/releases/2022/01/220128141333.htm

Classification and Biological Structure

Cell Wall

Gram-positive bacteria Lactobacillus contain complex structures of peptide bonded glycopolymers and integral membrane bound proteins. This consists of a multi-polymer chain of peptidoglycan sacculus that covers the inner cell membrane, and a cytoplasmic membrane regulating the osmotic pressure around the cell. These components of the membrane are significant to comprehend in order to understand the microbes capability of cell division and proliferation, ability to interact with its external environment, and ability of defending itself from external factors. However, despite a wide range of biological structure similarities between gram positive bacteria, Lactic Acid bacteria contains a cell wall that demonstrates unique properties differentiating it from other gram positive bacilli. For instance, lactobacillus’ cell wall contains several membrane receptors significant in the binding of bacteriophages, preventing significant fermentation processes in the food industry. In addition, as stated by “Cell wall structure and function in lactic acid bacteria”, they proposed the presence of Type IV pili present on Lactobacillus that act as adhesive factors, allowing it to proliferate in the gut microbiota of organisms. Lactobacillus’ adhesive properties also derive from an S-layer protein allowing it to bind to the dendritic specific site of human dendritic cells, modulating the immune response of the host. The S-layer of the microbial surface of Lactobacillus is linked by non-covalent intermolecular forces with the cell wall, in a spontaneous entropical process of reformation. However, in contrast to other gram positive bacteria, the s-layer of Lactobacillus is diminished in size, and is capable of absorbing high concentrations of Propidium Iodide. Despite having scarce research on the S-layer of lactobacillus, it has been identified of its major significance for not only its pathogenicity, but ability to proliferate and integrate in the gastrointestinal microbiota of mammals, having the ability to adhere to host cells, or extracellular proteins, as well as facilitate enzyme expression and maintain cell structure. https://link.springer.com/article/10.1007/s00253-013-4962-2 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4155827/

Peptidoglycan and Resistance

Peptidoglycan of lactobacillus, like any gram-positive bacteria, contains the majority of composition of the cell wall. The peptidoglycan sacculus consists of a repeating chain of alternating N-acetylglucosamine, and N-acetylmuramic, binded by specific peptide bonds classified as 𝛃-1,4 peptide chains. This alternating peptide polymer chain is what gives the 3-dimensional structure of the bacteria cell and ensures lactobacillus’ integrity. In lactic acid bacteria, such as lactobacillus, there is also an amino acid sequence of peptides that stem from the peptidoglycan sacculus, classified as carboxypeptidases, and endopeptidases. With an addition of D-Lac residues at the terminal ends of polysaccharide chains in the peptidoglycan layer of lactobacillus is what prompts vancomycin-resistance of the microbe, allowing it to thrive in the gut microbiota, and also be classified to have pathogenic factors, labelling it as a nosocomial infective pathogen. https://www.sciencedirect.com/science/article/pii/S107455210000116

Allochthonous vs. Autochthonous?

The clear distinction between allochthonous and autochthonous bacterium is its role in the microbiome of organisms and how it colonises the gastrointestinal tract. Lactobacillus has been identified to originate from fermented products, and are not actually considered native inhabitants of the mammalian intestinal microbiome. It has been found that despite not being a true inhabitant of the gut microbiome, lactobacillus is capable of colonising in high concentrations, adhering to the cilia surface of the epithelial lining and forming biofilms.

Metabolism and Interaction with the Environment

Overall

Lactobacillus is a member of the lactic acid bacteria that synthesises lactic acid as an end product of carbohydrate metabolism. Furthermore, Lactic Acid Bacteria, including Lactobacillus, have the capability of metabolising polysaccharides and macromolecules present in synthetic foods, that the gut microbiota of mammalian organisms is incapable of breaking down. (https://www.frontiersin.org/articles/10.3389/fbioe.2021.612285/full) Lactic acid is also capable of producing byproducts of short-chain fatty acids, amines, bacteriocins, vitamins, and exopolysaccharides during carbohydrate metabolism, other than lactic acid. The number of varying products formed by lactic acid bacteria is what allows it to be expansively used in the food industry, such as the flavour of fermented foods, reducing harmful microbes, and probiotics to improve the health of the gastrointestinal tract. In addition, lactic acid bacteria are capable of hydrolyzing proteins, synthesising viscous exopolysaccharides, and inhibiting bacterial proliferation of surrounding microbes, allowing them to be applied to biotechnological practices involved.

Degredation of Synthetic Food and Substrates in Gut Microbiota

Polysaccharides are classified as polymer chains of monomers known as monosaccharides, binded by alpha or beta glycosidic bonds. https://www.frontiersin.org/articles/10.3389/fbioe.2021.612285/full#B63 The degradation of different polysaccharides in synthetic foods by lactobacillus allows the formation of different monosaccharides, including lactic acid (the monomer of lactose), improving the quality of food. Furthermore, probiotics, including lactobacillus, is affected by the levels of polysaccharides present in the microbiome (saccharides that are classified as prebiotics). Lactic acid bacteria is also capable of transporting, degrading, and catalysing proteins present in the gut microbiota. The metabolic pathways of lactobacillus are then further divided based on chemical reactions initiated by protein expression in the bacteria This includes the expression of proteinase in the cell envelope that promotes the proteolysis in lactobacillus, degrading proteins into oligopeptides, one of the three transport systems for amino acids. The two others include dipeptide, and tripeptide transport systems. https://www.frontiersin.org/articles/10.3389/fbioe.2021.612285/full

Prebiotics and Efficiency of Lactobacillus

Prebiotics are classified as organic, or nonorganic compounds that have chemical properties that promote the proliferation and efficiency of probiotic bacteria. This includes dietary fibres and inulin, which can be considered anti-carcinogenic compounds, enhancing the capability of Lactobacillus plantarum in inhibiting carcinogenesis of tumour cells. Prebiotics can also be used as nutrients in the human intestinal tract, where lactobacillus is capable of metabolising complex carbohydrates that are usually non-binding by mammals enzymes.

Impact on Cancer and Chemotherapies=

General

Probiotic is a term that has originated from Greek roots, meaning “for life.” The term was initially used by Lilley and Stillwell in 1965 for fermented products such as cheese, bread and wine, for their medical properties. It has then been identified that probiotics are sourced from a varying degree of microbes including fungus and bacteria. The most commonly used probiotics are lactic acid bacteria, including Lactobacillus. Probiotics have been researched to treat several health disorders and diseases, such as inflammation, diarrhoea, obesity, urogenital infections, but most recently, cancers. Probiotics, including lactobacillus, have the capability of regulating cancer signalling. Through regulation of the cell cycle, including promotion of apoptosis of tumour cells, inhibition of mutagenic activity, or inhibition of oncogene expression. Probiotics promote the apoptosis of tumorous cells, initiating the fragmentation of DNA, reduction of cytoplasm and solutions in the cell, and prevention of lysis, by modulating specific expression of Bax/Bcl2, resulting the formation of small pores around the cell membrane, inducing the programmed death of the cell. Probiotics such as Lactobacillus acidophilus and Bifidobacterium bifidum have the ability of increasing the mRNA expression of hBD-2 genes inducing apoptosis of cancer cells, and promoting complete cell cycle arrest. These probiotic properties are based on the structure, metabolism, and cell communication expressed by the bacterial organism, also known as metabiotics. Metabiotics of probiotic organisms such as lactobacillus can be divided based on the structural components, and metabolic components. Lactobacillus is capable of inducing apoptosis of carcinogenic cells, act as antimutagens preventing lethal mutations in cell DNA, and a reactivator of Tumour suppressor reactivators.

Colorectal Cancer

According to Lactobacillus gallinarum modulates the gut microbiota and produces anti-cancer metabolites to protect against colorectal tumorigenesis, a primary research paper published by Naoki Sugimura, through metagenomic sequencing of lactobacillus gallinarum in mice diagnosed with colorectal cancer, they were able to identify a significant correlation between lactobacillus populations in the gut microbiota and number of tumours present. Using mice as a model organism, Naoki Sugimura and a team of researchers were able to identify the anticarcinogenic properties of lactobacillus, reducing size significantly in the number of tumours in the intestine. They tested this by either injecting the mouse with MG1655 E. Coli, or Lactobacillus gallinarum, to identify the effects of microbiome bacteria as probiotics. With a colonoscopy are they able to identify the state of the test subject's gastrointestinal tract and identify the number of neoplastic lesions (lesions caused by abnormal cell growth.) Their research concluded with identifying Lactobacillus’ possible use as a probiotic in order to inhibit the synthesis of intestinal tumours and treat colorectal cancer. Lactobacillus as a probiotic has been identified to be capable of cytotoxic and anticarcinogenic effects on cervical, gastric, colon, melanoma, and breast cancer. With mice as a model organism for mammals, scientists are able to identify how lactobacillus impacts the activation/enhancing of immunal responses, as well as directly attack cancer cells, and promote arrest in the cell cycle. https://gut.bmj.com/content/early/2021/12/21/gutjnl-2020-323951

Cervical Cancer

As stated by Anti-tumor activities of probiotics in cervical cancer, Cervical cancer is considered to be a critical global health issue, classified to be the fourth common disease affecting women worldwide. Prior to the new concept of using probiotics, such as lactobacillus, to treat cervical cancer, there was a multi step process including invasive surgeries, radiation, and chemotherapy, in hopes of treating the cancer. The most common treatment was concurrent chemoradiation, with a combination of prescribed anticarcinogenic medication. Only 40% of the treatments were completely successful. Probiotics, including L. Crispatus, L jensenii, and L. gasseri has shown to prevent the expression of CDK2, cyclin A, and HPV oncogenes, responsible for cervical cancer, and promoted anticancer activities against specifically HeLa cervical cancer. Lactobacillus also is capable of promoting the upregulation of apoptotic genes, including caspase3, caspase8, caspase9, BAX and BAD, promoting the programmed cell death of tumour cells, and acting as a anti-carcinogen.

Breast Cancer

In breast cancer, it has been discovered that long-term administration of the lactic acid bacteria strain: Lactobacillus plantarum, has shown effective anticarcinogenic effects, limiting the amount of tumour necrosis factors alpha in breast cancer, and increasing the number of CD4+ T-cells to combat carcinogenic infections. According to Wang et. Al Inhibitory effect of vaginal Lactobacillus supernatants on cervical Cancer cells, 3 strains of Lactobacillus (crispatus, jensenii, gasseria), has the capability of inhibiting the growth of Caski cells, altering its physical structure, decreasing the number of tumour cells present that lack tumour suppressors, and expression of oncogenes. Furthermore, Lactobacillus has the ability to mute expression of CDL2 and Cyclin A, decreasing the overall expression of HPV oncogenes, and decreasing the number of tumour cells.

Impact on Gut-Brain Axis and Alzheimers

The gut-microbiota has been identified to have the capability of moderating brain function and expression, through an interconnected signal system known as the gut-brain axis. The gut-brain axis is a signal mainly transmitted from the enteric and central nervous system through the vagal nerve through several different neurotransmitters such as serotonin, dopamine, GABA, and glutamate. Each neurotransmitter can be enhanced through expression of probiotics. Because of Lactobacillus’ beneficial impact on the gut microbiome of mammalian organisms, it has been researched how Lactobacillus as a probiotic supplement could be applied to alter the gut brain axis and help contribute to treatments for cognitive disorders such as Alzheimers. Alzheimers is a progressive chronic condition that is caused by A𝛃 plagues, inducing neuron damage and loss. Different treatments for Alzheimers focuses on promoting Tramiprosate and ALZ-801, that inhibit the synthesis of A𝛃 plagues. It also has been shown that a loss of biodiversity in the gastrointestinal tract, can lead to the development of Alzheimer's disease, altering the homeostasis of the brain's neurotransmitters and damage of the gut brain axis. This is caused by impairs in the gastrointestinal epithelial layer, inducing inflammation of neurons in the gut brain axis, and accelerating Alzheimers. Lactobacillus is capable of suppressing inflammation of the neurons through promotion of the synthesis of indole-3-aldehyde, and indole-3-propionic acid, which is transferred through the Brain Blood Barrier. https://microbiologyjournal.org/impact-of-gut-microbiome-lactobacillus-spp-in-brain-function-and-its-medicament-towards-alzheimers-disease-pathogenesis/

References

Authored for BIOL 238 Microbiology, taught by Joan Slonczewski, 2022, Kenyon College