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 classified into 2 distinct classification groups based on metabolic pathways: Obligately Facultatively homofermentative, and Obligately heterofermentative. Each group is unique in the 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 defenses targeting tumor 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 defenses of mammals. https://www.sciencedaily.com/releases/2022/01/220128141333.htm

Classification and Biological Structure

Cell Wall

Gram-positive bacteria, such as 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. 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, labeling it as a nosocomial infective pathogen. https://www.sciencedirect.com/science/article/pii/S107455210000116

Metabolism and Interaction with the Environment

Lactobacillus is a member of the lactic acid bacteria that synthesizes lactic acid as an end product of carbohydrate metabolism. Furthermore, Lactic Acid Bacteria, including Lactobacillus, have the capability of metabolizing 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 flavor 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, synthesizing viscous exopolysaccharides, and inhibiting bacterial proliferation of surrounding microbes, allowing them to be applied to biotechnological practices involved.

References

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