The phagocyte microbicidal system provides bacteriostatic and bactericidal actions, protecting the body against pathogens. It is divided into two key subsystems: oxygen-dependent and oxygen-independent.
Two subsystemsOxygen-dependent and oxygen-independent
MyeloperoxidaseAzurophilic granules of neutrophils and macrophage lysosomes
Respiratory burstProcess of reactive oxygen species (ROS) generation
LysozymeDestroys the muramic acid of the bacterial cell wall
Oxygen-Dependent Subsystem
This part of the microbicidal system relies on reactive oxygen species and enzymes generated during phagocytosis.
Reactive Oxygen Species (ROS): Formed during the respiratory burst (oxidative stress). These include superoxide radicals, hydrogen peroxide, singlet oxygen, and hydroxyl radicals, which exhibit high bactericidal efficacy.
Myeloperoxidase: Located in azurophilic granules of neutrophils and lysosomes of monocytes/macrophages. Its activity increases sharply in the presence of hydrogen peroxide and halides, leading to halide oxidation, chlorination, and destruction of bacterial envelopes.
Catalase: Reacts with hydrogen peroxide and halides to form strong oxidants. Together with myeloperoxidase, it destroys bacteria, viruses, fungi, and mycoplasmas.
Oxygen-Independent Subsystem
Includes a number of factors and compounds that function independently of oxygen metabolism:
Lysozyme (muramidase): Cleaves muramic acid in the peptidoglycan of the cell wall. Gram-positive microorganisms are most sensitive to it.
Lactoferrin: Localized in phagosomes. In its unsaturated state, it binds (chelates) iron, depriving bacteria of an essential growth factor and providing a bacteriostatic effect.
Cationic proteins: Exert bactericidal action primarily against Gram-positive microbes within phagolysosomes.
Acidosis (pH 4.0–6.5): The creation of an acidic environment suppresses the surface charge of bacteria and inhibits membrane processes. An excess of hydrogen ions promotes the formation of nitrites, chloramines, aldehydes, and singlet oxygen, as well as increases the permeability of lysosomal membranes.
Enzymatic Digestion
The final stage of destroying engulfed microorganisms involves intracellular structures.
Lysosomal hydrolases: More than 30 types of these enzymes are present in primary lysosomes in an inactive state.
Activation: Increases sharply under conditions of acidosis during phagocytosis.
Function: Carry out complete degradation of microbial components down to elementary compounds — amino acids, peptides, fatty acids, and nucleotides.
Mnemonic
Oxygen mechanisms work via the respiratory burst and enzymes (myeloperoxidase, catalase), while oxygen-independent mechanisms rely on acidosis, lysozyme, lactoferrin, and cationic proteins.
Frequently asked questions
Which specific cationic proteins provide oxygen-independent bactericidal activity?
Oxygen-independent bactericidal activity of phagocytes is provided by cationic proteins.
These include:
Serprocidins — cathepsin G, elastase, azurocidin, and proteinase-3; act together synergistically.
Lysozyme — destroys the bacterial cell wall.
Lactoferrin — binds iron ions, creating a deficiency for microorganisms; primary action is bacteriostatic.
BPI proteins (Bactericidal/Permeability-Increasing protein) — cationic proteins with bactericidal activity that increase the permeability of the bacterial wall.
What are the two main subsystems of the phagocyte microbicidal system?
The oxygen-dependent and oxygen-independent subsystems, each utilizing a unique set of enzymes and chemical factors.
Where is myeloperoxidase localized?
Myeloperoxidase is located in the azurophilic granules of neutrophils and the lysosomes of monocytes/macrophages.
What is the mechanism of action of lactoferrin?
Lactoferrin in phagosomes is unsaturated with iron ions and chelates (binds) iron, depriving bacteria of a factor necessary for their growth.
Go deeper
Age-related features of lysozyme content
Susceptibility spectrum of various microorganisms to lysozyme
Effect of acidosis on lysosomal membrane permeability