MCB 100 Exam 2 Lecture 12
Intracellular versus Extracellular Bacteria
Intracellular Pathogens:
Definition: Microbes that can enter and reside inside host cells.
Characteristics:
Can evade the host cell immune system.
Can avoid mechanisms intended to kill them.
Extracellular Pathogens:
Definition: Microbes that reside outside or on the surface of host cells or in the surrounding medium.
Characteristics:
Often produce toxins and other enzymes to combat the host immune system for survival.
Acne: Skin Infection with Normal Microbiota
Major Resident Microbes:
Gram (+) Bacteria:
Staphylococcus epidermidis: Commonly found in pores.
Propionibacterium acnes (P. acnes):
Classification: Gram-positive, anaerobic bacillus.
Habitat: Colonizes sebaceous (fat) glands.
Function: Consumes oils in sebum, generating a low pH that protects against other microbes.
Comedone Formation:
Definition: A skin condition caused by dried sebum plugging hair follicles.
Blackhead: Open plug formed by air-oxidized sebum.
Whitehead: Closed plug due to skin covering the follicle.
Consequences: Anaerobic conditions enhance bacterial growth, leading to infections and inflammation manifesting as acne.
Manifestations of Acne:
Typical symptoms include large pimples (referred to as ‘zits’), and in severe cases, cysts or boils.
Treatment:
Common methods include antibiotics and/or benzoyl peroxide targeting P. acnes and S. epidermidis.
Extracellular Bacterial Pathogens
Gastritis, Ulcers, and Cancer: Helicobacter pylori
Helicobacter pylori:
Classification: Helical-shaped, motile (4-6 flagella), Gram-negative bacterium.
Adaptation: Survives in the harsh acidic environment of the stomach (pH 2) by:
Using flagella to penetrate the mucus lining to reach epithelial cells.
Producing urease to degrade urea into carbon dioxide (CO2) and ammonia (a base) to neutralize acidity.
Tissue Penetration and Dissemination
Pathogenic Bacteria Spreading Factors:
Many produce enzymes that facilitate spread within body tissues.
DNases:
Function: Degrade DNA in pus, making the environment less viscous and facilitating pathogen movement.
Hyaluronidases:
Type: Glycosidases that degrade hyaluronic acid in connective tissue, allowing bacterial spread.
Collagenases and Elastases:
Function: Proteases that degrade connective tissue proteins, facilitating dissemination.
Plasminogen Activator-like Proteases:
Function: Degrade fibrin clots, aiding bacterial escape from blood clots.
Streptokinase:
Secreted by Streptococcus species; it cleaves and activates host serine protease plasminogen to degrade fibrin.
Example: Staphylococcus aureus (MRSA)
Pathogenicity:
Can cause serious infections including toxic shock syndrome and necrotizing fasciitis (flesh-eating skin infections).
Enzyme Production:
Produces various proteolytic enzymes and toxins that promote its spread:
Staphylokinase: Dissolves blood clots.
Hyaluronidase: Degrades hyaluronic acid.
V8 Protease: Cleaves human immunoglobulin G (IgG).
Exfoliative Toxins: Causes scalded skin syndrome leading to blistering and peeling of the skin, allowing bacteria to further invade.
Other conditions include impetigo, toxic shock syndrome, and necrotizing fasciitis.
Survival in the Host: Nutrient Acquisition
General Mechanism:
Pathogens secrete enzymes that degrade host macromolecules for nutrient absorption.
Types of Enzymes:
Glycosidases: Hydrolyze sugars/polysaccharides.
Neuraminidases/Sialidases: Cleave sialic acid from glycolipids or glycoproteins.
Hyaluronidases: Degrade hyaluronic acid in connective tissue.
Proteases: Degrade proteins; collagenases and elastases act as “meat tenderizers” for connective tissue and immunoglobulins.
Nucleases: Degrade DNA/RNA in pus from damaged host cells.
Phospholipases: Cleave lipids in membranes, lysing host cells and releasing nutrients.
Toxins: Some pathogens secrete toxins that disrupt host cells to release cytosolic contents.
Example: Brucella abortus
Classification: Gram-negative bacterium.
Disease: Causes brucellosis, resulting in spontaneous abortion in domestic animals and occasionally in humans.
Growth Characteristics: Grows slowly in most tissues but rapidly in the placenta due to high erythritol concentration, which it metabolizes efficiently.
Iron Acquisition
Iron Availability:
Free iron concentrations are very low ( to M, varying by body site) due to specific proteins binding it:
Lactoferrin: Secreted at mucosal surfaces and in milk.
Transferrin: Synthesized in the liver and found in serum.
Ferritin: Stores iron intracellularly.
Heme: Approximately 70% of total human iron is in hemoglobin.
Bacterial Strategies for Iron Acquisition:
Many bacteria secrete siderophores (catechols or hydroxamates) with high iron affinity.
The iron-siderophore complex is internalized by bacteria using specific receptors, releasing iron upon entry.
Some bacteria utilize receptor-mediated uptake of transferrin, lactoferrin, ferritin, or hemoglobin, or degrade them to release iron ().
Notably, Borrelia burgdorferi uses instead of iron.
Some bacteria produce cytolytic toxins that lyse host cells to release iron, generally regulated by available iron.
Normal Clearance of Pathogens from Body
Most pathogens are effectively cleared from the host.
Obligate Intracellular Bacteria
Two Forms:
Infectious Elementary Body (EB): Metabolically inactive.
Noninfectious Reticulate Body (RB): Metabolically active, reproduces, later transforms into EB for release.
Salmonella enterica – Food-borne Pathogen
Overview:
Causes Salmonellosis, a significant disease in humans and animals.
Serovars: Multiple strains; e.g., S. enterica serovar Typhi causes typhoid fever, S. enterica serovars Typhimurium and Enteritidis are responsible for gastroenteritis.
Epidemiology:
Responsible for approximately 30% of food-borne deaths in the USA, with around 1.4 million documented cases, 16,000 hospitalizations, and 600 deaths annually.
Infection Mechanism:
Enters macrophages, preventing phagolysosomal fusion, allowing survival and replication within specialized Salmonella-containing vacuoles.
Intracellular Bacterial Pathogens
Mycobacterium tuberculosis
Strategy: Binds to CR3 on macrophage surfaces, taken up in vesicles.
Brucella abortus:
Recruits host proteins to the phagosome surface, preventing lysosomal fusion.
Prevents endocytic acidification and has reduced oxidative burst; bacteria replicate within the vacuole.
Listeria monocytogenes – Food-borne Pathogen
Overview:
Causes listeriosis, which is a severe disease in humans and animals.
Mechanism of Action:
Upon host cell entry, secretes pore-forming and membrane-degrading toxins to escape into the cytosol.
Survival Strategies:
Utilizes cell-to-cell spread through actin-based motility, allowing rapid dissemination without exposure to the host's humoral immune system.
Intracellular versus Extracellular Bacteria
Intracellular Pathogens:
Definition: Microbes that have the unique ability to enter, reside, and often replicate inside host cells.
Characteristics:
Immune Evasion: By residing within host cells, they can effectively hide from circulating antibodies, complement proteins, and phagocytes in the bloodstream or extracellular spaces.
Survival Mechanisms: They can avoid or neutralize host defense mechanisms intended to kill them, such as lysosomal fusion or reactive oxygen species (ROS) within phagocytic cells.
Extracellular Pathogens:
Definition: Microbes that primarily reside outside or on the surface of host cells, either in tissue fluids, the bloodstream, or on mucosal surfaces.
Characteristics:
Immune Combat: Often produce a wide array of toxins (e.g., exotoxins, endotoxins) and other enzymes (e.g., proteases, nucleases) to directly combat and often overwhelm host immune responses for their survival and dissemination.
Acne: Skin Infection with Normal Microbiota
Major Resident Microbes:
Gram (+) Bacteria:
Staphylococcus epidermidis: A common commensal found in pores and on the skin surface, which can become opportunistic under certain conditions.
Propionibacterium acnes (P. acnes):
Classification: A Gram-positive, anaerobic bacillus.
Habitat: Primarily colonizes lipid-rich sebaceous (fat) glands within hair follicles.
Function: Consumes sebum, particularly triglycerides, breaking them down into free fatty acids. This metabolic activity of P. acnes contributes to creating a low pH on the skin surface, which historically was thought to protect against colonization by other, potentially more harmful microbes. However, its own metabolic byproducts can also be proinflammatory.
Comedone Formation:
Definition: A non-inflammatory skin lesion, a primary sign of acne, formed when dried sebum, keratinocytes, and bacteria plug a hair follicle.
Blackhead: An open comedone where the follicular plug is exposed to air, leading to the oxidation of melanin and lipids, which gives it a dark appearance.
Whitehead: A closed comedone where the follicular plug remains covered by a thin layer of skin, trapping sebum and bacteria beneath the surface.
Consequences: The blocked follicle creates an anaerobic microenvironment, which highly favors the proliferation of P. acnes. The subsequent breakdown of sebum and bacterial byproducts triggers an inflammatory response, leading to the characteristic red, swollen lesions of acne, including papules, pustules, nodules, and cysts.
Manifestations of Acne: Typical symptoms include the formation of large inflammatory lesions such as papules (small, red bumps), pustules (pus-filled lesions or ‘zits’), and in severe cases, deep, painful cysts or boils (nodules) that can lead to scarring.
Treatment: Common methods include topical or systemic antibiotics (e.g., tetracyclines, erythromycin) to reduce bacterial load (specifically targeting P. acnes and S. epidermidis) and/or topical retinoids and benzoyl peroxide. Benzoyl peroxide works by releasing oxygen into the follicle, killing anaerobic bacteria, and also has mild keratolytic effects, helping to unclog pores.
Extracellular Bacterial Pathogens
Gastritis, Ulcers, and Cancer: Helicobacter pylori
Helicobacter pylori:
Classification: A helical-shaped, highly motile (possessing 4-6 polar flagella), Gram-negative bacterium.
Adaptation: Exhibits remarkable survival strategies in the extremely harsh acidic environment of the stomach (with a pH as low as 2):
Mucus Penetration: Utilizes its powerful flagella to rapidly propel itself through the viscous, less acidic gastric mucus layer, allowing it to reach the more neutral pH environment near the epithelial cell surface.
Acid Neutralization: Produces large amounts of the enzyme urease, which catalyzes the hydrolysis of urea (present in gastric juice) into carbon dioxide () and ammonia (). Ammonia is a potent base that effectively neutralizes the surrounding stomach acid, creating a protective alkaline microenvironment around the bacterium. This localized pH modification is crucial for its colonization and survival.
Pathogenesis: Persistent colonization can lead to chronic gastritis, peptic ulcers, and is a major risk factor for gastric adenocarcinoma and MALT lymphoma.
Tissue Penetration and Dissemination
Pathogenic Bacteria Spreading Factors: Many pathogenic bacteria produce an array of extracellular enzymes that degrade host tissues, thereby facilitating their spread within body tissues and allowing them to access deeper sites or the bloodstream.
DNases (Deoxyribonucleases):
Function: These enzymes degrade extracellular DNA released from dead or dying host cells, particularly abundant in pus (which is composed of dead neutrophils and host cell debris). By breaking down the long, sticky DNA strands, DNases reduce the viscosity of pus and other exudates, enabling the pathogen to move more freely through the tissue and disseminate.
Hyaluronidases:
Type: These are glycosidases that specifically degrade hyaluronic acid.
Function: Hyaluronic acid is a major component of the extracellular matrix (ECM) and serves as a 'cement' that holds cells together in connective tissue. By hydrolyzing hyaluronic acid, hyaluronidases break down this intercellular cement, increasing tissue permeability and allowing bacterial cells to spread more easily through host tissues.
Collagenases and Elastases:
Function: These are specific proteases that degrade collagen and elastin, respectively. Collagen is the most abundant protein in connective tissue, providing structural integrity, while elastin provides elasticity. By degrading these crucial connective tissue proteins, bacteria can break down physical barriers, facilitating deeper tissue invasion and dissemination.
Plasminogen Activator-like Proteases:
Function: These enzymes activate host plasminogen into plasmin, a broad-spectrum protease that degrades fibrin clots. Fibrin clots are host defenses designed to wall off infections and prevent pathogen spread. By degrading these clots, bacteria can effectively escape localized infection sites and disseminate into the bloodstream.
Streptokinase:
Secretion: Secreted primarily by certain Streptococcus species (e.g., S. pyogenes).
Mechanism: It acts as a plasminogen activator. Streptokinase binds to and cleaves host plasminogen, converting it into active plasmin. Plasmin then degrades fibrin, breaking down blood clots that could otherwise entrap and localize the bacteria, allowing for systemic spread.
Example: Staphylococcus aureus (MRSA)
Pathogenicity: Staphylococcus aureus, including its antibiotic-resistant strains like Methicillin-Resistant S. aureus (MRSA), is a highly versatile pathogen capable of causing a wide range of serious infections, from superficial skin infections to life-threatening systemic diseases like toxic shock syndrome, bacteremia, endocarditis, and severe necrotizing fasciitis (often referred to as 'flesh-eating' skin infections).
Enzyme Production: S. aureus produces a formidable arsenal of proteolytic enzymes and toxins that collectively promote its survival, tissue invasion, and dissemination:
Staphylokinase: Similar to streptokinase, it activates plasminogen to plasmin, leading to the dissolution of blood clots and facilitating bacterial spread from localized lesions.
Hyaluronidase: Degrades hyaluronic acid in the extracellular matrix, breaking down connective tissue barriers and allowing the bacteria to penetrate deeper into tissues.
V8 Protease (Staphylococcus aureus metalloprotease): A highly specific protease that cleaves human immunoglobulin G (IgG), thereby impairing the host's antibody-mediated immune response.
Exfoliative Toxins (ETA and ETB): These serine proteases target desmoglein-1, a protein involved in cell-to-cell adhesion in the superficial layers of the epidermis. Their action leads to the characteristic blistering and peeling of the skin seen in conditions like staphylococcal scalded skin syndrome (SSSS), creating portals for further bacterial invasion and dissemination.
Other conditions: Beyond SSSS and necrotizing fasciitis, S. aureus is a leading cause of impetigo (a superficial skin infection), folliculitis, carbuncles, and furuncles.
Survival in the Host: Nutrient Acquisition
General Mechanism: Pathogens, both extracellular and intracellular, must acquire essential nutrients from the host to survive and replicate. They frequently secrete a diverse array of enzymes that degrade host macromolecules into smaller, absorbable components.
Types of Enzymes:
Glycosidases: These enzymes hydrolyze glycosidic bonds in complex carbohydrates (sugars/polysaccharides), breaking them into simpler sugars that can be metabolized by the bacterium.
Neuraminidases/Sialidases: These specific glycosidases cleave sialic acid residues from glycolipids or glycoproteins on host cell surfaces. Sialic acid is often a component of host cell receptors or masks antigens; its removal can expose underlying host structures for binding or release nutrients.
Hyaluronidases: As previously mentioned, they degrade hyaluronic acid in connective tissue, not only aiding spread but also potentially releasing sugars for nutrient acquisition.
Proteases: A broad class of enzymes that degrade proteins into peptides and amino acids. Collagenases and elastases specifically target structural proteins of connective tissue, acting as “meat tenderizers” to facilitate tissue breakdown and nutrient release. Other proteases can degrade immunoglobulins (antibodies) and complement proteins, interfering with immune defenses while providing amino acids.
Nucleases (DNases/RNases): Degrade host DNA and RNA, particularly abundant in areas of host cell damage or in pus, releasing nucleotides and nucleosides that can be used for bacterial nucleic acid synthesis.
Phospholipases: These enzymes cleave ester bonds in phospholipids, the main components of host cell membranes. By degrading membrane lipids, phospholipases can lyse host cells, releasing their cytosolic contents, which are rich in nutrients.
Toxins: Some pathogens secrete cytolytic toxins (e.g., hemolysins, leukocidins) that directly disrupt the integrity of host cell membranes, causing cell lysis and releasing the cell's rich cytosolic contents for bacterial scavenging.
Example: Brucella abortus
Classification: A Gram-negative, facultative intracellular bacterium.
Disease: The causative agent of brucellosis, a zoonotic disease that frequently leads to spontaneous abortions in domestic animals (cattle, goats, sheep) and can cause undulating fever and systemic illness in humans.
Growth Characteristics: While it can grow slowly in various host tissues, B. abortus exhibits particularly rapid growth in the placenta. This accelerated replication is attributed to the high concentration of erythritol (a sugar alcohol) present in the placenta of susceptible animals, which B. abortus can efficiently metabolize as a carbon and energy source, thus creating a preferential niche for bacterial proliferation.
Iron Acquisition
Iron Availability: Iron is an essential cofactor for many bacterial enzymes and metabolic processes. However, free iron concentrations in the human body are meticulously controlled and kept extremely low (ranging from M to M, depending on the body site) to limit pathogen growth. This is achieved through specific host proteins that tightly bind iron:
Lactoferrin: An iron-binding protein secreted at mucosal surfaces (e.g., saliva, tears, breast milk) and found in neutrophils, acting as a bacteriostatic agent by sequestering iron.
Transferrin: The primary iron-transport protein in serum, synthesized in the liver, which binds two ions and transports iron throughout the body.
Ferritin: An intracellular protein primarily responsible for iron storage within cells.
Heme: Approximately 70% of total human iron is incorporated into heme within hemoglobin (in red blood cells) and myoglobin.
Bacterial Strategies for Iron Acquisition: To overcome this iron scarcity, bacteria have evolved sophisticated mechanisms:
Siderophores: Many bacteria secrete low-molecular-weight organic compounds called siderophores (e.g., catechols or hydroxamates). These molecules have an extremely high affinity for ferric iron ()—significantly higher than host iron-binding proteins. The siderophore-iron () complex is then specifically recognized and internalized by bacteria via dedicated outer membrane receptors, after which the iron is released inside the bacterial cytosol, often by reduction to or enzymatic degradation of the siderophore.
Receptor-mediated Uptake/Degradation: Some bacteria directly express surface receptors that bind to host iron-binding proteins like transferrin, lactoferrin, or ferritin, or even hemoglobin. They then internalize these complexes or proteolytically degrade them on the cell surface to release iron () for uptake.
Alternative Metals: Notably, Borrelia burgdorferi, the causative agent of Lyme disease, has adapted its metabolism to use manganese () instead of iron for many enzymatic functions, thereby circumventing the host's iron limitation strategy.
Cytolytic Toxins: Some bacteria produce cytolytic toxins (e.g., hemolysins) that lyse host cells (like red blood cells), releasing iron-containing compounds (e.g., heme, hemoglobin) which the bacteria can then scavenge. The production of these toxins is often tightly regulated by the concentration of available iron, increasing when iron is scarce.
Normal Clearance of Pathogens from Body
Most pathogens are confronted by a robust and multi-layered host immune system (innate and adaptive) and are effectively cleared from the host before establishing a significant infection or causing severe disease.
Obligate Intracellular Bacteria
Definition: A distinct group of bacteria that cannot reproduce outside of host cells and rely on host cellular machinery for essential nutrients and energy.
Two Forms (e.g., Chlamydia, Rickettsia):
Infectious Elementary Body (EB): This is the metabolically inactive, robust, and environmentally stable form of the bacterium. EBs are specialized for extracellular survival and host-cell attachment and entry, representing the infectious stage that is taken up by the host cell.
Noninfectious Reticulate Body (RB): Once inside the host cell, the EB transforms into the metabolically active and replicative Reticulate Body. RBs actively grow and reproduce by binary fission within a membrane-bound vacuole (inclusion body) inside the host cell. Later, RBs differentiate back into EBs, which are then released from the cell to infect new host cells.
Salmonella enterica – Food-borne Pathogen
Overview: Salmonella enterica is a highly significant food-borne pathogen responsible for leading causes of bacterial gastroenteritis (salmonellosis) in humans and animals, as well as more severe systemic diseases.
Serovars: Comprises over 2,500 serovars, with distinct disease presentations. For example:
S. enterica serovar Typhi: The causative agent of typhoid fever, a life-threatening systemic illness characterized by high fever, malaise, and often leads to prolonged bacteremia and invasion of lymphoid tissues.
S. enterica serovars Typhimurium and Enteritidis: These are the primary serovars responsible for most cases of acute gastroenteritis (non-typhoidal salmonellosis), characterized by diarrhea, abdominal cramps, and fever.
Epidemiology: Globally, Salmonella causes millions of illnesses annually. In the USA, it is responsible for a significant burden of food-borne disease, with approximately 1.4 million documented cases, leading to around 16,000 hospitalizations and 600 deaths annually.
Infection Mechanism: After ingestion, Salmonella invades host cells, particularly macrophages and epithelial cells. Within macrophages, it employs sophisticated mechanisms to survive and replicate inside a specialized membrane-bound compartment called the Salmonella-containing vacuole (SCV). It actively prevents the fusion of the SCV with destructive lysosomes, thereby avoiding degradation and creating an intracellular niche for replication and persistence. This allows the bacteria to effectively evade intracellular killing mechanisms and contribute to systemic infection.
Intracellular Bacterial Pathogens
Mycobacterium tuberculosis
Strategy: Mycobacterium tuberculosis, the pathogen causing tuberculosis, initially binds to complement receptor 3 (CR3) (and other receptors like mannose receptors) on the surface of macrophages, which are its primary host cells. It is then taken up by phagocytosis into membrane-bound vesicles (phagosomes).
Survival/Replication: Within the macrophage, M. tuberculosis actively prevents the maturation of the phagosome into a phagolysosome by arresting lysosomal fusion and acidification. This allows the bacteria to survive and replicate within an early endosomal compartment inside the macrophage, avoiding the harsh antimicrobial environment of a mature phagolysosome.
Brucella abortus
Intracellular Survival: Brucella abortus is a classic facultative intracellular pathogen. After being internalized by phagocytic cells (macrophages), it employs sophisticated mechanisms to subvert host defenses.
Phagosome Modulation: It actively recruits various host proteins (e.g., components of the endoplasmic reticulum) to the surface of its phagosome, which critically prevents the normal fusion of the phagosome with lysosomes. This creates a modified membrane-bound compartment that is non-fusogenic with the lysosomal system.
Reduced Host Response: It also actively prevents endocytic acidification within its vacuole and has mechanisms to reduce the oxidative burst (production of reactive oxygen species) by the host cell. These combined strategies create a protected, permissive environment where the bacteria can survive and replicate within the host cell vacuole without being subjected to the full microbicidal assault of the phagolysosome.
Listeria monocytogenes – Food-borne Pathogen
Overview: Listeria monocytogenes is a rod-shaped, Gram-positive, facultative anaerobic bacterium. It is the causative agent of listeriosis, a severe food-borne disease that can cause meningitis, meningoencephalitis, septicemia, and abortion, particularly in immunocompromised individuals, pregnant women, and neonates.
Mechanism of Action: Listeria is a master of intracellular pathogenesis. Upon entry into a host cell (e.g., by invasion of non-phagocytic cells or uptake by phagocytes), it rapidly escapes the phagosome.
Pore-forming Toxins: It secretes the pore-forming toxin Listeriolysin O (LLO) and two phospholipases (PlcC and PlcB). LLO creates pores in the phagosomal membrane, while the phospholipases degrade the membrane, allowing the bacterium to escape into the host cell's nutrient-rich cytosol.
Survival Strategies:
Cytosolic Replication: Once in the cytosol, Listeria replicates rapidly.
Actin-based Motility: To spread efficiently without exposing itself to extracellular immune components, Listeria utilizes host cell actin polymerization. It expresses a surface protein called ActA, which hijacks the host cell's actin polymerization machinery, forming an actin 'comet tail' behind the bacterium. This propels Listeria directly into adjacent host cells, forming pseudopod-like protrusions which are then engulfed by neighboring cells. This cell-to-cell spread allows for rapid dissemination throughout tissues (e.g., from intestinal cells to hepatocytes to brain cells) while remaining hidden from the host's humoral immune system (antibodies) and complement.