BSCI 20021: Host-Microbe Interactions, Pathogenesis, Transmission, and Antimicrobials

Comprehensive vocabulary flashcards covering basic and clinical principles of host-microbe interactions, virulence factors, toxins, antimicrobial mechanisms, and the chain of disease transmission.

Pathogenicity

The ability of a microbe to cause disease, representing a binary state where an organism either has the potential to cause disease or cannot.

Virulence

The degree or extent of disease caused by a pathogen, which is a comparative measure determined by both microbial factors and host susceptibility.

Virulence Factor

Any trait or molecule produced by a pathogenic microbe that enhances its ability to adhere, invade tissues, acquire nutrients, or evade the host immune system.

Cytokine Storm

A severe, hyper-potent immune reaction characterized by an excessive release of cytokines that can cause significant damage to host tissues; a major cause of mortality in young, immunocompetent individuals during the 1918 influenza A pandemic.

Infectious Dose ($ID_{50}$)

An objective measure of virulence defined as the number of microbial cells or virions required to establish infection in $50\%$ of exposed hosts; a lower value indicates higher infectivity.

Lethal Dose ($LD_{50}$)

An objective measure of toxin or pathogen potency defined as the dose or number of organisms required to kill $50\%$ of infected host organisms; lower values indicate greater virulence and lethality.

Endotoxin

A toxin that is an intrinsic structural component of the outer membrane of Gram-negative bacteria, consisting of lipopolysaccharide (specifically Lipid A), which is released upon cell lysis or death.

Exotoxin

A toxic, soluble protein synthesized inside a living microbial cell and actively secreted into the surrounding environment or released upon cell lysis.

Type 1 Exotoxin (Cell-Surface-Active Toxin)

A class of exotoxin that binds to specific receptors on host cell plasma membranes without entering the cell, triggering intracellular signaling cascades that alter cellular physiology (e.g., Escherichia coli enterotoxin).

Superantigen

A type 1 exotoxin that directly cross-links MHC class II molecules on antigen-presenting cells to T-cell receptors without typical antigen processing, inducing massive non-specific T-cell activation and cytokine release (e.g., Staphylococcus aureus enterotoxin).

Type 2 Exotoxin (Membrane-Damaging Toxin)

A class of exotoxin that disrupts host cell plasma membrane integrity by forming transmembrane pores or enzymatically hydrolyzing membrane lipids, leading to osmotic lysis and cell death.

Hemolysin

A type 2 exotoxin that punctures holes in the plasma membrane of red blood cells (erythrocytes) and other host cells, inducing osmotic swelling and lysis.

Type 3 Exotoxin (A-B Toxin)

An intracellular exotoxin composed of two structural units: a binding subunit (B) that attaches to a specific host cell surface receptor, and an active enzymatic subunit (A) that enters the cytoplasm to exert toxic activity.

Pertussis Toxin

A type 3 A-B exotoxin produced by Bordetella pertussis that enters host respiratory epithelial cells, alters cellular pathways, stimulates excessive mucus secretion, and triggers the characteristic paroxysmal whooping cough.

Botulinum Toxin

A type 3 neurotoxin produced by Clostridium botulinum that blocks the presynaptic release of acetylcholine at the neuromuscular junction, preventing muscle contraction and resulting in flaccid paralysis.

Tetanus Toxin

A type 3 neurotoxin produced by Clostridium tetani that binds inhibitory interneurons and blocks the release of inhibitory neurotransmitters (such as glycine), preventing muscle relaxation and causing spastic paralysis (lockjaw).

Fimbriae

Short, numerous, bristle-like protein appendages on bacterial surfaces tipped with adhesins that facilitate attachment to host cellular receptors and surfaces.

Pili

Long, thick, hair-like protein appendages on bacterial surfaces, typically fewer in number than fimbriae, that mediate adhesion to host tissues and surface motility.

Capsule

A well-organized, structured polysaccharide layer firmly attached to the external bacterial cell wall that confers a sharp border microscopically and serves in surface adhesion and immune evasion.

Slime Layer

An unorganized, loosely associated polysaccharide glycocalyx surrounding a bacterial cell that confers a fuzzy microscopic border and facilitates non-specific adhesion to surfaces.

Biofilm

A coordinated, structured multicellular microbial community enclosed within a self-produced sticky matrix of extracellular polymeric substances (EPS) that adheres to surfaces and provides protection against shear forces and immune attacks.

Invasins

Microbial surface proteins or extracellular enzymes that break down host extracellular matrix, structural proteins, or cellular barriers to facilitate pathogen penetration and spread through tissues.

Coagulase

A bacterial invasin/enzyme produced by strains of Staphylococcus aureus that triggers fibrin blood clot formation around the bacteria, shielding them from host immune defenses.

Streptokinase

An extracellular enzyme (kinase) produced by Streptococcus species that digests fibrin blood clots, allowing trapped bacteria to escape and disseminate deeper into host tissues.

Hyaluronidase

An extracellular enzyme that digests hyaluronic acid, the polysaccharide "glue" that binds adjacent cells together in connective tissue, enabling pathogens to pass between cells.

Collagenase

An extracellular enzyme that degrades collagen fibers within connective tissue and extracellular matrices, facilitating invasive tissue migration by pathogens.

Cytopathic Effect (CPE)

Visible structural and pathological damage or destruction inflicted upon host cells resulting from microbial toxins, viral replication, or direct pathogen infection.

Antigenic Masking

An immune evasion mechanism in which a pathogen coats its cell surface with host-derived self-molecules (such as fibronectin, collagen, or fibrinogen) to hide from immune recognition.

Antigenic Mimicry

An immune evasion strategy wherein a pathogen produces cell-surface components that structurally resemble host molecules, such as the sialic acid capsule of Neisseria meningitidis mimicking human glycoproteins.

Antigenic Variation

A mechanism of immune evasion where a pathogen periodically alters its surface antigens via rapid gene conversion or mutation, rendering existing host antibodies and memory lymphocytes ineffective.

Antibiosis

An antagonistic or inhibitory biological interaction between organisms where one produces chemical substances (antibiotics or toxins) that kill or prevent the growth of another.

Bactericidal Agent

An antimicrobial drug or chemical substance that directly causes bacterial cell death, resulting in a rapid drop in viable cell count.

Bacteriostatic Agent

An antimicrobial agent that reversibly inhibits bacterial cellular growth and replication without directly lysing the cells, relying on the host immune system to eliminate the arrested bacteria.

Selective Toxicity

The property of an antimicrobial medication to inhibit or destroy a microbial pathogen effectively without causing significant harm to the host's cells and tissues.

Nephrotoxicity

Toxic damage, cellular impairment, or functional deterioration inflicted upon kidney tissues as a consequence of drug exposure or overdose.

Hepatotoxicity

Chemical- or medication-induced damage or toxic dysfunction occurring in the liver.

Broad-Spectrum Antibiotic

An antibacterial drug effective against a wide taxonomic variety of bacteria, including both Gram-positive and Gram-negative organisms.

Narrow-Spectrum Antibiotic

An antibacterial medication that targets only a specific, limited subset or family of bacteria, minimizing disruption to the normal microbiota.

Beta-Lactam Ring

A core four-membered cyclic amide structure found in penicillins, cephalosporins, carbapenems, and monobactams that binds to transpeptidase enzymes and halts bacterial cell wall peptidoglycan cross-linking.

Penicillinase (Beta-Lactamase)

A bacterial resistance enzyme that hydrolyzes the internal amide bond of the four-membered beta-lactam ring, converting active penicillin into inactive penicilloic acid.

Vancomycin

A large glycopeptide antibiotic that inhibits cell wall synthesis through a non-beta-lactam mechanism by binding directly to D-alanyl-D-alanine termini, commonly reserved for resistant Gram-positive infections like MRSA.

Macrolides

A class of bacteriostatic antibacterial drugs (e.g., azithromycin) that inhibit bacterial protein synthesis by reversibly binding to the $50S$ ribosomal subunit and blocking amino acid chain elongation.

Colistin

A polypeptide antimicrobial that binds to lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria, disrupting membrane integrity and lysing the cell.

Amphotericin B

A polyene antifungal drug that interacts hydrophobically with ergosterol in fungal plasma membranes to create transmembrane pores, which causes ion leakage and cell death, carrying a risk of nephrotoxicity.

Fluoroquinolones

A class of bactericidal antibiotics that target bacterial DNA replication and transcription by inhibiting two key nuclear enzymes: DNA gyrase and topoisomerase IV.

Sulfa Drugs (Sulfonamides)

Synthetic antimetabolite drugs that competitively inhibit the enzymatic biosynthesis of folic acid in bacteria, starving them of the precursor needed to synthesize purines, pyrimidines, and amino acids.

Chain of Infection

An epidemiological model outlining the series of interconnected steps required for a communicable disease to spread from an infectious agent to a susceptible host.

Endemic Disease

A disease or infectious agent that is constantly and habitually present at a stable baseline rate within a specific geographic area or population.

Epidemic

A widespread and sudden increase in the number of cases of an infectious disease above what is normally expected in a defined human population within a specific region.

Pandemic

An epidemic that has spread over multiple countries, continents, or globally, typically affecting a large proportion of the human population.

Reservoir

The primary natural ecological habitat, environment, or organism in which an infectious agent normally lives, survives, and replicates.

Source (of Infection)

The specific physical object, bodily fluid, environmental medium, or individual from which an infectious agent is directly acquired by a new host.

Carrier

An infected person who harbors a specific pathogen without exhibiting overt symptoms of clinical illness and who can serve as an unwitting source of transmission to others.

Zoonosis (Zoonotic Disease)

An infectious disease that is transmissible under natural conditions between vertebrate animals and humans (e.g., rabies, cat-scratch disease).

Portal of Exit

The anatomical route or pathway by which a pathogen departs its reservoir or infected host, such as the respiratory tract, GI tract, urogenital tract, skin, or blood.

Direct Contact Transmission

Disease transmission requiring physical contact between the source of the pathogen and the susceptible host, including touching, kissing, sexual contact, or animal bites.

Vertical Transmission

The direct transfer of a pathogen from a mother to her offspring in utero across the placenta, during vaginal delivery, or postpartum through breast milk.

Fomite

An inanimate object or environmental surface (e.g., doorknobs, contaminated needles, utensils) that inadvertently harbors infectious agents and facilitates indirect contact transmission.

Airborne Transmission

A mode of indirect transmission where infectious particles, fine droplet nuclei, or dust remain suspended in the air over distance and time and are inhaled by a susceptible host.

Biological Vector

An arthropod or living organism in which a pathogen undergoes a mandatory developmental cycle or multiplication before being transmitted to another host (e.g., Anopheles mosquitoes and Plasmodium).

Mechanical Vector

An organism, such as a housefly or cockroach, that passively transports a pathogen on its body surfaces from one place to another without becoming biologically infected itself.

Portal of Entry

The specific anatomical site, tissue, or mucous membrane through which an infectious pathogen enters a host to initiate an infection.

Microbial Antagonism

The beneficial competitive mechanism whereby normal resident microbiota consume available nutrients, occupy physical attachment sites, and secrete waste metabolites to prevent colonization by incoming pathogens.

Dysbiosis

A disruption, imbalance, or pathological reduction in the diversity and normal composition of resident microbiota, frequently triggered by broad-spectrum antibiotic treatment and predisposing to opportunistic infections.

Opportunistic Pathogen

A microorganism that normally does not cause disease in a healthy individual but can cause infection when host defenses are suppressed or when normal microbiota are disrupted by dysbiosis.

Primary Pathogen

A pathogen capable of overcoming intact host immune defenses to cause disease in healthy, immunocompetent individuals.

Tropism

The preference or specificity of a pathogen for infecting particular host species, anatomical tissues, or cell types, determined by physiological growth conditions and cell surface receptor compatibility.