Pathogen–Host Interactions & Normal Flora – Key Vocabulary

A set of vocabulary flashcards covering the essential terms and definitions from the lecture on pathogen–host interactions and the normal human microbiota.

Pathogen

Any biological agent capable of causing disease.

Pathogenicity

The ability of a pathogen to produce infectious disease in a host.

Pathogenesis

The biological process by which a disease develops, including onset, progression and maintenance.

Virulence

The relative degree of pathogenic damage or infectiousness a pathogen can inflict.

Symbiosis

A close association in which at least one partner benefits; includes commensalism, mutualism and parasitism.

Commensalism

A symbiotic relationship in which one organism benefits and the other is neither harmed nor helped.

Mutualism

A symbiotic interaction where both partners benefit.

Parasitism

A relationship in which one organism (parasite) benefits at the expense of the host; includes bacterial pathogens in an ecological sense.

Opportunistic Pathogen

A microbe that causes disease only in a compromised host or when located in an unusual body site.

True (Primary) Pathogen

An organism able to cause disease in healthy individuals.

Virulence Factor

A microbial property (gene product) that enables colonisation or enhances the ability to cause disease.

Exotoxin

A secreted bacterial toxin that damages host cells, e.g. pore-forming toxins.

Pore-Forming Toxin (PFT)

Exotoxin that inserts into host membranes and assembles into a pore, disrupting the cell.

Haemolysin

Membrane-damaging pore-forming toxin identifiable by red-blood-cell lysis on blood agar.

Adhesin

A microbial surface ligand that mediates specific attachment to host-cell receptors.

Pili (Fimbriae)

Thin protein tubes projecting from many Gram-negative bacteria, enabling adhesion and sometimes conjugation.

Type 1 Pili

Uropathogenic E. coli fimbriae implicated mainly in lower urinary-tract infections.

P Pili (P-Pilus)

Composite fimbriae of UPEC whose PapG adhesin binds kidney receptors, leading to pyelonephritis.

Chaperone–Usher Pathway

Protein-secretion and assembly system that builds pili in Gram-negative bacteria.

Non-Fimbrial Adhesin

Afimbrial surface molecules (e.g. outer-membrane proteins, LPS, teichoic acids) mediating adhesion.

Receptor (Host)

Host molecule—often a carbohydrate-containing glycoconjugate—to which a microbial adhesin binds.

Motility

The ability of an organism to move; many rods and all spiral bacteria are motile.

Chemotaxis

Directed bacterial movement toward or away from chemical stimuli.

Flagellum

A rotary, whip-like protein filament that propels many motile bacteria.

Proton Motive Force (PMF)

Electrochemical proton gradient across a membrane; powers flagellar rotation and other processes.

Flagellar Hook

Flexible universal-joint structure that links flagellar filament to the motor and transmits torque.

Swarming Motility

Rapid, coordinated surface movement of hyper-flagellated bacteria, e.g. Proteus mirabilis.

Invasion

Entry of bacteria into host cells or tissues, often initiating infection.

Internalization

Process by which bacteria induce or survive host cell uptake, including endocytosis and phagocytosis.

Biofilm

Community of microbes embedded in extracellular polymeric matrix that aids adhesion and protection.

Pathogenicity Island (PAI)

Chromosomal gene cluster encoding virulence factors, present in pathogens but absent in commensals.

Siderophore

High-affinity iron-chelating compound secreted by bacteria to acquire iron from the host.

Capsule

Polysaccharide layer surrounding some bacteria, protecting against immune responses and desiccation.

Lipopolysaccharide (LPS)

Endotoxin component of Gram-negative outer membrane; can act as an adhesin or immune stimulant.

Protein Secretion System

Bacterial machinery that exports proteins such as toxins or pilus subunits outside the cytoplasm.

Normal Microbiota (Normal Flora)

The community of microbes that inhabit healthy humans without causing disease.

Microbiome

The full genetic and species complement of a host’s microbial community.

Transient Flora

Microorganisms that inhabit the body only temporarily before disappearing.

Hygiene Hypothesis

Concept that reduced early microbial exposure alters immune maturation and increases autoimmune disease risk.

Short-Chain Fatty Acids (SCFAs)

Metabolites produced by gut bacteria fermenting dietary fibre; influence host metabolism and immunity.

Opportunistic Infection

Disease caused when normal flora or environmental microbes exploit a weakened host or unusual site.

Respiratory Tract

Airway system from nose/mouth to alveoli; upper tract harbours microbiota, lower tract is normally sterile.

Lower Respiratory Tract Infection (LRTI)

Inflammation of trachea, bronchi, or lungs, e.g. bronchitis or pneumonia.

Pneumonia

Inflammatory condition of alveoli often caused by Streptococcus pneumoniae, leading to fluid-filled lungs.

Probiotic

Preparation containing live microbes intended to restore or support healthy gut microbiota.

Prebiotic

Indigestible nutrients formulated to promote growth of beneficial gut bacteria.

Clostridioides difficile

Gram-positive, spore-forming bacterium causing antibiotic-associated diarrhoea and colitis.

Faecal Microbiota Transplant (FMT)

Therapeutic transfer of stool microbiota from a healthy donor to treat recurrent C. difficile infection.

Sterile Site (Clinical)

Body area normally free of microbiota; detection of microbes here usually indicates infection.

Uropathogenic E. coli (UPEC)

E. coli strains possessing virulence factors such as P and Type 1 pili that cause urinary-tract infections.