Veterinary Microbiology & Parasitology: Sessions 1–4D Terms

Acellular

Not composed of cells.

Prokaryote / Prokaryotic

Cellular organism lacking a membrane-bound nucleus.

Eukaryotic

Cellular, with a membrane-bound nucleus; includes fungi, protozoa, helminths, and arthropods; can be unicellular or multicellular.

Virus

Acellular infectious agent whose replication depends on living host cells (obligate intracellular parasite); genome is DNA or RNA; lacks ribosomes or independent protein-synthesis machinery.

Bacterium

Unicellular prokaryote; no membrane-bound nucleus but has ribosomes and reproduces as a cell; classified by morphology, staining, and growth/culture requirements.

Fungus

Eukaryotic organism that obtains nutrients by absorption; yeast-like (unicellular) or filamentous forms; filamentous growth may form hyphae; reproductive structures aid dissemination.

Protozoan

Unicellular eukaryotic parasite with a nucleus; no multicellular adult body; often has an active feeding/motile stage (e.g., trophozoite) and a resistant stage (e.g., cyst).

Arthropod

Multicellular eukaryote with a segmented body, jointed appendages, and a chitinous exoskeleton; may be parasites or vectors.

Nematode

Cylindrical, unsegmented helminth (roundworm) with a complete digestive tract, staged larval development, and moulting; direct or indirect life cycles.

Cestode

Tapeworm; flattened, segmented helminth (scolex, neck, proglottids); no alimentary canal; typically an indirect life cycle.

Trematode

Fluke; typically a dorsoventrally flattened, unsegmented helminth; complex life cycle, usually with a molluscan (snail) intermediate host.

Ribosome

Cellular machinery that translates RNA into protein.

Nucleotide

Building block (monomer) of nucleic acids, composed of a sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base.

Gene

A nucleic acid sequence that is preserved and transmitted, and controls biological processes by coding for polypeptides/proteins or RNA.

Genomics

The study of the functions of genes.

Proteomics

The study of proteins.

DNA (deoxyribonucleic acid)

The genetic, informational molecule in every cellular organism; double-helix shape; found in the nucleus of eukaryotes and the cytoplasm of prokaryotes.

RNA (ribonucleic acid)

Nucleic acid found in many forms, mostly used in protein synthesis; is the molecule of inheritance in some viruses; some RNA (ribozymes) has enzymatic function.

mRNA (messenger RNA)

Carries the coded genetic message from DNA.

tRNA (transfer RNA)

Transfers amino acids to ribosomes.

rRNA (ribosomal RNA)

Main structural/functional component of ribosomes.

Central Dogma

The framework describing information flow: replication (DNA copied from DNA), transcription (RNA copied from DNA), translation (protein synthesized from RNA).

Replication

DNA → DNA; copies DNA so genetic information can be transmitted.

Transcription

DNA → RNA; produces an RNA copy from DNA.

Translation

RNA → protein; uses RNA information to build a protein.

Reverse transcription

RNA → DNA; an important replication strategy for retroviruses.

Reverse transcriptase

Special enzyme in retroviruses that converts viral RNA into DNA, reversing the usual direction of the central dogma.

Prokaryotic chromosome

Typically small and circular; packaged by DNA-binding proteins into a nucleoid (no membrane); may be accompanied by additional plasmids.

Plasmid

Small, circular 'naked' DNA molecule, additional to the main chromosome, found in bacteria.

Nucleoid

The region in a prokaryotic cell where the circular chromosome is packaged (not membrane-bound, unlike a nucleus).

Maintenance host

Host population capable of sustaining pathogen transmission without repeated outside introduction (e.g., unmanaged possums maintaining bovine TB in New Zealand).

Spillover host

Non-maintenance host that acquires infection from a maintenance population/sustaining source without normally sustaining it itself.

Definitive (final) host

Host in which the adult or sexually reproductive parasite stage occurs.

Intermediate host

Required host in which parasite development occurs before the definitive host.

Paratenic host

Transport host that carries a parasite without the required developmental progression of an intermediate host.

Vector

Living organism (often an arthropod) that transmits an infectious agent between hosts; a biological vector supports pathogen development/replication, a mechanical vector provides simple transport.

Fomite

Inanimate object capable of passively carrying infectious material between hosts.

Environmental source

Non-host environmental location or material (soil, water, pasture) from which exposure occurs, depending on the pathogen's structural resistance.

Reservoir (epidemiological)

Broadly, any host complex in which a disease persists indefinitely and that can act as a source for a target species/population.

Basic reproductive rate (R0)

The number of new/secondary infection cases within a host population that are, on average, generated by a single infected animal in a totally susceptible population.

Spillover transmission

Inter-species transmission of a pathogen from a maintenance host to a non-maintenance (spillover) host.

Dead-end host

A spillover host species with R0 near zero for both intra- and inter-species transmission, contributing nothing to ongoing infection.

Apparent multi-host pathogen (system)

Multi-host system where inter-species R0 is intermediate to high but intra-species R0 is low/zero.

Amplifier host

A spillover host whose main epidemiological role is to increase (amplify) the total burden of infection in an area.

Secondary spillover

Transmission from a spillover host onward to another (third) species.

Spillback (reciprocal secondary spillover)

Transmission from a spillover host back into the maintenance host species from which it originally acquired the infection.

True multi-host pathogen (system)

System with two or more maintenance hosts where inter-specific transmission is also high.

Potential emerging infectious disease (system)

System with two maintenance hosts present but very low inter-specific transmission.

Density-dependent transmission

Transmission dynamics in which host status (maintenance vs. spillover) depends on host population density relative to a threshold for disease persistence.

Prevalence

Epidemiological measure of existing disease cases in a population at a point in time.

Symbiosis

A close, long-term biological relationship/interaction between different species; can be mutualistic, commensal, or parasitic.

Symbiogenesis

The evolution of new physiological processes, tissues, or organs as a result of a symbiotic relationship.

Endosymbiosis theory

Theory that mitochondria and chloroplasts originated as free-living bacteria engulfed by an ancestral eukaryotic cell.

Commensalism

Symbiotic relationship in which one partner benefits while the other is neither helped nor harmed.

Mutualism

Symbiotic relationship in which both partners benefit.

Parasitism

Symbiotic relationship in which one partner (the parasite) benefits at the expense of the other (the host), which is harmed.

Endosymbiotic

Describes a symbiont living inside the body/cells of its host.

Ectosymbiotic

Describes a symbiont living on the outer surface of its host.

Episymbiont

A symbiont that lives on the surface of an animal host.

Endosymbiont

Microbial partner in a long-term host-associated symbiosis.

Horizontal transmission (of symbionts)

Transfer of a symbiont between individuals of the host species (or even to other species), not from parent to offspring.

Vertical transmission

Transmission of a symbiont from mother to egg/embryo (parent to offspring).

Primary symbiont

Symbiont transmitted maternally, living in a specialized host organ (bacteriome).

Secondary symbiont

Symbiont that can be transmitted horizontally in addition to maternal transmission.

Bacteriome

Specialized organelle/organ in some insects that houses bacterial endosymbionts.

Virulence

Disease-induced mortality (or severity of harm) caused by a pathogen in its host.

Colonization resistance

Beneficial function of gut microbiota that prevents non-native/pathogenic bacteria from colonizing the host's gut.

Cytoplasmic incompatibility

Reproductive manipulation strategy used by Wolbachia bacteria.

Coinfection

Simultaneous infection of a host by multiple pathogens/parasite strains.

Obligate association

A symbiotic relationship required for the organism's survival.

Facultative association

A symbiotic relationship the organism can also exist independently of.

Variation

Members of a pathogen population are not necessarily genetically or phenotypically identical.

Mutation

A source of genetic change; not automatically beneficial.

Fitness

A measure of an individual's expected reproductive output and survival; relative success in survival and reproduction/replication under particular conditions.

Selection

Differences among variants affect their contribution to later generations.

Adaptation

Population-level change in which traits associated with greater success become more common over generations.

LTEE (long-term evolution experiment)

A long-term evolution experiment conducted on 12 populations of Escherichia coli, running for tens of thousands of generations.

Stationary phase

A physiological state that cells enter when nutrients have been exhausted.

Mutator

A bacterial strain with an elevated mutation rate.

Phenotype

The physiological or morphological state of a cell.

Epistasis

Any interaction between mutations that affects fitness — the effect of one mutation depends on the presence of others.

Fitness landscape

A function on the high-dimensional space of all possible genomes that assigns a fitness value to each genome.

Clone

A genetically homogeneous population derived from a single cell.

(Bacterio)phage

A virus that infects bacteria.

Diminishing returns epistasis

Pattern in which the higher the background fitness, the lower the expected fitness gain from an additional mutation.

Evolutionary trade-off

Improved performance/fitness under one set of conditions that adversely affects performance under another set of conditions.

Stochastic switch

A molecular mechanism allowing cells to spontaneously switch phenotypic states without a sensory signaling pathway; produces heritable phenotypic variation that selection can act on.

Persister cells

A slow-growing subpopulation of cells (maintained by a stochastic switch) that is highly tolerant of antibiotics and other stresses, enabling survival under fluctuating adverse conditions.

Competence (bacterial)

A phenotypic state, activated stochastically under starvation in some bacteria (e.g., Bacillus subtilis), that allows cells to take up DNA from the environment.

Aging (microbial/evolutionary)

The reduction in average reproductive output as a function of an individual cell's age; can result from accumulation of damage (e.g., misfolded proteins).

Asymmetric damage partitioning

A cellular strategy for coping with damage in which one daughter cell receives less damage ('rejuvenated') than the other at division.

Biofilm

A dense, surface-bound, structured microbial community.

Cooperation (microbial)

Collective cellular activity (e.g., swarming, biofilm formation, light production) that benefits the group, potentially exploitable by cheaters.

Cheater

An individual/strain that does not pay the physiological cost of a cooperative activity but still benefits from it (free-rides on a public good).

Public good

A beneficial molecule or resource produced by some cells (at a cost) that becomes available to the wider population (e.g., extracellular invertase products).

Quorum sensing

Genetic signaling pathway by which bacteria sense their own population density using small-molecule signals (autoinducers) and regulate gene expression accordingly.

Autoinducer

Small-molecule signal produced, exported, and sensed by bacteria as part of quorum-sensing networks.

Quorum quenching

Process by which one species specifically destroys the quorum-sensing signaling molecules of another species.

Horizontal gene transfer

Transfer of genetic material between organisms other than by reproduction (parent to offspring); shown by metagenomic analyses to be more prevalent in bacteria than previously thought; involves mobile genetic elements such as transposons, phages, and plasmids.

Mobile genetic elements

DNA elements (transposons, phages, plasmids) that can move within genomes and between cells, influencing adaptation.