Overview of Microorganisms: Bacteria, Fungi, Viruses, and Prions

Comprehensive vocabulary flashcards covering the biological classification, morphology, pathogenicity, oral implications, antimicrobial pharmacology, diagnostics, and virology of major microorganisms from the National Dental Nurse Training Programme.

Microbiology

The study of microscopic organisms (microbes) that are invisible to the naked eye ($\mu\text{m}$ in size), typically consist of a single cell, and require a microscope to be visualized.

Taxonomy

The science of biological classification that categorizes organisms into hierarchical taxa: Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species.

Prokaryotes

Unicellular organisms measuring $1\text{--}10\,\mu\text{m}$ (Bacteria and Archaea) that lack a membrane-bound nucleus and internal organelles, containing a single circular chromosome, ribosomes, and a peptidoglycan cell wall.

Eukaryotes

Organisms measuring $10\text{--}100\,\mu\text{m}$, mostly multicellular, characterized by DNA contained within a membrane-bound nucleus, multiple chromosomes, membrane-bound organelles, and cell walls of cellulose or chitin (when present).

Pathogen

A microorganism or infectious agent (such as a bacterium, virus, fungus, or protozoan parasite) capable of causing disease in a host.

Infectious Disease

A disease resulting from the invasion and growth of pathogenic microorganisms within a host that can spread from one individual to another, directly or indirectly.

Koch's Postulates

A set of four criteria establishing microbial disease causation: (1) the microorganism must be present in every diseased host but absent in healthy individuals, (2) it must be isolated and grown in pure culture, (3) inoculation into a healthy host must cause the same disease, and (4) it must be re-isolated from the experimentally diseased host.

Carrier

A typically healthy individual who has survived an infection or remains asymptomatic, harboring the pathogen and capable of transmitting it to other susceptible hosts (e.g., Mary Mallon / "Typhoid Mary").

Gram Stain

A differential staining procedure utilizing crystal violet, iodine, alcohol decolorization, and safranin to distinguish bacteria into Gram-positive (purple) and Gram-negative (pink) based on cell wall architecture.

Bacterial Cell Wall Architecture

Structural distinction where Gram-positive bacteria possess a thick peptidoglycan layer outside the plasma membrane, whereas Gram-negative bacteria possess a thin peptidoglycan layer, a periplasmic space, and an outer lipid membrane.

Dichotomous Key for Cocci

A step-by-step diagnostic workflow using Gram stain, catalase reaction, coagulase production, novobiocin/optochin/bacitracin susceptibility, and hemolysis patterns to differentiate genera such as Staphylococcus, Streptococcus, and Neisseria.

Streptococcus mutans

A Gram-positive, chain-forming oral coccus that rapidly ferments dietary sugars into acid, recognized as a primary causative agent of dental caries and capable of causing infective endocarditis if introduced into the bloodstream.

Actinomyces

A genus of anaerobic, Gram-positive filamentous bacteria growing as branching rods; normal oral commensals strongly associated with root surface caries.

Staphylococcus aureus

A major pathogenic Gram-positive coccus arranged in grape-like clusters, producing skin and soft-tissue infections, food poisoning, toxic shock syndrome, pneumonia, and endocarditis.

MRSA (Methicillin-resistant Staphylococcus aureus)

Strains of Staphylococcus aureus that acquired resistance to beta-lactam antibiotics via horizontal gene transfer of the $mec$ gene (e.g., $mecA$ carried on SCC$mec$).

Red Complex Bacteria

A group of anaerobic Gram-negative bacilli found in subgingival dental plaque—consisting of Prevotella, Porphyromonas, and Tannerella—consistently implicated in the pathogenesis and progression of periodontal disease.

Dimorphic Fungi

Fungi that switch between two morphological forms depending on environmental factors: a single-celled yeast form (typically reproducing by budding) and a filamentous hyphal form (enabling tissue invasion).

Candida albicans

A dimorphic, eukaryotic single-cell yeast with a cell wall of mannan, glucan, and chitin; the most frequent causative agent of superficial oral thrush (pseudomembranous candidiasis) and systemic candidiasis in immunocompromised hosts.

Candida auris

An emerging multidrug-resistant fungal pathogen first discovered in 2009 that causes healthcare-associated invasive systemic infections and transmits readily between hospitalized patients.

Dental Plaque

A complex polymicrobial biofilm on the tooth surface composed of bacteria (e.g., Streptococcus, Actinomyces, Fusobacterium, Veillonella) that causes caries and periodontitis when microbial dysbiosis occurs.

Dysbiosis

A disruptive ecological shift or microbial imbalance in the composition of resident normal microflora triggered by environmental stresses such as high sugar intake or acidic pH.

Efflux Pumps

Transmembrane transport protein mechanisms utilized by bacterial and fungal cells to actively expel antimicrobial agents out into the extracellular space, preventing intracellular accumulation.

Virus

An acellular, obligate intracellular parasite containing either a DNA or RNA genome enclosed within a protein coat (capsid), devoid of cytoplasm, ribosomes, and autonomous metabolic machinery.

Nucleocapsid

The core viral structure composed of the viral nucleic acid genome enclosed within the capsid, organized in helical, icosahedral, or complex symmetry.

Bacteriophage

A virus that infects bacteria, typically exhibiting a complex structure consisting of an icosahedral capsid head enclosing DNA, a collar, a contractile sheath/tail, a baseplate, spikes, and tail fibers.

Viral Replication Cycle

The sequential process of host infection consisting of: (1) attachment to host cell receptors, (2) penetration, (3) uncoating to release nucleic acid, (4) replication and viral protein synthesis, (5) capsid assembly, and (6) release via cytolysis or budding.

Human Immunodeficiency Virus (HIV) Genome Organization

A retroviral architecture characterized by flanking long terminal repeats (LTR) and core genes: $gag$ (encoding structural core proteins p24, p17, p9, p7), $pol$ (reverse transcriptase, integrase, protease), and $env$ (gp120 attachment and gp41 fusion glycoproteins).

Human Papillomavirus (HPV)

A non-enveloped, icosahedral dsDNA virus ($52\text{--}55\,\text{nm}$ diameter) with an $8000\,\text{bp}$ genome and a 72-pentamer capsid composed of L1 ($55\,\text{kDa}$) and L2 ($70\,\text{kDa}$) proteins, responsible for cutaneous/mucosal warts and cervical, oropharyngeal, and anogenital cancers.

Gardasil 9

A recombinant 9-valent vaccine produced in yeast using recombinant L1 capsomer proteins, providing protection against high- and low-risk HPV types 6, 11, 16, 18, 31, 33, 45, 52, and 58.

SARS-CoV-2 Structure

An enveloped RNA coronavirus characterized by surface Spike (S) glycoproteins that bind the ACE2 host receptor, membrane (M) proteins, envelope (E) proteins, and an internal nucleocapsid (N) protein enclosing positive-sense single-stranded genomic RNA.

Mpox (Monkeypox / Orthopoxvirus)

A zoonotic, enveloped double-stranded DNA virus endemic to Central and West Africa, characterized by an incubation period of approximately 12 days, systemic prodromal symptoms, prominent lymphadenopathy, and progressive umbilicated pustular skin lesions.

Prions

Infectious misfolded proteins lacking nucleic acids ($\text{PrP}^{\text{Sc}}$) that self-propagate by binding to and inducing abnormal conformational switching of normal membrane-anchored cellular prion proteins ($\text{PrP}^{\text{C}}$), causing transmissible spongiform encephalopathies such as Creutzfeldt-Jakob disease (CJD).

Polymerase Chain Reaction (PCR)

A molecular diagnostic technique that amplifies minute amounts of target DNA sequences using sequence-specific primers, producing distinct amplimers that are detected in real time or via agarose gel electrophoresis.

Enzyme-linked Immunosorbent Assay (ELISA)

An immunological diagnostic assay using antibodies linked to enzyme reporters to detect and quantify antigens or antibodies in clinical specimens via chromogenic substrate conversion.