Lecture 2: Host-Microbe Interactions

Vocabulary practice flashcards covering host-microbe interactions, oral niches, symbiosis vs. dysbiosis, opportunistic oral infections, antimicrobial resistance, and molecular methods for identifying oral microorganisms from Lecture 2 (Microbiology, DBS1).

Dental Plaque

The community of microorganisms found on the tooth surface as a biofilm embedded in a self-produced matrix of polymers of salivary and bacterial origin.

Oral Environment pH Range

The controlled pH range of the hydrated oral cavity under healthy conditions, typically spanning from $6.2$ to $7.6$.

Bacterial Load of Dental Niches

The quantification of colonising bacteria on teeth, comprising $1\text{--}33 \times 10^6$ bacteria per fissure in fissure plaque and $10^3\text{--}10^6$ bacteria per crevice in sub-gingival plaque.

Two-Tone Plaque Disclosing

A visual diagnostic staining method that differentiates mature dental plaque (which stains purple) from newly formed plaque (which stains pink).

Desquamation

The continuous shedding of epithelial cells from oral mucosal surfaces, which maintains a low microbial load on keratinised and non-keratinised surfaces.

Tongue Dorsum Niche

A specialised oral habitat characterised by a papillary surface covered in microscopic projections, harbouring approximately $100$ bacteria per epithelial cell.

Saliva

A primary nutrient fluid for oral microbes consisting of $99\%$ water, $0.3\%$ protein, and $0.2\%$ inorganic molecules, lipids, and hormones, containing approximately $10^8$ viable microbes per millilitre.

Gingival Crevicular Fluid (GCF)

A serum-like exudate located at the junctional epithelium of the gingivae that supplies rich nutrients to subgingival bacteria and carries host inflammatory components.

Dysbiosis

An ecological imbalance in a microbial community characterised by the disruption of the normal composition, diversity, and symbiotic relationship between microbiota and host.

Pathogen

Any microbe capable of causing host damage, tissue injury, or clinical disease.

Pathbiont

A resident member of the normal microbiota that is typically harmless but promotes or causes disease following a disruption in host-microbe balance (dysbiosis).

Symbiont

A member of the resident microbiota that maintains homeostasis and confers health benefits to the host.

Mutualism

A form of symbiotic interaction in which both the host and the microorganism derive benefit, such as bacteria producing nutritional metabolites or protective hydrogen peroxide.

Commensalism

A symbiotic relationship where one organism benefits while the host remains unharmed, exemplified by early colonising oral bacteria such as Streptococci.

Parasitism

A symbiotic relationship in which a microorganism benefits at the expense and detriment of the host, exemplified by Porphyromonas gingivalis degrading host tissues for nutrients.

Opportunistic Pathogen

An organism that does not normally provoke disease in a healthy host but causes infection when host barriers or immune defences are compromised, or when local ecology is disrupted.

Caries-Associated Microorganisms

Opportunistic oral bacteria driving enamel demineralisation and tooth decay, specifically Streptococcus mutans, Lactobacillus spp., and Actinomyces spp.

Periodontal Disease Pathogens

A consortium of dysbiotic, tissue-damaging bacteria in subgingival plaque, including Porphyromonas gingivalis, Tannerella forsythia, Aggregatibacter actinomycetemcomitans (A. actinomycetemcomitans), and Spirochaetes.

Infective Endocarditis

A life-threatening systemic infection of the heart valves frequently caused by oral bacteria entering the vascular system, primarily $\alpha$-haemolytic streptococci and Staphylococcus spp.

Bacterial Immune Evasion Mechanisms

Strategies employed by oral bacteria to escape host defenses, including biofilm barrier protection, proteolytic degradation of secretory immunoglobulins (e.g. IgA), phagocyte inactivation, and resistance to antimicrobial peptides (AMPs).

16S rRNA Gene Structure

A bacterial gene comprising universally conserved sequence regions for broad primer annealing alongside nine hypervariable regions ($\text{V1}\text{--}\text{V9}$) utilized for genus- and species-level taxonomic identification.

Denaturing Gradient Gel Electrophoresis (DGGE)

An electrophoretic profiling technique that separates 16S rRNA gene fragments of identical length according to their melting behavior as they migrate through an increasing chemical denaturant gradient.

DNA-DNA Checkerboard Hybridization

A high-throughput molecular method that uses a membrane grid of species-specific labeled DNA probes (commonly targeting $40$ plaque-associated species) to simultaneously identify and quantify bacteria across numerous samples.

Human Oral Microbe Identification Microarray (HOMIM)

A glass slide microarray containing 16S rRNA-directed probes capable of concurrently profiling approximately $300$ known oral bacterial species via fluorescence intensity readouts.

DNA-FISH (Fluorescence In Situ Hybridisation)

A molecular imaging technique using fluorophore-tagged complementary DNA probes to detect, localize, and map the spatial architecture of specific bacterial taxa within intact oral biofilms under a specialised microscope.

Amplicon Sequence Variants (ASVs)

High-resolution operational taxonomic units resolved down to individual nucleotide variations from high-throughput 16S rRNA sequencing datasets without requiring arbitrary percent-similarity clustering.

Metagenomics

The untargeted sequencing and computational reconstruction of all collective genomic DNA extracted directly from an environmental or clinical sample without prior targeted gene amplification.

MinION

A portable, USB-powered long-read sequencing device developed by Oxford Nanopore Technologies that determines DNA/RNA sequences in real time by sensing disruptions in ionic current across a nanoscale protein pore.

Antimicrobial Resistance (AMR) 2050 Forecast

Global epidemiological projection estimating that deaths from antibiotic-resistant bacterial infections will rise from approximately $700,000$ per year to $10\text{ million}$ per year by $2050$, heavily affecting Asia ($4,730,000$) and Africa ($4,150,000$).