microbiome quiz 1
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What is the biological definition of a microorganism (microbe)?
A tiny living organism or biological entity that exists as a single cell or a small group of cells, but can include viruses.
Which major biological groups count as microorganisms?
Bacteria, viruses, fungi, protozoa, algae (including microalgae), and archaea.
Who was the first scientist to directly observe and describe microscopic organisms ('animalcules') in 1673?
Anton van Leeuwenhoek
Why is the bacterium Epulopiscium fishelsoni notable regarding microbial size?
At approximately 250 x 30 micrometers, it is large enough to be visible to the naked eye, proving that not all microbes are microscopic.
According to Koch's Postulates, what are the first two criteria required to establish a microbe as the causative agent of a disease?
"1. The microorganism must be present in all cases of the disease. 2. The pathogen must be isolated from the diseased host and grown in pure culture."
According to Koch's Postulates, what are the third and fourth criteria required to prove disease causation?
"3. The pathogen from pure culture must cause disease when inoculated into a healthy, susceptible host. 4. The same pathogen must be reisolated from the experimentally infected host."
In microbiology, what defines a microbial population?
A group of individual microbes belonging to the same species living in the same place at the same time.
In microbiology, how does a microbial community differ from a microbial population?
A microbial community consists of two or more diverse populations of different species (e.g., bacteria, fungi, archaea, viruses) coexisting, interacting, and sharing a habitat.
How does a planktonic microbial lifestyle differ from a sessile microbial lifestyle
Planktonic cells are free-swimming or single floating cells in liquid media, whereas sessile cells are surface-attached and encased in a community matrix.
Identify the four sequential phases of bacterial growth depicted in this population curve: Bacterial growth curve graph highlighting four sequential growth phases
Lag Phase 2. Exponential (Log) Phase 3. Stationary Phase 4. Death Phase
mathematical formulas for total generations (n) during exponential bacterial growth?
Total generations: n = t/g (where (t) is total growth time and (g) is generation time)
mathematical formulas for final population size (Nt) during exponential bacterial growth?
Final population: Nt = N0 times 2n (where N0 is the initial cell count).
How does a spectrophotometer estimate bacterial population growth?
It measures Optical Density (OD). Bacteria scatter light passing through the cuvette, reducing the light that reaches the photoelectric cell and increasing the measured OD value (OD > 0).
How did Robert Koch's development of pure cultures transform microbiology as a discipline?
It transformed microbiology from a descriptive science into an experimental science, enabling isolation of specific disease agents, genetic analysis, and metabolic mapping.
Why do standard antibiotic susceptibility tests conducted on planktonic pure cultures often fail to predict patient outcomes in chronic infections?
Because over 99% of bacteria in natural and clinical settings live in complex biofilms, where structural matrices and specialized dormant cells make them significantly more drug-resistant.
What is a bacterial biofilm?
A surface-attached microbial community enclosed within a self-produced slime matrix of extracellular polymeric substances (EPS).
What key survival advantages encourage bacteria to form biofilm communities?
- Protection against antibiotics, immune cells, and disinfectants 2. Firm surface attachment preventing physical washout 3. Nutrient sharing and chemical signal exchange 4. Defense against environmental stress (pH, temperature, dehydration)"
What occurs during Step 1 (Reversible Attachment) of the biofilm lifecycle?
Free-swimming planktonic bacteria synthesize pili, flagella, and adhesion factors to make initial, weak, transient interactions with a surface.
What characterizes Step 2 (Colonization) in biofilm development?
Bacteria anchor irreversibly to the surface, cluster into microcolonies, and initiate stable cell-to-cell signaling.
What molecular shift defines Step 3 (High EPS Production / Maturation I) of a biofilm?
Cells heavily upregulate secretion of extracellular polymeric substances (EPS), thickening the protective matrix around cell clusters.
What structural features develop in Step 4 (Biofilm Formation / Maturation II)?
The biofilm forms a mature 3D architecture with tower/mushroom pillars, an internal water channel network, active efflux pumps, and persister cell sub-populations.
What structural breakdown occurs during Steps 5 and 6 of the biofilm lifecycle?
EPS production declines, followed by enzymatic EPS degradation, surfactant production, cell lysis, and matrix weakening.
What is the biological goal of Step 7 (Microbial Dispersion) in a biofilm?
To release motile planktonic cells that swim away to colonize new sites and spread the infection or community.
What defines the sessile cell phenotype in a biofilm?
Surface-attached, metabolically active cells that exhibit continuous growth and readily form colonies when cultured on routine agar plates.
What are persister cells in a bacterial biofilm?
Dormant phenotypic variants with very low metabolic activity that tolerate lethal antibiotic exposure and rapidly resume normal growth when stress is removed.
What characterizes Viable But Not Culturable (VBNC) bacteria?
Cells entering deep stress-induced dormancy with minimal metabolic activity; they remain alive but cannot form colonies on routine laboratory agar.
How do Sessile, Persister, and VBNC bacteria compare in their ability to grow on routine agar media?
Sessile: Grows directly. Persister: Grows directly after stress removal or revival. VBNC: Does NOT grow on routine agar without specific resuscitation triggers.
What is Extracellular Polymeric Substance (EPS), and what proportion of a biofilm does it comprise?
A sticky, self-produced matrix made of polysaccharides, proteins, lipids, and nucleic acids that constitutes 65% to 95% of total biofilm mass.
What structural role does extracellular DNA (eDNA) play within the biofilm EPS matrix?
eDNA acts as an essential structural scaffold, cross-linking matrix proteins and polysaccharides to stabilize biofilm integrity.
What are microcolonies within biofilm architecture?
Dense, organized clusters of bacterial cells that serve as the primary structural building blocks of a biofilm.
What functional advantage do tower and mushroom 3D structures provide to a mature biofilm?
They extend the community upward into surrounding fluid streams, maximizing surface area for nutrient capture and oxygen absorption.
What are interstitial voids and fluid channels within a biofilm?
Open, liquid-filled pathways weaving between microcolonies that act as a internal plumbing network ('microbial streets') to circulate water, nutrients, oxygen, and enzymes.
How does fluid circulation through interstitial channels protect inner biofilm inhabitants?
Continuous fluid movement flushes out toxic metabolic waste products, preventing self-poisoning in deep biofilm layers.
What mechanism enables division of labor within a single-species bacterial biofilm?
Distinct cellular subpopulations execute unique transcriptional programs, expressing specific genes tailored to localized micro-environments.
In biofilm division of labor, what is the role and location of Breathers (Oxygen Processors)?
Positioned at the outer boundary (high O2), Breathers consume incoming oxygen to power rapid metabolism and metabolic defense.
In biofilm division of labor, what is the key responsibility of Feeders (Metabolic Cooks)?
Located in outer-to-middle layers, Feeders break down complex primary nutrients into simple food molecules for inner cells.
In biofilm division of labor, what is the function of Makers (Matrix Producers)?
Situated in the middle biofilm layer, Makers synthesize extracellular polymeric substances (EPS) to build the physical protective matrix.
In biofilm division of labor, what primary role do Cleaners (Waste Managers) execute?
Located in middle-to-deep layers, Cleaners degrade toxic metabolic waste products and neutralize harmful chemical threats.
In biofilm division of labor, what function do Spies (Signalers) fulfill?
Distributed colony-wide, Spies synthesize autoinducers (quorum sensing signals) to coordinate group behaviors across the entire community.
In biofilm division of labor, what is the task of Scouts (Dispersers)?
Positioned at the edge and surface, Scouts regain motility, detach from the matrix, and migrate to colonize new environments.
In biofilm division of labor, where are Sleepers (Persister cells) located and what is their function?
Located in the deep core (low O2), Sleepers enter metabolic dormancy to survive harsh antibiotic attacks and preserve the lineage.
What is quorum sensing in bacteria?
A density-dependent cell-to-cell communication process wherein bacteria produce, release, and detect chemical signal molecules (autoinducers) to coordinate group behaviors.
In standard Gram-negative quorum sensing circuits, what are the distinct roles of LuxI and LuxR?
LuxI: Autoinducer synthase enzyme that synthesizes signal molecules. LuxR: Transcriptional receptor protein that binds autoinducers at high cell density to trigger target gene expression."
How do chemical signaling molecules (autoinducers) differ between Gram-negative bacteria, Gram-positive bacteria, and inter-species interactions?
Gram-negative: Acyl-homoserine lactones (AHLs / AI-1) Gram-positive: Autoinducing peptides (AIPs) Polymicrobial (Inter-species): Autoinducer-2 (AI-2, universal language)"