,CONTENT Chapter 19: Rickettsia, Ehrlichia,
Anaplasma, and Coxiella
Chapter 1: Introduction to Microbiology
Chapter 20: Fungi
Chapter 2: Normal Flora
Chapter 21: Protozoa
Chapter 3: Pathogenicity of
Microorganisms Chapter 22: Helminths
Chapter 4: Diagnostic Microbiology Chapter 23: Introduction to the Viruses
Chapter 5: Vaccines and Antimicrobial Chapter 24: Nonenveloped DNA Viruses
Agents
Chapter 25: Enveloped DNA Viruses
Chapter 6: Bacterial Structure, Growth,
and Metabolism Chapter 26: Hepatitis B and Hepatitis D
(Delta) Viruses
Chapter 7: Bacterial Genetics
Chapter 27: Positive-Strand RNA
Chapter 8: Staphylococci Viruses
Chapter 9: Streptococci Chapter 28: Retroviruses
Chapter 10: Gram-Positive Rods Chapter 29: Negative-Strand RNA
Viruses
Chapter 11: Neisseriae
Chapter 30: Double-Stranded RNA
Chapter 12: Gastrointestinal Gram- Viruses: Reoviridae
Negative Rods
Chapter 31: Unconventional Infectious
Chapter 13: Other Gram-Negative Rods Agents
Chapter 14: Clostridia and Other Chapter 32: Quick Review of Clinically
Anaerobic Rods Important Microorganisms
Chapter 15: Spirochetes Chapter 33: Disease Summaries
Chapter 16: Mycoplasma Chapter 34: Illustrated Case Studies
Chapter 17: Chlamydiae
Chapter 18: Mycobacteria and
Actinomycetes
, Ch. 1: Introduction to Microbiology
Advanced university-level multiple-choice questions.
Each question aligned to the chapter theme (overview of microbes, classification,
medical importance). Four options (A–D). The correct option is shown only as
Answer: X. A deep rationale and keywords follow each question.
Q1
Which molecular marker was foundational to Woese’s three-domain classification
and remains the standard for prokaryotic phylogenetics?
A. Peptidoglycan cross-linking pattern
B. 16S ribosomal RNA gene sequence
C. Fatty-acid composition of membrane lipids
D. Presence or absence of an outer membrane
Answer: B
Rationale: Carl Woese used small subunit ribosomal RNA sequences (16S rRNA
in prokaryotes) to reveal deep evolutionary relationships and to define the three
domains (Bacteria, Archaea, Eukarya). 16S rRNA is universally conserved with
variable regions useful for phylogeny. Peptidoglycan structure and membrane
lipids are informative for physiology and grouping (and distinguish Bacteria vs
Archaea), but they are not as broadly conserved or phylogenetically reliable as
rRNA sequences. Presence/absence of an outer membrane is a phenotypic trait that
does not capture evolutionary distances as robustly.
Keywords: 16S rRNA, Woese, phylogeny, three-domain system
Q2
The biochemical basis for the classical Gram stain dichotomy (Gram-positive vs
Gram-negative) is best explained by which of the following?
,A. Presence of teichoic acids alone
B. Ability to form endospores
C. Retention of the crystal violet-iodine complex by a thick peptidoglycan layer
D. Outer membrane lipopolysaccharide (LPS) composition
Answer: C
Rationale: Gram positivity results from retention of the crystal violet–iodine
complex within a thick, highly crosslinked peptidoglycan layer during the
decolorization step. This physical retention—not teichoic acids alone—accounts
for the purple color. Teichoic acids are associated with Gram-positive cells but are
not the direct reason for dye retention. The outer membrane/LPS is characteristic
of Gram-negative bacteria and explains why Gram-negatives are decolorized, but
the immediate staining mechanism is the peptidoglycan’s ability to trap the dye
complex. Endospore formation is unrelated to Gram staining.
Keywords: Gram stain, peptidoglycan, crystal violet, cell wall
Q3
Which membrane feature is characteristic of archaeal cells and distinguishes them
from typical bacterial membranes?
A. Phospholipids with ester-linked straight-chain fatty acids
B. Sterol-containing bilayers
C. Ether-linked isoprenoid chains that may form monolayers
D. Lipopolysaccharide (LPS) in the outer leaflet
Answer: C
Rationale: Archaeal membranes contain ether linkages between glycerol and
isoprenoid side chains; some archaeal lipids form a covalently linked monolayer
(tetraether lipids) conferring increased stability in extreme environments. Bacterial
membranes typically use ester-linked straight-chain fatty acids. Sterols are
common in eukaryotic membranes (and some bacteria) but not a defining archaeal
feature. LPS is a component of Gram-negative bacterial outer membranes and is
absent from archaeal membranes.
Keywords: Archaea, ether linkage, isoprenoids, lipid monolayer
,Q4
Under the Baltimore classification, which class corresponds to positive-sense
single-stranded RNA viruses whose genomes function directly as mRNA?
A. Class I
B. Class II
C. Class IV
D. Class VI
Answer: C
Rationale: In the Baltimore system, Class IV viruses are positive-sense single-
stranded RNA (+ssRNA) viruses; their genomes can be directly translated by host
ribosomes as mRNA. Class I = dsDNA, Class II = single-stranded DNA, and Class
VI = (+)ssRNA viruses that replicate through a DNA intermediate (retroviruses).
The distinction is functional (how genome becomes mRNA) rather than
taxonomic.
Keywords: Baltimore classification, +ssRNA, mRNA, viral replication
Q5
Which statement best captures the currently accepted model for prion infectivity?
A. Prions are small RNA viruses that evade detection by PCR.
B. Prions are misfolded host proteins that template a conformational change in
normal isoforms.
C. Prions are bacteria lacking cell walls that produce exotoxins.
D. Prions are viroids composed of circular DNA.
Answer: B
Rationale: The prion hypothesis states that an abnormal conformation of a host-
encoded protein (PrP^Sc) can induce misfolding of the normal cellular isoform
(PrP^C), leading to aggregation and neurodegeneration. Prions are proteinaceous
infectious agents without nucleic acid. They are not viruses, bacteria, or DNA-
,based viroids. Viroids are small infectious RNAs (plant pathogens), not circular
DNA. This protein-only mechanism explains transmissibility and resistance to
nucleic acid–targeting disinfectants.
Keywords: prion, PrP^Sc, protein misfolding, protein-only hypothesis
Q6
Which pair of organisms are typically not resolvable by standard bright-field light
microscopy and therefore require electron microscopy or molecular methods for
direct visualization/identification?
A. Bacillus anthracis and Candida albicans
B. Mycobacterium tuberculosis and Nocardia spp.
C. Viruses and prions
D. Escherichia coli and Staphylococcus aureus
Answer: C
Rationale: Viruses (generally 20–300 nm) are below the resolution limit (~200
nm) of light microscopy and require electron microscopy or molecular detection.
Prions are even smaller (misfolded proteins) and cannot be visualized by light
microscopy; diagnosis is made by biochemical or histopathologic markers and
advanced imaging. Bacteria and fungi (options A, B, D) are typically large enough
to be seen with light microscopy, though specialized stains or culture may be
needed for weakly staining or intracellular organisms.
Keywords: microscopy resolution, electron microscopy, viruses, prions
Q7
Which mechanism best explains “colonization resistance” provided by the normal
human microbiota?
A. Adaptive immunity generating pathogen-specific antibodies
B. Production of bacteriocins, competition for nutrients and niches, and stimulation
of mucosal defenses
,C. Permanent sterilization of mucosal surfaces by commensals
D. Horizontal gene transfer from commensals to pathogens to increase virulence
Answer: B
Rationale: Colonization resistance arises when resident microbiota limit pathogen
establishment through direct mechanisms (competition for nutrients and
attachment sites, secretion of antimicrobial peptides/bacteriocins) and indirect
mechanisms (stimulation of host innate and mucosal immune defenses). Adaptive
immunity (A) is a host response but not the principal mechanism of immediate
colonization resistance. Commensals do not sterilize surfaces; they occupy niches.
Horizontal gene transfer can change pathogen traits but generally undermines
resistance rather than constitutes it.
Keywords: microbiome, colonization resistance, bacteriocins, competition
Q8
Koch’s postulates historically linked microbes to disease. Which clinical scenario
most clearly violates the original classical postulates, prompting revisions for
modern microbiology?
A. An obligate pathogen that causes disease when introduced to a healthy
susceptible host
B. An organism that cannot be cultured in vitro using current methods yet is
detected by molecular probes in diseased tissue
C. A microbe that, when isolated and reintroduced to an animal model, reproduces
the same disease
D. A pathogen that is never found in diseased tissue
Answer: B
Rationale: The original Koch’s postulates require isolation and cultivation of the
organism; obligate intracellular or unculturable microbes (e.g., some viruses,
Mycobacterium leprae historically, and many newly discovered agents) violate this
requirement. Molecular detection of an agent in diseased tissue without
culturability forced modern revisions (molecular Koch’s postulates) that allow
nucleic acid evidence and correlations. Option D is nonsensical. Option A and C
conform to classical postulates.
, Keywords: Koch’s postulates, uncultivable organisms, molecular criteria,
causation
Q9
Relapsing fever caused by some Borrelia species is an example of immune evasion
driven primarily by which molecular mechanism?
A. Antigenic drift via polymerase point mutations akin to influenza
B. Horizontal gene transfer via conjugative plasmids
C. Gene conversion-mediated antigenic variation of surface-exposed lipoprotein
genes
D. Biofilm formation in the bloodstream
Answer: C
Rationale: Borrelia species causing relapsing fever undergo antigenic variation
through gene conversion of variable major protein (VMP) loci, periodically
changing surface lipoproteins and allowing immune escape and recurrent
bacteremia. Antigenic drift (A) is a mechanism for some RNA viruses (influenza)
but not the episodic relapses seen here. Conjugation and biofilm formation are not
the principal mechanisms of the relapsing pattern in Borrelia.
Keywords: Borrelia, antigenic variation, gene conversion, relapsing fever
Q10
Which of the following best exemplifies an opportunistic pathogen in the context
of human microbiota?
A. Mycobacterium tuberculosis colonizing and infecting healthy lungs
B. Candida albicans causing oral thrush in a patient receiving broad-spectrum
antibiotics and corticosteroids
C. Vibrio cholerae causing cholera after ingesting contaminated water
D. Streptococcus pyogenes causing primary pharyngitis in healthy children
Answer: B
Anaplasma, and Coxiella
Chapter 1: Introduction to Microbiology
Chapter 20: Fungi
Chapter 2: Normal Flora
Chapter 21: Protozoa
Chapter 3: Pathogenicity of
Microorganisms Chapter 22: Helminths
Chapter 4: Diagnostic Microbiology Chapter 23: Introduction to the Viruses
Chapter 5: Vaccines and Antimicrobial Chapter 24: Nonenveloped DNA Viruses
Agents
Chapter 25: Enveloped DNA Viruses
Chapter 6: Bacterial Structure, Growth,
and Metabolism Chapter 26: Hepatitis B and Hepatitis D
(Delta) Viruses
Chapter 7: Bacterial Genetics
Chapter 27: Positive-Strand RNA
Chapter 8: Staphylococci Viruses
Chapter 9: Streptococci Chapter 28: Retroviruses
Chapter 10: Gram-Positive Rods Chapter 29: Negative-Strand RNA
Viruses
Chapter 11: Neisseriae
Chapter 30: Double-Stranded RNA
Chapter 12: Gastrointestinal Gram- Viruses: Reoviridae
Negative Rods
Chapter 31: Unconventional Infectious
Chapter 13: Other Gram-Negative Rods Agents
Chapter 14: Clostridia and Other Chapter 32: Quick Review of Clinically
Anaerobic Rods Important Microorganisms
Chapter 15: Spirochetes Chapter 33: Disease Summaries
Chapter 16: Mycoplasma Chapter 34: Illustrated Case Studies
Chapter 17: Chlamydiae
Chapter 18: Mycobacteria and
Actinomycetes
, Ch. 1: Introduction to Microbiology
Advanced university-level multiple-choice questions.
Each question aligned to the chapter theme (overview of microbes, classification,
medical importance). Four options (A–D). The correct option is shown only as
Answer: X. A deep rationale and keywords follow each question.
Q1
Which molecular marker was foundational to Woese’s three-domain classification
and remains the standard for prokaryotic phylogenetics?
A. Peptidoglycan cross-linking pattern
B. 16S ribosomal RNA gene sequence
C. Fatty-acid composition of membrane lipids
D. Presence or absence of an outer membrane
Answer: B
Rationale: Carl Woese used small subunit ribosomal RNA sequences (16S rRNA
in prokaryotes) to reveal deep evolutionary relationships and to define the three
domains (Bacteria, Archaea, Eukarya). 16S rRNA is universally conserved with
variable regions useful for phylogeny. Peptidoglycan structure and membrane
lipids are informative for physiology and grouping (and distinguish Bacteria vs
Archaea), but they are not as broadly conserved or phylogenetically reliable as
rRNA sequences. Presence/absence of an outer membrane is a phenotypic trait that
does not capture evolutionary distances as robustly.
Keywords: 16S rRNA, Woese, phylogeny, three-domain system
Q2
The biochemical basis for the classical Gram stain dichotomy (Gram-positive vs
Gram-negative) is best explained by which of the following?
,A. Presence of teichoic acids alone
B. Ability to form endospores
C. Retention of the crystal violet-iodine complex by a thick peptidoglycan layer
D. Outer membrane lipopolysaccharide (LPS) composition
Answer: C
Rationale: Gram positivity results from retention of the crystal violet–iodine
complex within a thick, highly crosslinked peptidoglycan layer during the
decolorization step. This physical retention—not teichoic acids alone—accounts
for the purple color. Teichoic acids are associated with Gram-positive cells but are
not the direct reason for dye retention. The outer membrane/LPS is characteristic
of Gram-negative bacteria and explains why Gram-negatives are decolorized, but
the immediate staining mechanism is the peptidoglycan’s ability to trap the dye
complex. Endospore formation is unrelated to Gram staining.
Keywords: Gram stain, peptidoglycan, crystal violet, cell wall
Q3
Which membrane feature is characteristic of archaeal cells and distinguishes them
from typical bacterial membranes?
A. Phospholipids with ester-linked straight-chain fatty acids
B. Sterol-containing bilayers
C. Ether-linked isoprenoid chains that may form monolayers
D. Lipopolysaccharide (LPS) in the outer leaflet
Answer: C
Rationale: Archaeal membranes contain ether linkages between glycerol and
isoprenoid side chains; some archaeal lipids form a covalently linked monolayer
(tetraether lipids) conferring increased stability in extreme environments. Bacterial
membranes typically use ester-linked straight-chain fatty acids. Sterols are
common in eukaryotic membranes (and some bacteria) but not a defining archaeal
feature. LPS is a component of Gram-negative bacterial outer membranes and is
absent from archaeal membranes.
Keywords: Archaea, ether linkage, isoprenoids, lipid monolayer
,Q4
Under the Baltimore classification, which class corresponds to positive-sense
single-stranded RNA viruses whose genomes function directly as mRNA?
A. Class I
B. Class II
C. Class IV
D. Class VI
Answer: C
Rationale: In the Baltimore system, Class IV viruses are positive-sense single-
stranded RNA (+ssRNA) viruses; their genomes can be directly translated by host
ribosomes as mRNA. Class I = dsDNA, Class II = single-stranded DNA, and Class
VI = (+)ssRNA viruses that replicate through a DNA intermediate (retroviruses).
The distinction is functional (how genome becomes mRNA) rather than
taxonomic.
Keywords: Baltimore classification, +ssRNA, mRNA, viral replication
Q5
Which statement best captures the currently accepted model for prion infectivity?
A. Prions are small RNA viruses that evade detection by PCR.
B. Prions are misfolded host proteins that template a conformational change in
normal isoforms.
C. Prions are bacteria lacking cell walls that produce exotoxins.
D. Prions are viroids composed of circular DNA.
Answer: B
Rationale: The prion hypothesis states that an abnormal conformation of a host-
encoded protein (PrP^Sc) can induce misfolding of the normal cellular isoform
(PrP^C), leading to aggregation and neurodegeneration. Prions are proteinaceous
infectious agents without nucleic acid. They are not viruses, bacteria, or DNA-
,based viroids. Viroids are small infectious RNAs (plant pathogens), not circular
DNA. This protein-only mechanism explains transmissibility and resistance to
nucleic acid–targeting disinfectants.
Keywords: prion, PrP^Sc, protein misfolding, protein-only hypothesis
Q6
Which pair of organisms are typically not resolvable by standard bright-field light
microscopy and therefore require electron microscopy or molecular methods for
direct visualization/identification?
A. Bacillus anthracis and Candida albicans
B. Mycobacterium tuberculosis and Nocardia spp.
C. Viruses and prions
D. Escherichia coli and Staphylococcus aureus
Answer: C
Rationale: Viruses (generally 20–300 nm) are below the resolution limit (~200
nm) of light microscopy and require electron microscopy or molecular detection.
Prions are even smaller (misfolded proteins) and cannot be visualized by light
microscopy; diagnosis is made by biochemical or histopathologic markers and
advanced imaging. Bacteria and fungi (options A, B, D) are typically large enough
to be seen with light microscopy, though specialized stains or culture may be
needed for weakly staining or intracellular organisms.
Keywords: microscopy resolution, electron microscopy, viruses, prions
Q7
Which mechanism best explains “colonization resistance” provided by the normal
human microbiota?
A. Adaptive immunity generating pathogen-specific antibodies
B. Production of bacteriocins, competition for nutrients and niches, and stimulation
of mucosal defenses
,C. Permanent sterilization of mucosal surfaces by commensals
D. Horizontal gene transfer from commensals to pathogens to increase virulence
Answer: B
Rationale: Colonization resistance arises when resident microbiota limit pathogen
establishment through direct mechanisms (competition for nutrients and
attachment sites, secretion of antimicrobial peptides/bacteriocins) and indirect
mechanisms (stimulation of host innate and mucosal immune defenses). Adaptive
immunity (A) is a host response but not the principal mechanism of immediate
colonization resistance. Commensals do not sterilize surfaces; they occupy niches.
Horizontal gene transfer can change pathogen traits but generally undermines
resistance rather than constitutes it.
Keywords: microbiome, colonization resistance, bacteriocins, competition
Q8
Koch’s postulates historically linked microbes to disease. Which clinical scenario
most clearly violates the original classical postulates, prompting revisions for
modern microbiology?
A. An obligate pathogen that causes disease when introduced to a healthy
susceptible host
B. An organism that cannot be cultured in vitro using current methods yet is
detected by molecular probes in diseased tissue
C. A microbe that, when isolated and reintroduced to an animal model, reproduces
the same disease
D. A pathogen that is never found in diseased tissue
Answer: B
Rationale: The original Koch’s postulates require isolation and cultivation of the
organism; obligate intracellular or unculturable microbes (e.g., some viruses,
Mycobacterium leprae historically, and many newly discovered agents) violate this
requirement. Molecular detection of an agent in diseased tissue without
culturability forced modern revisions (molecular Koch’s postulates) that allow
nucleic acid evidence and correlations. Option D is nonsensical. Option A and C
conform to classical postulates.
, Keywords: Koch’s postulates, uncultivable organisms, molecular criteria,
causation
Q9
Relapsing fever caused by some Borrelia species is an example of immune evasion
driven primarily by which molecular mechanism?
A. Antigenic drift via polymerase point mutations akin to influenza
B. Horizontal gene transfer via conjugative plasmids
C. Gene conversion-mediated antigenic variation of surface-exposed lipoprotein
genes
D. Biofilm formation in the bloodstream
Answer: C
Rationale: Borrelia species causing relapsing fever undergo antigenic variation
through gene conversion of variable major protein (VMP) loci, periodically
changing surface lipoproteins and allowing immune escape and recurrent
bacteremia. Antigenic drift (A) is a mechanism for some RNA viruses (influenza)
but not the episodic relapses seen here. Conjugation and biofilm formation are not
the principal mechanisms of the relapsing pattern in Borrelia.
Keywords: Borrelia, antigenic variation, gene conversion, relapsing fever
Q10
Which of the following best exemplifies an opportunistic pathogen in the context
of human microbiota?
A. Mycobacterium tuberculosis colonizing and infecting healthy lungs
B. Candida albicans causing oral thrush in a patient receiving broad-spectrum
antibiotics and corticosteroids
C. Vibrio cholerae causing cholera after ingesting contaminated water
D. Streptococcus pyogenes causing primary pharyngitis in healthy children
Answer: B
