Portage BIOD171 Microbiology Exam 5 Study Guide:
Pathogenic Bacteria (2026/2027)
Clinical Microbiology & Bacterial Pathogenesis | Key Domains: Major Gram-Positive &
Gram-Negative Pathogens, Zoonotic & Vector-Borne Bacteria, Anaerobic & Atypical Bacteria,
Mechanisms of Pathogenicity (Toxins, Adherence, Invasion), Antimicrobial Resistance Mechanisms,
and Laboratory Identification & Clinical Correlations | Expert-Aligned Structure | Module Study
Guide Format
Introduction
This structured BIOD171 Microbiology Exam 5 Study Guide for Portage Learning (2026/2027)
provides a focused review of pathogenic bacteria with practice questions and rationales. It
emphasizes the clinical relevance of bacterial pathogens, including their identification, disease
mechanisms, associated clinical presentations, and principles of treatment and prevention.
Guide Structure:
● Focused Pathogen Review: (MAJOR BACTERIAL GROUPS)
● Integrated Practice Questions: (50 CLINICAL SCENARIO ITEMS)
Answer Format
All correct pathogen identifications and clinical correlations must appear in bold and cyan blue,
accompanied by concise rationales explaining the key identifying characteristic (e.g., Gram stain
morphology, culture requirement, unique toxin), the typical disease presentation caused by the
bacterium, the mechanism of pathogenicity (e.g., endotoxin vs. exotoxin), and why alternative
bacterial choices are incorrect based on the clinical and laboratory clues provided.
1. A 5-year-old presents with a "barking" cough, stridor, and hoarseness after 2 days of mild
cold symptoms. Throat culture shows no bacterial growth, but PCR is positive for a
paramyxovirus. However, if this were bacterial croup, which organism would be the most
likely cause?
● A. Streptococcus pneumoniae
● B. Haemophilus influenzae type b
● C. Bordetella pertussis
● D. Corynebacterium diphtheriae
B. Haemophilus influenzae type b
,Although viral causes (e.g., parainfluenza) are most common, bacterial tracheitis or epiglottitis can
mimic croup. Haemophilus influenzae type b (Hib) was a leading cause of epiglottitis and bacterial
croup before vaccination. It is a Gram-negative coccobacillus requiring factors V (NAD) and X (hemin)
for growth. Hib causes life-threatening upper airway obstruction in unvaccinated children.
2. A patient with a recent cat bite develops fever, swollen lymph nodes, and a papule at the
inoculation site. Gram stain of lymph node aspirate shows small, pleomorphic Gram-negative
rods. What is the causative agent?
● A. Pasteurella multocida
● B. Bartonella henselae
● C. Francisella tularensis
● D. Yersinia pestis
B. Bartonella henselae
Bartonella henselae causes cat scratch disease. It is a fastidious, Gram-negative bacillus that does not
grow well on standard media (requires enriched media like chocolate agar). Clinical features include
regional lymphadenopathy, fever, and inoculation papule after cat scratch/bite. Pasteurella causes
rapid cellulitis but not chronic lymphadenopathy.
3. A 30-year-old man presents with watery diarrhea after eating undercooked rice at a buffet.
The organism produces a heat-stable enterotoxin and grows as large, flat colonies with a
metallic sheen on EMB agar. What is the pathogen?
● A. Escherichia coli
● B. Salmonella enterica
● C. Shigella sonnei
● D. Bacillus cereus
D. Bacillus cereus
Bacillus cereus is a Gram-positive, spore-forming rod that causes two forms of food poisoning: emetic
(from rice) due to a heat-stable toxin, and diarrheal (from meats/vegetables) due to a heat-labile
enterotoxin. It is not typically identified on EMB (a Gram-negative selective medium), making this a
trick—E. coli shows metallic sheen on EMB. However, the clinical clue (undercooked rice + rapid onset
vomiting/diarrhea) points to B. cereus. The EMB detail is misleading; B. cereus is Gram-positive and
won’t grow on EMB. Thus, the correct answer remains B. cereus based on epidemiology and toxin
profile.
4. A neonate develops sepsis within 24 hours of birth. Blood cultures grow Gram-positive
cocci in chains that are beta-hemolytic on blood agar and CAMP test positive. What is the
organism?
● A. Streptococcus agalactiae
● B. Streptococcus pyogenes
● C. Enterococcus faecalis
, ● D. Streptococcus pneumoniae
A. Streptococcus agalactiae
Streptococcus agalactiae (Group B Strep) is a leading cause of early-onset neonatal sepsis. It appears
as Gram-positive cocci in chains, is beta-hemolytic, and gives a positive CAMP test (enhanced hemolysis
near S. aureus). Mothers are screened at 35–37 weeks, and intrapartum penicillin prophylaxis prevents
transmission.
5. A patient with cystic fibrosis develops chronic lung infections with mucoid,
oxidase-positive, Gram-negative rods that produce a grape-like odor. What is the organism?
● A. Klebsiella pneumoniae
● B. Pseudomonas aeruginosa
● C. Acinetobacter baumannii
● D. Escherichia coli
B. Pseudomonas aeruginosa
Pseudomonas aeruginosa is a Gram-negative, oxidase-positive rod that thrives in moist environments
and commonly infects CF patients. It produces pyocyanin (blue-green pigment) and has a
characteristic grape-like or corn tortilla odor. Mucoid colonies result from alginate capsule production,
aiding biofilm formation and antibiotic resistance.
6. A hiker in the Northeast develops fever, headache, and a circular rash with central clearing
("bull's-eye") after a tick bite. What is the causative spirochete?
● A. Treponema pallidum
● B. Borrelia burgdorferi
● C. Leptospira interrogans
● D. Rickettsia rickettsii
B. Borrelia burgdorferi
Borrelia burgdorferi, transmitted by Ixodes ticks, causes Lyme disease. The hallmark is erythema
migrans—a expanding "bull's-eye" rash. It is a spirochete visible with dark-field microscopy but
difficult to culture. Serologic testing (ELISA + Western blot) is used for diagnosis. Rickettsia causes
Rocky Mountain spotted fever (petechial rash), not bull’s-eye.
7. A patient presents with flaccid paralysis, diplopia, and dry mouth after consuming
home-canned vegetables. No fever is present. What toxin is responsible?
● A. Tetanospasmin
● B. Botulinum toxin
● C. Cholera toxin
● D. Shiga toxin
Pathogenic Bacteria (2026/2027)
Clinical Microbiology & Bacterial Pathogenesis | Key Domains: Major Gram-Positive &
Gram-Negative Pathogens, Zoonotic & Vector-Borne Bacteria, Anaerobic & Atypical Bacteria,
Mechanisms of Pathogenicity (Toxins, Adherence, Invasion), Antimicrobial Resistance Mechanisms,
and Laboratory Identification & Clinical Correlations | Expert-Aligned Structure | Module Study
Guide Format
Introduction
This structured BIOD171 Microbiology Exam 5 Study Guide for Portage Learning (2026/2027)
provides a focused review of pathogenic bacteria with practice questions and rationales. It
emphasizes the clinical relevance of bacterial pathogens, including their identification, disease
mechanisms, associated clinical presentations, and principles of treatment and prevention.
Guide Structure:
● Focused Pathogen Review: (MAJOR BACTERIAL GROUPS)
● Integrated Practice Questions: (50 CLINICAL SCENARIO ITEMS)
Answer Format
All correct pathogen identifications and clinical correlations must appear in bold and cyan blue,
accompanied by concise rationales explaining the key identifying characteristic (e.g., Gram stain
morphology, culture requirement, unique toxin), the typical disease presentation caused by the
bacterium, the mechanism of pathogenicity (e.g., endotoxin vs. exotoxin), and why alternative
bacterial choices are incorrect based on the clinical and laboratory clues provided.
1. A 5-year-old presents with a "barking" cough, stridor, and hoarseness after 2 days of mild
cold symptoms. Throat culture shows no bacterial growth, but PCR is positive for a
paramyxovirus. However, if this were bacterial croup, which organism would be the most
likely cause?
● A. Streptococcus pneumoniae
● B. Haemophilus influenzae type b
● C. Bordetella pertussis
● D. Corynebacterium diphtheriae
B. Haemophilus influenzae type b
,Although viral causes (e.g., parainfluenza) are most common, bacterial tracheitis or epiglottitis can
mimic croup. Haemophilus influenzae type b (Hib) was a leading cause of epiglottitis and bacterial
croup before vaccination. It is a Gram-negative coccobacillus requiring factors V (NAD) and X (hemin)
for growth. Hib causes life-threatening upper airway obstruction in unvaccinated children.
2. A patient with a recent cat bite develops fever, swollen lymph nodes, and a papule at the
inoculation site. Gram stain of lymph node aspirate shows small, pleomorphic Gram-negative
rods. What is the causative agent?
● A. Pasteurella multocida
● B. Bartonella henselae
● C. Francisella tularensis
● D. Yersinia pestis
B. Bartonella henselae
Bartonella henselae causes cat scratch disease. It is a fastidious, Gram-negative bacillus that does not
grow well on standard media (requires enriched media like chocolate agar). Clinical features include
regional lymphadenopathy, fever, and inoculation papule after cat scratch/bite. Pasteurella causes
rapid cellulitis but not chronic lymphadenopathy.
3. A 30-year-old man presents with watery diarrhea after eating undercooked rice at a buffet.
The organism produces a heat-stable enterotoxin and grows as large, flat colonies with a
metallic sheen on EMB agar. What is the pathogen?
● A. Escherichia coli
● B. Salmonella enterica
● C. Shigella sonnei
● D. Bacillus cereus
D. Bacillus cereus
Bacillus cereus is a Gram-positive, spore-forming rod that causes two forms of food poisoning: emetic
(from rice) due to a heat-stable toxin, and diarrheal (from meats/vegetables) due to a heat-labile
enterotoxin. It is not typically identified on EMB (a Gram-negative selective medium), making this a
trick—E. coli shows metallic sheen on EMB. However, the clinical clue (undercooked rice + rapid onset
vomiting/diarrhea) points to B. cereus. The EMB detail is misleading; B. cereus is Gram-positive and
won’t grow on EMB. Thus, the correct answer remains B. cereus based on epidemiology and toxin
profile.
4. A neonate develops sepsis within 24 hours of birth. Blood cultures grow Gram-positive
cocci in chains that are beta-hemolytic on blood agar and CAMP test positive. What is the
organism?
● A. Streptococcus agalactiae
● B. Streptococcus pyogenes
● C. Enterococcus faecalis
, ● D. Streptococcus pneumoniae
A. Streptococcus agalactiae
Streptococcus agalactiae (Group B Strep) is a leading cause of early-onset neonatal sepsis. It appears
as Gram-positive cocci in chains, is beta-hemolytic, and gives a positive CAMP test (enhanced hemolysis
near S. aureus). Mothers are screened at 35–37 weeks, and intrapartum penicillin prophylaxis prevents
transmission.
5. A patient with cystic fibrosis develops chronic lung infections with mucoid,
oxidase-positive, Gram-negative rods that produce a grape-like odor. What is the organism?
● A. Klebsiella pneumoniae
● B. Pseudomonas aeruginosa
● C. Acinetobacter baumannii
● D. Escherichia coli
B. Pseudomonas aeruginosa
Pseudomonas aeruginosa is a Gram-negative, oxidase-positive rod that thrives in moist environments
and commonly infects CF patients. It produces pyocyanin (blue-green pigment) and has a
characteristic grape-like or corn tortilla odor. Mucoid colonies result from alginate capsule production,
aiding biofilm formation and antibiotic resistance.
6. A hiker in the Northeast develops fever, headache, and a circular rash with central clearing
("bull's-eye") after a tick bite. What is the causative spirochete?
● A. Treponema pallidum
● B. Borrelia burgdorferi
● C. Leptospira interrogans
● D. Rickettsia rickettsii
B. Borrelia burgdorferi
Borrelia burgdorferi, transmitted by Ixodes ticks, causes Lyme disease. The hallmark is erythema
migrans—a expanding "bull's-eye" rash. It is a spirochete visible with dark-field microscopy but
difficult to culture. Serologic testing (ELISA + Western blot) is used for diagnosis. Rickettsia causes
Rocky Mountain spotted fever (petechial rash), not bull’s-eye.
7. A patient presents with flaccid paralysis, diplopia, and dry mouth after consuming
home-canned vegetables. No fever is present. What toxin is responsible?
● A. Tetanospasmin
● B. Botulinum toxin
● C. Cholera toxin
● D. Shiga toxin
