Bacteriology MCQs: Key Characteristics of Bacterial Pathogens

Ten multiple choice questions on the defining features of important bacterial pathogens, with a full answer key and explanations at the end. Write down your answers first, then check against the key.

1. A diagnosis of diphtheria is confirmed by:

A) Microscopic appearance of organisms stained with methylene blue
B) Isolation of a typical colony on Tinsdale's agar
C) Isolation of typical organisms from materials such as blood, showing invasiveness
D) Detection of beta phage plaques in cultures of suspicious isolates E) Demonstration of toxin production by a suspicious isolate

2. Listeria monocytogenes shows which of the following characteristics?

A) It can grow at refrigerator temperatures (4°C)
B) It is an extracellular pathogen
C) It is catalase-negative
D) It is a gram-negative coccus
E) It is strictly a human pathogen

3. Which one of the following is characteristic of N. meningitidis but not N. gonorrhoeae?

A) Ferments glucose
B) Contains a polysaccharide capsule
C) Is oxidase-positive
D) Most isolates show resistance to penicillin
E) No effective vaccines are available

4. Which of the following is true of Haemophilus influenzae?

A) Invasive infections are most commonly associated with encapsulated strains
B) Most invasive infections occur in infants during the neonatal period
C) Most human infections are acquired from domestic pets
D) The organism can be readily cultured on sheep blood agar in an environment of elevated CO2
E) Older adults are rarely at risk for infection with this organism because they typically have a high level of immunity

5. For which of the following organisms is there no known animal reservoir?

A) Francisella tularensis
B) Pasteurella multocida
C) Bordetella pertussis
D) Brucella melitensis
E) Yersinia pestis

6. A distinguishing feature of human mycoplasma species is that they:

A) Stain well with Giemsa, but not by Gram stain
B) Contain no bacterial peptidoglycan
C) Are not immunogenic because they mimic host cell membrane components
D) Cannot be cultivated in vitro
E) Are dependent on host sources of ATP

7. Which one of the following is most characteristic of Mycoplasma pneumoniae infection?

A) Infection results in a fever of sudden onset accompanied by a productive cough B) Infection most commonly occurs in the upper respiratory tract
C) Infection is definitively diagnosed by direct microscopic examination of sputum D) Re-infection is rare and less severe than primary infection
E) Infection causes extensive scarring and calcification of affected lung tissue

8. Which one of the following organisms is a common cause of urinary tract infection and characteristically produces a strong urease, raising urine pH and predisposing to struvite (staghorn) kidney stones?

A) Escherichia coli
B) Proteus mirabilis
C) Enterococcus faecalis
D) Staphylococcus saprophyticus
E) Klebsiella pneumoniae

9. A gram-positive, catalase-positive, coagulase-negative coccus is isolated from the urine of a sexually active young woman with cystitis. It is novobiocin-resistant. The most likely organism is:

A) Staphylococcus aureus
B) Staphylococcus epidermidis
C) Staphylococcus saprophyticus
D) Enterococcus faecalis
E) Streptococcus agalactiae

10. Which one of the following statements about Pseudomonas aeruginosa is correct?

A) It is a fastidious organism that grows only on enriched media
B) It produces a blue-green pigment and a characteristic grape-like or corn-tortilla odor
C) It is a gram-positive rod
D) It is oxidase-negative
E) Infections are easily treated with most oral antibiotics

Answer key

1. E. Demonstration of toxin production by a suspicious isolate
2. A. It can grow at refrigerator temperatures (4°C)
3. B. Contains a polysaccharide capsule
4. A. Invasive infections are most commonly associated with encapsulated strains
5. C. Bordetella pertussis
6. B. Contain no bacterial peptidoglycan
7. B. Infection most commonly occurs in the upper respiratory tract
8. B. Proteus mirabilis
9. C. Staphylococcus saprophyticus
10. B. It produces a blue-green pigment and a characteristic grape-like or corn-tortilla odor

Why these are the answers

1. Demonstration of toxin production. Diphtheria is confirmed by showing that the isolate produces diphtheria toxin, because it is the toxin, not the mere presence of the organism, that defines toxigenic Corynebacterium diphtheriae. Methylene blue microscopy (A) and a typical colony on Tinsdale agar (B) are only presumptive. C. diphtheriae is noninvasive, so it is recovered from surface sites such as the oropharynx and skin lesions, not from blood (C). The beta phage carries the tox gene, but it is a temperate phage, so it does not form lytic plaques (D).

2. It can grow at refrigerator temperatures. Listeria monocytogenes grows optimally at 30 to 37°C but can also grow at refrigerator temperature (4°C), which is why refrigeration does not reliably stop its growth in contaminated food. This is the basis of listeriosis outbreaks from cold deli products and soft cheeses. It is catalase-positive, gram-positive, and a facultative intracellular pathogen, so options B, C, and D are wrong. It is found in cattle, other warm-blooded animals, and fish, so it is not strictly a human pathogen (E).

3. Contains a polysaccharide capsule. Neisseria meningitidis has a polysaccharide capsule and N. gonorrhoeae does not, which is the distinction the question asks for. Both are oxidase-positive, so C is shared, not distinguishing. On sugars, N. meningitidis ferments both glucose and maltose while N. gonorrhoeae ferments only glucose. Most meningococci remain penicillin-sensitive (though resistance is rising), and effective capsular vaccines exist for meningococcal serogroups A, C, W-135, and Y, but not for gonococcus.

4. Invasive infections are most commonly associated with encapsulated strains. Invasive Haemophilus influenzae disease is caused mostly by encapsulated (typeable, especially type b) strains, because the capsule is antiphagocytic and lets the organism spread through the blood. Maternal IgG protects most neonates, so invasive disease peaks after that passive immunity wanes, not during the neonatal period (B). Humans are the only natural host, so C is wrong. On growth: H. influenzae needs hemin (factor X) and NAD (factor V); on plain blood agar factor V is unavailable, but heating the blood lyses the red cells, releases the factors, and destroys the NAD-degrading enzyme, giving chocolate agar. The organism does prefer elevated CO2.

5. Bordetella pertussis. It is a strictly human pathogen with no known animal reservoir, so whooping cough is sustained only by person-to-person spread. The other four are zoonoses with animal reservoirs: Francisella tularensis (wild and domestic animals, birds, pets), Pasteurella multocida (mammals and birds), Brucella melitensis (sheep and goats), and Yersinia pestis (rodents and other mammals). Only Bordetella fits "no animal reservoir."

6. Contain no bacterial peptidoglycan. The defining feature of the mycoplasmas is that they have no cell wall at all, and therefore no peptidoglycan. This single fact explains most of their other properties: they are not seen on Gram stain, they take irregular (pleomorphic) shapes, and they are intrinsically resistant to cell-wall-active antibiotics such as the beta-lactams. The options about ATP dependence or non-immunogenicity are not the distinguishing feature.

7. Infection most commonly occurs in the upper respiratory tract. Mycoplasma pneumoniae infection most often involves the upper respiratory tract and ear, more commonly than the well-known atypical ("walking") pneumonia. When pneumonia does occur, it comes on gradually with a scanty, nonproductive cough, not the sudden fever and purulent sputum of typical bacterial pneumonia (A is wrong). Direct microscopy of sputum is of little value, because the sputum is scanty and the organism, lacking a cell wall, stains poorly or not at all with standard bacteriologic stains. This is the exact point behind the common exam phrasing that M. pneumoniae cannot be cultured or seen from sputum by routine methods (C is wrong). Reinfection tends to be more severe, not milder (D is wrong), and recovery is typically slow but complete, without the scarring or calcification described in E.

8. Proteus mirabilis. Proteus mirabilis is a common urinary pathogen and a strong urease producer. The urease splits urea into ammonia, which raises urine pH and drives precipitation of magnesium ammonium phosphate (struvite), forming the large branching staghorn calculi that can fill the renal pelvis. E. coli and Klebsiella cause UTIs but do not typically alkalinize urine this way. A clinical clue to carry: alkaline urine in a UTI, with stones, points to a urease-positive organism such as Proteus. Proteus also swarms across blood agar and is rarely part of the normal nasal flora, which is why a Proteus from the nose would be an unusual finding.

9. Staphylococcus saprophyticus. All staphylococci are gram-positive and catalase-positive; the coagulase-negative result rules out S. aureus. Among the coagulase-negative staphylococci, novobiocin resistance separates Staphylococcus saprophyticus (novobiocin-resistant) from Staphylococcus epidermidis (novobiocin-sensitive). S. saprophyticus is a leading cause of uncomplicated cystitis in young, sexually active women, second only to E. coli in that group. The novobiocin disk is the classic bench test that distinguishes the two skin staphylococci once coagulase is negative.

10. It produces a blue-green pigment and a characteristic grape-like or corn-tortilla odor. Pseudomonas aeruginosa is a non-fastidious, oxidase-positive, gram-negative rod that grows on ordinary media. It produces pyocyanin (blue-green) and pyoverdine (yellow-green) pigments and gives off a sweet, grape-like or corn-tortilla odor that experienced technologists recognize on the plate. It is intrinsically resistant to many antibiotics because of low outer-membrane permeability and efflux pumps, so infections are often difficult to treat and are not reliably covered by common oral agents. Options A, C, D, and E each state the opposite of a defining Pseudomonas feature.