Bacteriology MCQs: Zoonoses, STIs and Spirochetes

Twenty clinical multiple choice questions on zoonotic bacteria, sexually transmitted infections, and spirochetes, with a full answer key and explanations at the end. Write down your answers first, then check against the key.

Zoonoses and high-consequence bacteria

1. Which of the following is generally not considered a potential agent of bioterrorism and biologic warfare?

a. Yersinia pestis
b. Botulinum toxin
c. Streptococcus pyogenes
d. Brucella species
e. Bacillus anthracis

2. Which of the following species of Pasteurella has been associated with infections of the female genital tract and a newborn?

a. Pasteurella multocida
b. Pasteurella pneumotropica
c. Pasteurella ureae
d. Pasteurella bettyae

3. In a patient who has the bubonic form of plague, all of the following specimens are acceptable for diagnosis except:

a. Stool culture on Hektoen Enteric agar
b. Blood culture using routine laboratory media
c. Culture of a lymph node aspirate on blood or MacConkey agar
d. Acute and convalescent serology
e. Immunohistochemical staining of lymph node tissue

4. An 8-year-old boy was bitten by a stray cat. Two days later, the wound was red, swollen, and draining purulent fluid. Pasteurella multocida was cultured from the wound. The drug of choice to treat this infection is:

a. Amikacin
b. Erythromycin
c. Gentamicin
d. Penicillin G
e. Clindamycin

5. All of the following statements regarding the epidemiology of infections caused by Yersinia enterocolitica are correct except:

a. Most human infections are caused by serotypes O:3 and O:9
b. Humans acquire the infection from ingesting food or drinks contaminated by animals or animal products
c. Person-to-person spread is quite common
d. A large inoculum is required to cause infection
e. Infection is more prevalent in persons with histocompatibility antigen HLA-B27

6. Intimate contacts of patients with suspected plague pneumonia should receive which of the following agents as chemoprophylaxis?

a. Gentamicin
b. Cefazolin
c. Rifampin
d. Penicillin
e. Doxycycline

7. Which of the following subspecies of Francisella tularensis is the most virulent for humans?

a. tularensis
b. holarctica
c. mediasiatica
d. novicida

8. All the following statements regarding the etiologic agent of chancroid are correct except:

a. The organism is a small gram-negative rod
b. The organism requires X factor but not V factor
c. The organism grows well on standard chocolate agar
d. On Gram stain of lesions, the organism occurs in strands
e. The organism is susceptible to erythromycin

9. Which of the following cell components produced by Neisseria gonorrhoeae is responsible for attachment to host cells?

a. Lipooligosaccharide
b. Pili (fimbriae)
c. IgA1 protease
d. Outer membrane porin protein
e. Iron binding protein

10. An 8-year-old girl was playing in a slowly moving stream when she cut her foot on a sharp object. Three days later, she was brought to the emergency room of Tribhuvan University Teaching Hospital because of pain and swelling at the site of the wound and drainage of pus from it. The most likely cause of the infection is:

a. Vibrio vulnificus
b. Escherichia coli
c. Aeromonas hydrophila
d. Proteus mirabilis
e. Salmonella Typhimurium

Sexually transmitted infections and spirochetes

11. A 70-year-old woman with severe chronic lung disease presented in the OPD of Tribhuvan University Teaching Hospital with fever, productive cough with purulent sputum, and worsening hypoxemia. A sputum sample was collected and sent to the microbiology lab. The lab reported plenty of polymorphonuclear leukocytes with both intracellular and extracellular gram-negative diplococci. This organism grows well on 5% sheep blood agar and chocolate agar. Later, a butyrate esterase test was done and found to be positive. What is the most likely organism involved in this illness?

a. Haemophilus influenzae
b. Moraxella catarrhalis
c. Neisseria gonorrhoeae
d. Neisseria lactamica
e. Neisseria meningitidis

12. A 32-year-old woman who is 10 weeks pregnant presents to the Obstetrics clinic for prenatal care. She has a history of treatment for syphilis 7 years previously. The results of serologic tests for syphilis are: non-treponemal test (RPR) nonreactive; treponemal test (TP-PA) reactive. Which of the following statements is most correct?

a. The baby is at high risk for congenital syphilis
b. The mother needs a lumbar puncture and a VDRL test of her CSF for neurosyphilis
c. The mother needs to be treated again for syphilis
d. The mother's previous treatment for syphilis was effective

13. A 26-year-old sexually active woman presents to the OPD with purulent vaginal discharge and dysuria. She reports unprotected sexual intercourse with a new partner a week ago. Which of the following diagnostic tests will be the most sensitive method to find the etiologic agents involved in this case?

a. An enzyme immunoassay
b. Bacterial culture on selective media
c. Gram's stain
d. Molecular diagnostic methods
e. Serology

14. A 20-year-old woman who reports unprotected sex with a new partner 2 weeks previously develops fever and left lower quadrant abdominal pain, with onset in association with her menstrual period. Neisseria gonorrhoeae is cultured from her endocervix. The diagnosis is gonococcal pelvic inflammatory disease. What is a common sequela of this infection?

a. Cancer of the cervix
b. Infertility
c. Urethral stricture
d. Uterine fibroid tumors
e. Vaginal-rectal fistula

15. A 40-year-old man is suffering from multiple episodes of disseminated gonococcal infection over the last few years. Cultures of his urethra and knee fluid yield Neisseria gonorrhoeae. What is the most likely cause for this recurrence?

a. A polymorphonuclear cell chemotactic factor deficiency
b. Absence of lymphocyte adenosine deaminase activity
c. Deficiency of a late-acting complement component (C5, C6, C7, or C8)
d. Myeloperoxidase deficiency
e. Selective IgA deficiency

16. Infection with which of the following agents can result in a biological false-positive non-treponemal (VDRL or RPR) test for syphilis?

a. Borrelia burgdorferi
b. Epstein-Barr virus
c. Streptococcus pyogenes
d. Varicella-zoster virus
e. All of the above

17. A false-positive non-treponemal (VDRL or RPR) test for syphilis is a common diagnostic problem. In which of the following conditions can you expect false-positive VDRL or RPR results?

a. Leprosy
b. Lupus erythematosus
c. Malaria
d. Measles
e. All of the above

18. A 22-year-old woman presents with a 2 cm ulcer on her labia majora. The lesion is painless and has a raised border. The differential diagnosis of this lesion includes:

a. Adenovirus infection
b. Chlamydia trachomatis infection
c. Neisseria gonorrhoeae infection
d. Papillomavirus infection
e. Treponema pallidum infection

19. After returning from a community posting in a rural village in Nepal, a 20-year-old medical student was admitted to the hospital with fever (39°C) and headache. He had been swimming in a pond where cows are also bathed. Based on clinical examination, leptospirosis is suspected. Which of the following tests will help to confirm the diagnosis?

a. Culture of CSF on blood and chocolate agar
b. Culture of urine on human diploid fibroblast cells
c. Testing acute and convalescent-phase sera for anti-leptospiral antibodies
d. Testing acute and convalescent-phase sera using the RPR test
e. Testing serum by dark-field examination for the presence of leptospires

20. Which of the following animals is a reservoir or source of Leptospira interrogans?

a. Cattle
b. Dogs
c. Mice
d. Swine
e. All of the above

Answer key

1. c. Streptococcus pyogenes
2. d. Pasteurella bettyae
3. a. Stool culture on Hektoen Enteric agar
4. d. Penicillin G
5. c. Person-to-person spread is quite common
6. e. Doxycycline
7. a. tularensis (Type A)
8. c. The organism grows well on standard chocolate agar
9. b. Pili (fimbriae)
10. c. Aeromonas hydrophila
11. b. Moraxella catarrhalis
12. d. The mother's previous treatment for syphilis was effective
13. d. Molecular diagnostic methods (NAAT)
14. b. Infertility
15. c. Deficiency of a late-acting complement component (C5, C6, C7, or C8)
16. e. All of the above
17. e. All of the above
18. e. Treponema pallidum infection
19. c. Testing acute and convalescent-phase sera for anti-leptospiral antibodies
20. e. All of the above

Why these are the answers

1. Streptococcus pyogenes. High-consequence pathogens are categorized by the CDC as potential bioweapons: Bacillus anthracis (anthrax), Yersinia pestis (plague), and botulinum toxin are Category A agents, and Brucella species are Category B. Streptococcus pyogenes (group A strep), though it causes severe suppurative infection and necrotizing fasciitis, is a ubiquitous community pathogen and is not classified as a biological warfare agent.

2. Pasteurella bettyae. While Pasteurella multocida is overwhelmingly linked to animal bites and scratches, Pasteurella bettyae is distinct because it is isolated mainly from human genitourinary infections and Bartholin gland abscesses, and it can be transmitted vertically to a newborn during delivery, causing neonatal septicemia.

3. Stool culture on Hektoen Enteric agar. Bubonic plague is characterized by painful, swollen regional lymph nodes (buboes), where Yersinia pestis multiplies rapidly before spilling into the blood. So lymph node aspirate culture, routine blood culture, serology, and tissue immunohistochemistry are all valid. Hektoen Enteric agar is a selective medium for intestinal pathogens such as Salmonella and Shigella; it has no role in bubonic plague, because the organism is not shed in feces in this form of the disease.

4. Penicillin G. Pasteurella multocida is an exception to the usual gram-negative treatment pattern, because it stays highly sensitive to narrow-spectrum penicillins. For animal bites empirically, a beta-lactamase-inhibitor combination such as amoxicillin-clavulanate is preferred, because it also covers the mouth anaerobes in the wound. But for a confirmed, isolated P. multocida infection, penicillin G (or oral amoxicillin) is the definitive drug of choice. It responds poorly and unpredictably to macrolides such as erythromycin and to lincosamides such as clindamycin, which is why those distractors are wrong.

5. Person-to-person spread is quite common. This is the false statement, so it is the answer to an "except" question. Yersinia enterocolitica is a zoonotic enteric pathogen acquired by the fecal-oral route, classically from undercooked pork or contaminated water; direct person-to-person spread is rare. The other statements are true: most human infections are caused by serotypes O:3 and O:9, a relatively large inoculum is needed, and there is a well-documented link between Y. enterocolitica reactive arthritis and the HLA-B27 tissue type.

6. Doxycycline. Pneumonic plague spreads by respiratory droplets and is rapidly fatal if untreated, so close contacts need post-exposure prophylaxis. Oral doxycycline or a fluoroquinolone such as ciprofloxacin is the standard choice to halt infection before it establishes. Aminoglycosides such as gentamicin are reserved for treating active, systemic plague rather than for prophylaxis, which is why gentamicin is the tempting but wrong option here.

7. tularensis (Type A). Francisella tularensis is divided into subspecies of differing virulence. Subspecies tularensis (Type A) is the most lethal for humans, often causing severe disease from an infectious dose of fewer than 10 organisms. Subspecies holarctica (Type B) causes a milder tularemia, and novicida is generally of low virulence in immunocompetent people. This extremely low infectious dose is also why F. tularensis is handled as a high-risk laboratory pathogen.

8. The organism grows well on standard chocolate agar. This is the false statement and therefore the answer. The cause of chancroid is Haemophilus ducreyi, an extremely fastidious organism that does not grow well on standard chocolate agar. It needs specially enriched media (supplemented with fetal bovine serum and vancomycin to suppress competing genital flora) incubated under humid, capnophilic conditions. The true statements: it is a small gram-negative rod, it requires exogenous X factor (hemin), it lines up in a "school of fish" or parallel-strand pattern on Gram stain of lesions, and it responds to macrolide therapy.

9. Pili (fimbriae). Pili are hair-like appendages projecting from the outer membrane of Neisseria gonorrhoeae, and they are the primary virulence factor for initial attachment to the microvilli of non-ciliated urogenital epithelium. The distractors each have a different job: lipooligosaccharide (LOS) drives endotoxic inflammation, porin proteins (PorB) modulate phagosome fusion, and IgA1 protease cleaves mucosal antibody, but attachment is the pilus's role. This is also why a gonococcus that loses its pili loses much of its ability to establish infection.

10. Aeromonas hydrophila. This is a classic freshwater wound infection. Aeromonas species live in fresh water (streams, rivers, lakes), so when a wound is exposed to fresh water or mud, A. hydrophila can colonize it rapidly and cause aggressive cellulitis with purulent drainage. Contrast this with Vibrio vulnificus, which causes similar aggressive wound infection but after exposure to warm salt or brackish (marine) water. The freshwater history points to Aeromonas.

11. Moraxella catarrhalis. Moraxella catarrhalis is a gram-negative diplococcus that commonly causes respiratory infection in elderly patients with COPD. Under the microscope it resembles Neisseria, but it grows on standard blood and chocolate agar (unlike the fastidious pathogenic Neisseria such as N. gonorrhoeae), and a positive butyrate esterase test separates it cleanly from Neisseria species. Both features in the vignette point to M. catarrhalis.

12. The mother's previous treatment for syphilis was effective. Treponemal tests (TP-PA, FTA-ABS) detect antibody to Treponema pallidum antigens and usually stay reactive for life, whether or not treatment succeeded. Non-treponemal tests (RPR, VDRL) track anti-cardiolipin antibody, which falls with disease activity and reverts to nonreactive after cure. A nonreactive RPR with a reactive TP-PA in someone treated years ago is the pattern of past, successfully treated syphilis, so there is no current risk of congenital transmission and no need to re-treat.

13. Molecular diagnostic methods (NAAT). Nucleic acid amplification tests are the most sensitive method for urogenital gonorrhea and chlamydia. They outperform culture (they do not depend on keeping the organism alive during transport) and vastly outperform a cervical Gram stain, which has poor sensitivity in women. NAAT is the current gold standard for these infections.

14. Infertility. Pelvic inflammatory disease occurs when a lower genital tract infection ascends to the endometrium and fallopian tubes, triggering inflammation, mucosal scarring, adhesions, and tubal occlusion. The most important long-term consequence of untreated or recurrent gonococcal PID is infertility, along with a raised risk of ectopic pregnancy from tubal scarring.

15. Deficiency of a late-acting complement component (C5-C8). The immune system relies on the membrane attack complex (MAC), built from the terminal complement components C5b through C9, to lyse Neisseria. People with a genetic deficiency of these late components cannot assemble the MAC, so they cannot lyse Neisseria gonorrhoeae or N. meningitidis directly, and they suffer recurrent, disseminated neisserial infections. Anyone with recurrent meningococcal or disseminated gonococcal disease should therefore be evaluated for terminal complement deficiency.

16. All of the above. Non-treponemal tests (RPR, VDRL) detect antibody to a cardiolipin-cholesterol-lecithin antigen rather than to the treponeme itself. Because cardiolipin is a phospholipid found in host cell membranes and in many microbes, diverse infections, including Lyme disease (Borrelia burgdorferi), infectious mononucleosis (EBV), acute varicella (chickenpox), and severe bacterial infections such as streptococcal disease, can trigger transient cross-reactive antibodies that produce a biological false-positive. This is exactly why a reactive non-treponemal test must always be confirmed with a specific treponemal test.

17. All of the above. Beyond acute infections, chronic conditions are a frequent cause of a biological false-positive VDRL or RPR: lepromatous leprosy, autoimmune disease such as systemic lupus erythematosus (through antiphospholipid antibodies), malaria, and acute exanthems such as measles all cause tissue inflammation and cell breakdown that release cardiolipin and mimic a reactive syphilis screen. Questions 16 and 17 together are worth learning as a pair: acute or infectious causes in one, chronic causes in the other.

18. Treponema pallidum infection. A 2 cm, indurated, painless ulcer with a clean base and a firm, raised border is a hard chancre, the hallmark of primary syphilis, caused by the spirochete Treponema pallidum. The key contrasts: chancroid (Haemophilus ducreyi) produces a painful, soft ulcer, and the primary lesion of lymphogranuloma venereum (Chlamydia trachomatis) is a small, painless ulcer that is transient and heals before the prominent inguinal lymphadenopathy appears. The painless, firm, indurated quality points to syphilis.

19. Testing acute and convalescent-phase sera for anti-leptospiral antibodies. Leptospirosis is confirmed most reliably by serology, tracking a fourfold or greater rise in specific antibody titer between acute and convalescent samples (by the microscopic agglutination test, MAT, or ELISA). Leptospires are fastidious and do not grow on routine blood or chocolate agar, nor on viral fibroblast cell lines, and dark-field microscopy of blood or serum gives many false results because fibrin strands are easily mistaken for spirochetes.

20. All of the above. Leptospira interrogans is a zoonotic spirochete that sets up chronic kidney infection in a wide range of wild and domestic animals, including cattle, dogs, rodents (mice and rats), and swine, all of which shed the organism in their urine. Humans are incidental hosts, infected through skin or mucous membrane contact with water, mud, or soil contaminated by that animal urine. The village pond shared with cattle in question 19 is a textbook exposure.