MCQ Bacteriology (51-60): Question with Answers

51. 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

52. 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

53. 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

54. 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

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

a. Most human infections are caused by serotype O:1

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

56. 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

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

a. tularensis

b. holarctica

c. mediasiatica

d. novicida

58. 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

59. 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

60. 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

Answer Key & Explanations

**51. Correct Answer: c (Streptococcus pyogenes)**

• Explanation: High-consequence pathogens are categorized by the CDC as potential bioweapons. Bacillus anthracis (anthrax), Yersinia pestis (plague), and Botulinum toxin are Category A agents, while Brucella species are Category B. Streptococcus pyogenes (Group A Strep), though capable of causing severe suppurative infections and necrotizing fasciitis, is a ubiquitous community pathogen and is not classified as a biological warfare agent.

**52. Correct Answer: d (Pasteurella bettyae)**

• Explanation: While Pasteurella multocida is overwhelmingly linked to animal bites and scratches, Pasteurella bettyae is distinct because it is primarily isolated from human infections involving the genitourinary tract, Bartholin gland abscesses, and can be transmitted vertically to newborns during delivery, leading to neonatal septicemia.

**53. Correct Answer: a (Stool culture on Hektoen Enteric agar)**

• Explanation: Bubonic plague is characterized by painful, swollen regional lymph nodes called buboes. Yersinia pestis proliferates rapidly within these sites and frequently escapes into the bloodstream. Therefore, lymph node aspirates and routine blood cultures are excellent diagnostic specimens. Serology and tissue immunohistochemistry are also standard. Stool culture on Hektoen Enteric agar is used for intestinal pathogens like Salmonella and Shigella; it has no role in diagnosing bubonic plague, as the pathogen is not shed in feces during this clinical presentation.

**54. Correct Answer: d (Penicillin G)**

• Explanation: Pasteurella multocida presents an exception to general gram-negative treatment patterns because it remains highly sensitive to narrow-spectrum penicillins. While broad-spectrum beta-lactamase inhibitor combinations (like amoxicillin-clavulanate) are preferred empirically for animal bites to cover oral anaerobes simultaneously, iv Penicillin G or oral amoxicillin is the definitive drug of choice for a confirmed, isolated P. multocida infection. It shows poor or unpredictable susceptibility to macrolides like erythromycin and lincosamides like clindamycin.

**55. Correct Answer: c (Person-to-person spread is quite common)**

• Explanation: This is the incorrect statement, making it the right exception. Yersinia enterocolitica is predominantly a zoonotic enteric pathogen. Humans typically acquire it through the fecal-oral pathway, notably by eating undercooked pork or drinking contaminated water. Person-to-person spread is exceedingly rare. Pathogenic human strains are typically associated with serotypes O:3, O:9, and O:8, rather than O:1. Furthermore, there is a well-documented genetic correlation between Y. enterocolitica reactive arthritis and individuals carrying the HLA-B27 tissue type.

**56. Correct Answer: e (Doxycycline)**

• Explanation: Pneumonic plague is highly contagious and spreads via respiratory droplets, carrying a nearly 100% mortality rate if untreated. For post-exposure prophylaxis of close or intimate contacts, oral Doxycycline (100 mg twice daily for 7 days) or a fluoroquinolone like ciprofloxacin is the standard guideline to halt infection. Aminoglycosides like gentamicin are reserved for active, systemic treatment rather than prophylaxis.

**57. Correct Answer: a (tularensis)**

• Explanation: Francisella tularensis is divided into subspecies with varying degrees of virulence. Subspecies tularensis (Type A) is the most lethal and highly virulent form for humans, often requiring an infectious dose of fewer than 10 organisms to cause severe disease. Subspecies holarctica (Type B) causes a milder form of tularemia, while novicida is generally considered non-pathogenic or of low virulence in immunocompetent individuals.

**58. Correct Answer: c (The organism grows well on standard chocolate agar)**

• Explanation: This statement is the exception. The causative agent of chancroid is Haemophilus ducreyi. It is an extremely fastidious organism that does not grow well on standard chocolate agar plates used for routine cultures. It requires specialized, highly enriched media supplemented with fetal bovine serum and vancomycin to suppress competing genital flora, incubated under humid, capnophilic conditions. It is a small gram-negative rod, requires exogenous X factor (hemin), displays a classic "school of fish" or parallel strand arrangement on Gram stains, and responds well to macrolide therapy.

**59. Correct Answer: b (Pili / Fimbriae)**

• Explanation: Pili (fimbriae) are hair-like appendages protruding from the outer membrane of Neisseria gonorrhoeae. They are primary virulence factors responsible for facilitating initial attachment to the microvilli of non-ciliated epithelial cells in the human urogenital tract. Lipooligosaccharide (LOS) drives endotoxic shock and inflammation, Porin proteins (PorB) modulate phagosome fusion, and IgA1 protease cleaves mucosal antibodies, but pili initiate the anchoring step.

**60. Correct Answer: c (Aeromonas hydrophila)**

• Explanation: This is a classic presentation of an Aeronomas hydrophila wound infection. Aeromonas species are ubiquitous inhabitants of freshwater environments (like streams, rivers, and lakes). When a wound is exposed to fresh water or mud, A. hydrophila can rapidly colonize the site, causing aggressive cellulitis and purulent drainage. Conversely, Vibrio vulnificus is associated with traumatic injuries in marine or estuarine saltwater environments, making Aeromonas the correct etiologic agent for a freshwater exposure.