Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2321: Which of the following organisms is the most common cause of urinary tract infections in sexually active women?

A. Escherichia coli (Correct Answer)
B. Staphylococcus saprophyticus
C. Klebsiella pneumoniae
D. Proteus mirabilis

Explanation: ***Escherichia coli*** - **Uropathogenic *E. coli*** (*UPEC*) is responsible for 75-95% of all **uncomplicated UTIs**, making it the most common pathogen. - Its virulence factors, such as **fimbriae (pili)**, enable it to adhere to and colonize the uroepithelium. *Staphylococcus saprophyticus* - While a common cause of UTIs, especially in **young, sexually active women**, it ranks second to *E. coli*. - It accounts for 5-15% of community-acquired UTIs in this demographic. *Klebsiella pneumoniae* - This organism is a common cause of **complicated UTIs**, particularly in hospitalized patients or those with catheterization. - It is less frequently the causative agent in **uncomplicated UTIs** in healthy, sexually active women. *Proteus mirabilis* - Known for its ability to produce **urease**, which hydrolyzes urea to ammonia, leading to alkaline urine and the formation of **struvite stones**. - More commonly associated with complicated UTIs, recurrent infections, and **kidney stones**, rather than typical uncomplicated UTIs in sexually active women.

Question 2322: Which pathogen is responsible for causing tuberculosis?

A. Mycobacterium tuberculosis (Correct Answer)
B. Streptococcus pneumoniae
C. Haemophilus influenzae
D. Mycobacterium leprae

Explanation: ***Mycobacterium tuberculosis*** - This bacterium is the **primary causative agent** of tuberculosis (TB), a serious infectious disease. - It predominantly affects the **lungs** but can also spread to other organs. *Streptococcus pneumoniae* - This pathogen is a common cause of **bacterial pneumonia**, as well as **meningitis** and **otitis media**. - It does not cause tuberculosis; its disease presentation is distinct and usually more acute. *Haemophilus influenzae* - This bacterium is frequently associated with **respiratory tract infections**, including **bronchitis**, **pneumonia**, and **otitis media**, especially in children. - It is not the causative agent of tuberculosis. *Mycobacterium leprae* - This bacterium is the causative agent of **leprosy**, a chronic infectious disease that primarily affects the skin, nerves, and upper respiratory tract. - While it belongs to the *Mycobacterium* genus, it causes a distinct disease from tuberculosis.

Question 2323: Which microorganism is most likely to cause a rapidly progressive, painful infection with gas formation in the tissues?

A. Clostridium perfringens (Correct Answer)
B. Staphylococcus aureus
C. Pseudomonas aeruginosa
D. Mycobacterium tuberculosis

Explanation: **Clostridium perfringens** *(CORRECT)* - This bacterium is the classic cause of **gas gangrene** (clostridial myonecrosis), characterized by rapid progression, severe pain, and **crepitus** (gas in the tissues). - It produces potent **exotoxins**, such as alpha-toxin, which cause extensive tissue destruction and hemolysis. - Key clinical feature: **Gas formation** in tissues is pathognomonic. *Staphylococcus aureus* - While *S. aureus* can cause severe and painful infections (e.g., **cellulitis**, **abscesses**, **necrotizing fasciitis**), it does not typically produce gas in the tissues. - It is a common cause of skin and soft tissue infections, often presenting with **purulent drainage**. *Pseudomonas aeruginosa* - This organism can cause rapidly progressive and severe infections, especially in immunocompromised individuals or burn patients, but **gas production is not a characteristic feature**. - Infections often have a **grape-like odor** and can produce a blue-green pigment (pyocyanin). *Mycobacterium tuberculosis* - *M. tuberculosis* causes **tuberculosis**, a chronic infection primarily affecting the lungs, and sometimes other organs. - It is characterized by slow progression, **granuloma formation**, and does not cause rapid, painful infections with gas formation.

Question 2324: Which of the following is the primary cause of infection in post-traumatic osteomyelitis?

A. Staphylococcus aureus (Correct Answer)
B. E. coli
C. Anaerobes
D. Pseudomonas aeruginosa

Explanation: ***Staphylococcus aureus*** - **Staphylococcus aureus** is the most common pathogen responsible for **post-traumatic osteomyelitis**. - It gains entry to the bone through the **open wound** or **surgical site** after trauma. *Pseudomonas aeruginosa* - **Pseudomonas aeruginosa** is more commonly associated with **puncture wounds** of the foot, especially through footwear, and infections in **immunocompromised** individuals or those with extensive burns. - While it can cause osteomyelitis, it is not the primary cause in general post-traumatic cases. *E. coli* - **E. coli** is typically associated with **urinary tract infections** and **intra-abdominal infections**. - It is a less common cause of osteomyelitis, usually seen in contiguous infections from the gastrointestinal or genitourinary tracts, or in **diabetic foot infections**. *Anaerobes* - **Anaerobic bacteria** can cause osteomyelitis, especially in cases of **deep, contaminated wounds** or **diabetic foot ulcers**. - However, they are generally part of a **polymicrobial infection** and are not as frequently identified as the sole primary cause compared to *Staphylococcus aureus* in typical post-traumatic scenarios.

Question 2325: A patient with chronic sinusitis has nasal discharge that is positive for gram-positive, coagulase-positive cocci. What is the most likely causative organism?

A. Staphylococcus aureus (Correct Answer)
B. Streptococcus pneumoniae
C. Haemophilus influenzae
D. Moraxella catarrhalis

Explanation: ***Staphylococcus aureus*** - **Gram-positive, coagulase-positive cocci** are classic microbiological findings for **Staphylococcus aureus**. - This organism is a common cause of **chronic sinusitis**, particularly in cases that are refractory to initial treatment. *Streptococcus pneumoniae* - While a common cause of **acute bacterial sinusitis**, it is typically identified as **Gram-positive cocci in pairs or short chains**, not primarily as coagulase-positive. - It is often **alpha-hemolytic** on blood agar, but coagulase testing is not a primary identifier. *Haemophilus influenzae* - This bacterium is a **Gram-negative coccobacillus**, which distinctly differentiates it from the Gram-positive cocci described in the question. - It is a common cause of both acute and chronic sinusitis. *Moraxella catarrhalis* - This organism is a **Gram-negative diplococcus**, which also differentiates it from the Gram-positive cocci described. - It is a frequent cause of acute otitis media and sinusitis, especially in children.

Question 2326: Which of the following statements about Vibrio cholerae is correct?

A. Cholera is more common in women than men.
B. El Tor strain has more asymptomatic carriers than the classical strain. (Correct Answer)
C. All statements are correct.
D. The classical strain of Vibrio cholerae provides immunity against El Tor strain.

Explanation: ***El Tor strain has more asymptomatic carriers than the classical strain.*** - The **El Tor biotype** of *Vibrio cholerae* is associated with a **significantly higher proportion of asymptomatic carriers** (10-20%) compared to the classical biotype (2-5%). - This higher carrier rate contributes to its **better epidemiological success** and has allowed it to cause the ongoing **7th cholera pandemic** since 1961. - Both biotypes can cause **equally severe disease** in symptomatic individuals, but El Tor's ability to persist in carriers and the environment gives it transmission advantages. *Cholera is more common in women than men.* - The incidence of cholera generally **does not show a significant sex predilection**. - Most epidemiological studies indicate a **relatively equal distribution** between sexes when controlling for exposure. *All statements are correct.* - This statement is incorrect because only the first option regarding asymptomatic carriers is correct. *The classical strain of Vibrio cholerae provides immunity against El Tor strain.* - While infection with one *Vibrio cholerae* serogroup (e.g., O1) can provide some **short-term protection** against re-infection with the same serogroup, **cross-protection between biotypes is incomplete and variable**. - Immunity is primarily **serotype-specific** (O1 vs O139) rather than biotype-specific, and does not guarantee protection against different biotypes.

Question 2327: An 18-year-old girl presents with watery diarrhea. Most likely causative agent -

A. Rotavirus
B. V. cholerae (Correct Answer)
C. Salmonella
D. Shigella

Explanation: ***V. cholerae*** - *Vibrio cholerae* is a classic cause of **acute, severe watery diarrhea** that can lead to rapid dehydration. - While other agents can cause watery diarrhea, *V. cholerae* is primarily associated with large-scale outbreaks of this symptom. *Rota virus* - While rotavirus causes **watery diarrhea**, it primarily affects **infants and young children** and is less common as the most likely cause in an 18-year-old in many settings due to widespread vaccination programs. - The diarrhea, though watery, is often accompanied by **fever and vomiting**. *Salmonella* - *Salmonella* typically causes **inflammatory diarrhea** (dysentery-like symptoms with blood/mucus in stool) or **food poisoning**, with diarrhea that may be watery but is often not as profuse or severe as cholera. - It is more commonly associated with **fever and abdominal cramps**. *Shigella* - *Shigella* causes **bacillary dysentery**, characterized by **bloody, mucoid stools**, abdominal cramps, and fever. - It is not typically associated with solely profuse watery diarrhea.

Question 2328: The following statements are true regarding botulism except:

A. Clostridium botulinum A, B, E and F cause human disease.
B. The gene for botulinum toxin is encoded by a bacteriophage.
C. Clostridium baratii may cause botulism.
D. Infant botulism is caused by ingestion of preformed toxin. (Correct Answer)

Explanation: ***Infant botulism is caused by ingestion of preformed toxin.*** - This statement is incorrect because **infant botulism** is caused by the *in vivo* production of **botulinum toxin** in the infant's intestine after *Clostridium botulinum* spores are ingested and colonize the gut. - Unlike foodborne botulism in adults, it is **not** due to consuming preformed toxin, as the infant's immature gut flora allows spore germination. *Clostridium botulinum A, B, E and F cause human disease.* - This statement is true; **neurotoxin types A, B, E, and F** are the most common causes of human botulism, with type A and B being responsible for the majority of cases worldwide. - These types produce potent **neurotoxins** that block acetylcholine release at neuromuscular junctions, leading to flaccid paralysis. *The gene for botulinum toxin is encoded by a bacteriophage.* - This statement is true. The genes for **botulinum neurotoxins** (BoNT) in *C. botulinum* are often found on **bacteriophages** or plasmids, which can transfer these virulence genes between bacteria. - This horizontal gene transfer contributes to the diversity of *C. botulinum* and the spread of toxin production capability. *Clostridium baratii may cause botulism.* - This statement is true. While *Clostridium botulinum* is the most common species, **atypical clostridia** such as *Clostridium baratii* and *Clostridium butyricum* have also been identified as rare causes of botulism, particularly infant botulism. - These species also produce **botulinum neurotoxins** (specifically type F in *C. baratii* and type E in *C. butyricum*), leading to similar clinical manifestations.

Question 2329: Which of the following diseases is caused by Borrelia?

A. Vincent angina
B. Weil's disease
C. Bejel
D. Lyme disease (Correct Answer)

Explanation: ***Lyme disease*** - **Lyme disease** is caused by the spirochete bacterium **_Borrelia burgdorferi_**, transmitted primarily by ticks. - Its classic early symptom is an **erythema migrans** rash, often described as a "bull's-eye" appearance. *Weil's disease* - **Weil's disease** is a severe form of **leptospirosis**, caused by the bacterium **_Leptospira interrogans_**, not _Borrelia_. - It presents with **jaundice**, **renal failure**, and **hemorrhage**. *Bejel* - **Bejel** (also known as endemic syphilis) is caused by **_Treponema pallidum subspecies endemicum_**, a bacterium closely related to the one causing syphilis, but not _Borrelia_. - It primarily affects the **skin**, **mucous membranes**, and **bones**, particularly in children in arid regions. *Vincent angina* - **Vincent angina** (also known as necrotizing ulcerative gingivitis or "trench mouth") is caused by a polymicrobial infection involving **anaerobic bacteria**, particularly **_Fusobacterium nucleatum_** and **_Treponema vincentii_**, not _Borrelia_. - It is characterized by **painful**, bleeding gums, and **halitosis**.

Question 2330: What is the generation time of Mycobacterium tuberculosis?

A. 10-15 hours
B. 1-2 hours
C. 5-10 hours
D. 15-20 hours (Correct Answer)

Explanation: ***15-20 hours*** - *Mycobacterium tuberculosis* is known for its **slow growth rate**, with a generation time of approximately **15-20 hours** (some sources cite up to 18-24 hours). - This unusually long generation time contributes to the **prolonged incubation period** required for culture (3-8 weeks) and makes its isolation and identification a time-consuming process. - The slow growth is due to its **complex, lipid-rich cell wall** and unique metabolic characteristics. *1-2 hours* - A generation time of 1-2 hours is typical for **rapidly growing bacteria** such as *Escherichia coli* (which has a generation time of approximately 20 minutes under optimal conditions). - *Mycobacterium tuberculosis* is characterized by its **much slower growth rate**, distinguishing it from most common bacterial pathogens. *10-15 hours* - While this is slower than typical bacteria, it is still at the **lower end** of the range for *M. tuberculosis*. - The more widely accepted generation time is **15-20 hours**, making this option slightly inaccurate. *5-10 hours* - A generation time of 5-10 hours is **too short** for *Mycobacterium tuberculosis*. - Members of the *Mycobacterium tuberculosis complex* are known for their **exceptionally long replication cycles** compared to most other bacteria.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

Practice Questions

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