Bacteriology Practice Questions

Q: Which of the following organisms is most likely represented?

Mycoplasma. ***Mycoplasma*** - This genus is characterized by the **absence of a cell wall**, rendering **beta-lactam antibiotics** like penicillin ineffective. - It is a common cause of **atypical pneumonia** (often called 'walking pneumonia') and typically requires diagnosis via **serology** or **PCR** rather than Gram stain, as they do not stain well. *Chlamydia* - These are **obligate intracellular** bacteria that require host cell ATP for energy, unlike the free-living *Mycoplasma*. - They are well-known to cause **urethritis** and **cervicitis** via Elementary and Reticulate body life cycles, which are distinct from the *Mycoplasma* life cycle. *Legionella* - This is a **Gram-negative rod** that typically resides in **aquatic environments** and contaminated **HVAC systems** or water supplies. - It requires **buffered charcoal yeast extract (BCYE) agar** with iron and cysteine for growth and often presents with gastrointestinal symptoms alongside pneumonia. *Ureaplasma* - Belonging to the family **Mycoplasmataceae**, it is distinguished by its ability to metabolize **urea** using the enzyme **urease**. - While closely related to *Mycoplasma*, it is classically associated with **urogenital infections** and adverse pregnancy outcomes, whereas *Mycoplasma pneumoniae* is primarily respiratory.

Q: A patient presents with a painless ulcer over the leg with a characteristic black eschar at the center, as shown in the image. There is surrounding edema with minimal pain. Which of the following is the most likely causative organism?

Bacillus anthracis. ***Bacillus anthracis*** - Cutaneous anthrax typically presents as a **painless ulcerative lesion** with a pathognomonic **central black eschar** (necrotic scab). - The lesion is classically surrounded by **extensive edema** and an erythematous rim, with notably disproportionate lack of pain. *Staphylococcus aureus* - Commonly causes **furuncles**, **carbuncles**, or **cellulitis**, which are typically very painful and tender. - These infections typically result in **purulent discharge** and abscess formation rather than a dry, black necrotic eschar. *Clostridium perfringens* - Primarily associated with **gas gangrene**, which presents with crepitus, severe pain, and rapidly spreading tissue necrosis. - This condition is characterized by **foul-smelling exudate** and systemic toxicity, unlike the localized, relatively painless presentation of anthrax. *Rickettsia conorii* - Etiological agent of **Mediterranean Spotted Fever**, which typically presents with a **tache noire** (black spot) at the site of the tick bite, accompanied by fever and rash. - Unlike the isolated lesion of cutaneous anthrax, this is generally associated with a **triad of fever**, **rash**, and the **eschar**.

Q: Which of the following bacteria possesses two circular DNA molecules (i.e., two distinct chromosomes) in its genome?

Vibrio cholerae. ***Vibrio cholerae*** - This pathogen is unique among many bacteria because its genome is organized into **two distinct circular chromosomes** (Chromosome 1 and Chromosome 2). - This multipartite genome structure is essential for its environmental adaptation and **virulence regulation**. *Escherichia coli* - The genome of this organism typically consists of a **single circular chromosome**. - While it may contain **extrachromosomal plasmids**, it does not possess multiple main chromosomes. *Salmonella typhi* - This bacterium has a genome organized as a **single circular chromosome**. - It is known for its ability to survive within **macrophages**, but this is not related to a multi-chromosome genome structure. *Shigella dysenteriae* - Its genome is characterized by a **single circular DNA molecule**. - Pathogenicity is primarily driven by the **Shiga toxin** and the **virulence plasmid** (pINV), not by having multiple chromosomes.

Q: Which of the following statements are true about diphtheria toxin? A. It inhibits EF-2 B. It is an A-B type exotoxin C. It has particular affinity for myocardial and nervous tissue D. Toxin is produced only by strains carrying a plasmid Choose the correct answer:

A, B and C. ***A, B and C*** - Diphtheria toxin acts by **ADP-ribosylation of Elongation Factor-2 (EF-2)**, which halts protein synthesis and leads to cell death. - It is classified as an **A-B exotoxin**, where the 'B' subunit facilitates binding to host cell receptors and the 'A' subunit possesses the enzymatic activity; furthermore, it demonstrates organ-specific toxicity with a high affinity for **cardiac muscle** (causing myocarditis) and **nerve cells** (causing demyelination and polyneuritis). *A and C* - While this statement correctly identifies that the toxin inhibits EF-2 and affects heart/nerves, it is incomplete because it omits the important classification of the toxin structure as an **A-B exotoxin**. - A comprehensive answer must include the structural mechanism, which is a fundamental characteristic of *Corynebacterium diphtheriae* pathogenicity. *All are correct* - This option is incorrect because the toxin production is not determined by a plasmid, but rather by a **lysogenic bacteriophage (beta-phage)**. - The *tox* gene is integrated into the bacterial chromosome via this bacteriophage, which is required for the bacteria to produce exotoxin. *B and D* - This option incorrectly identifies the genetic source of toxin production; as noted, the toxin is encoded by a **lysogenic beta-phage**, not a plasmid. - While it correctly acknowledges the **A-B exotoxin** structure, it fails to include the mechanism of action (EF-2 inhibition) and the specific organ affinity, which are critical features.

Q: What is the most common causative organism of acute pyelonephritis in pregnancy?

E. coli. **Explanation:** **1. Why E. coli is correct:** *Escherichia coli* is the most common causative organism for acute pyelonephritis in pregnancy, accounting for approximately **70–80% of cases**. The primary mechanism is the ascending route of infection. During pregnancy, physiological changes such as progesterone-induced smooth muscle relaxation (leading to ureteral dilatation) and mechanical compression of the ureters by the gravid uterus cause **urinary stasis**. This environment, combined with pregnancy-induced glycosuria and aminoaciduria, facilitates the upward migration of fecal flora (E. coli) from the perineum to the upper urinary tract. **2. Why the other options are incorrect:** * **B & C (Klebsiella and Enterobacter):** While these are Gram-negative bacilli that can cause UTIs, they are significantly less common than E. coli, typically accounting for only 3–5% of cases each. They are more frequently seen in recurrent infections or hospital-acquired settings. * **D (Staphylococcus group):** Gram-positive organisms like *Staphylococcus saprophyticus* or Group B Streptococcus (GBS) are occasional causes of UTI in pregnancy, but they represent a small minority of pyelonephritis cases compared to the overwhelming prevalence of E. coli. **3. NEET-PG High-Yield Pearls:** * **Most common medical complication** requiring hospitalization during pregnancy: Acute Pyelonephritis. * **Most common site:** Right side (due to dextrorotation of the uterus and protection of the left ureter by the sigmoid colon). * **Screening:** All pregnant women should be screened for **Asymptomatic Bacteriuria (ASB)** at the first prenatal visit (12–16 weeks). If untreated, 25–40% of ASB cases progress to acute pyelonephritis. * **Complications:** Can lead to preterm labor, maternal sepsis, and ARDS.

Question 1

Which of the following organisms is most likely represented?

Accepted Answer

Mycoplasma

Answer

Chlamydia

Answer

Legionella

Answer

Ureaplasma

Question 2

A patient presents with a painless ulcer over the leg with a characteristic black eschar at the center, as shown in the image. There is surrounding edema with minimal pain. Which of the following is the most likely causative organism?

Accepted Answer

Bacillus anthracis

Answer

Staphylococcus aureus

Answer

Clostridium perfringens

Answer

Rickettsia conorii

Question 3

Which of the following bacteria possesses two circular DNA molecules (i.e., two distinct
chromosomes) in its genome?

Accepted Answer

Vibrio cholerae

Answer

Escherichia coli

Answer

Salmonella typhi

Answer

Shigella dysenteriae

Question 4

Which of the following statements are true about diphtheria toxin?
A. It inhibits EF-2
B. It is an A-B type exotoxin
C. It has particular affinity for myocardial and nervous tissue
D. Toxin is produced only by strains carrying a plasmid
Choose the correct answer:

Accepted Answer

A, B and C

Answer

A and C

Answer

All are correct

Answer

B and D

Question 5

What is the most common causative organism of acute pyelonephritis in pregnancy?

Accepted Answer

E. coli

Answer

Klebsiella pneumoniae

Answer

Enterobacter

Answer

Staphylococcus group

Bacteriology — MCQs

Bacteriology — MCQs

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