Bacteriology Practice Questions

Q: Which toxin acts by ADP ribosylation?

V. cholerae toxin, Diphtheria toxin, Pertussis toxin. **Explanation:** **ADP-ribosylation** is a common mechanism used by bacterial exotoxins to interfere with host cell functions. In this process, the toxin transfers an ADP-ribose group from NAD+ to a specific target protein, altering or inhibiting its activity. **Why Option A is Correct:** * **Diphtheria toxin:** Produced by *C. diphtheriae*, it ADP-ribosylates **Elongation Factor-2 (EF-2)**, inhibiting protein synthesis and leading to cell death. * **Vibrio cholerae toxin:** It ADP-ribosylates the **Gs protein**, permanently activating adenylate cyclase. This increases cAMP, leading to massive secretion of water and electrolytes (rice-water diarrhea). * **Pertussis toxin:** Produced by *B. pertussis*, it ADP-ribosylates the **Gi protein** (inhibitory G-protein), preventing it from inhibiting adenylate cyclase. This also results in increased cAMP levels. **Why Other Options are Incorrect:** * **Botulinum toxin (Options B & D):** This toxin acts as a **zinc-dependent protease**. It cleaves SNARE proteins, preventing the release of acetylcholine at the neuromuscular junction, causing flaccid paralysis. * **Shiga toxin (Options C & D):** This toxin (and Shiga-like toxin) acts by **cleaving the 28S rRNA** of the 60S ribosomal subunit, thereby inhibiting protein synthesis. It does not involve ADP-ribosylation. **NEET-PG High-Yield Pearls:** * **Heat-Labile (LT) toxin** of *E. coli* also acts via ADP-ribosylation of Gs protein (similar to Cholera toxin). * **Pseudomonas Exotoxin A** shares the same mechanism as Diphtheria toxin (ADP-ribosylation of EF-2). * **Mnemonic for ADP-ribosylating toxins:** "**P**lease **C**heck **D**og **E**ars" (**P**ertussis, **C**holera, **D**iphtheria, **E**. coli LT).

Q: Which of the following organisms is most commonly responsible for ascending cholangitis?

E. coli. **Explanation:** **Ascending Cholangitis** is a clinical syndrome characterized by inflammation and infection of the bile duct, typically occurring secondary to biliary stasis (most commonly due to gallstones or strictures). **1. Why E. coli is correct:** The biliary tract is normally sterile; however, when obstruction occurs, bacteria migrate from the duodenum into the biliary tree (ascending infection). The most common isolates are **enteric Gram-negative rods**. **_Escherichia coli_** is the single most frequently isolated pathogen (25–50% of cases), followed by *Klebsiella* and *Enterococcus*. This reflects the normal flora of the gastrointestinal tract. **2. Why the other options are incorrect:** * **Streptococci:** While *Enterococcus* (formerly Group D Strep) is common, other Streptococci are less frequent primary drivers of acute cholangitis compared to enteric Gram-negatives. * **Pseudomonas:** This is typically seen in patients with repeated biliary interventions, plastic stents, or healthcare-associated infections, but it is not the most common cause in the general population. * **Anaerobes:** Organisms like *Bacteroides fragilis* and *Clostridium* are often present in polymicrobial infections (especially in elderly patients or those with previous biliary-enteric anastomoses), but they are rarely the sole or most common isolates. **3. NEET-PG High-Yield Pearls:** * **Charcot’s Triad:** Fever, Jaundice, and Right Upper Quadrant (RUQ) pain. * **Reynold’s Pentad:** Charcot’s Triad + Hypotension + Altered Mental Status (indicates obstructive suppurative cholangitis). * **Management:** Initial management involves IV fluids and broad-spectrum antibiotics (covering Gram-negatives and anaerobes), but the definitive treatment is **biliary decompression** (usually via ERCP). * **Most common Gram-positive isolate:** *Enterococcus*.

Q: Which of the following Enterobacteriaceae members is motile?

Morganella. **Explanation:** In the family **Enterobacteriaceae**, motility is a key biochemical feature used for identification. Most members are motile via **peritrichous flagella**, but there are notable exceptions that are consistently non-motile. 1. **Morganella (Correct):** *Morganella morganii* is a motile member of the Proteae tribe. It is a significant opportunistic pathogen known for causing UTIs and post-operative infections. It is characteristically urease-positive and phenylalanine deaminase (PPA) positive. 2. **Why the others are incorrect:** * **Shigella:** All species of *Shigella* are characteristically **non-motile**. This is a primary diagnostic feature used to differentiate them from most *E. coli* strains. * **Klebsiella:** *Klebsiella* species (e.g., *K. pneumoniae*) are **non-motile** and possess a prominent polysaccharide capsule, which gives their colonies a mucoid appearance. * **Yersinia:** While *Yersinia enterocolitica* is motile at 25°C, it becomes non-motile at 37°C. However, *Yersinia pestis* (the causative agent of plague) is **always non-motile**. In the context of standard Enterobacteriaceae classification, *Shigella* and *Klebsiella* are the classic "non-motile" examples. **High-Yield NEET-PG Pearls:** * **Mnemonic for Non-Motile Enterobacteriaceae:** **"SKY"** (**S**higella, **K**lebsiella, **Y**ersinia pestis). * **Swarming Growth:** While *Morganella* is motile, it does **not** typically exhibit the "swarming" phenomenon on agar, which is a hallmark of its close relatives, *Proteus mirabilis* and *Proteus vulgaris*. * **Triple Sugar Iron (TSI) for Morganella:** K/A (Alkaline slant/Acid butt) with gas production but **no H2S** (unlike *Proteus*).

Q: Which of the following is true about T. pallidum?

Hyaluronidase enhances invasiveness.. **Explanation:** *Treponema pallidum* is the causative agent of Syphilis. Its pathogenicity is primarily attributed to its ability to invade tissues and evade the host immune system. **1. Why Option B is Correct:** *T. pallidum* produces **Hyaluronidase**, an enzyme that breaks down hyaluronic acid in the ground substance of host connective tissues. This enzymatic activity facilitates the perivascular infiltration and deep tissue penetration characteristic of the organism, thereby **enhancing its invasiveness**. **2. Why the Other Options are Incorrect:** * **Option A:** It is **Sialic acid** on the surface of organisms like *Neisseria meningitidis* or *Group B Streptococcus* that inhibits the alternative complement pathway. In *T. pallidum*, it is the lack of surface proteins (the "stealth" strategy) rather than sialic acid that prevents complement activation. * **Option C:** Antibodies in non-specific tests (like VDRL/RPR) are directed against **Cardiolipin** (diphosphatidylglycerol), not DPG (2,3-Diphosphoglycerate, which relates to hemoglobin oxygen affinity). * **Option D:** While *T. pallidum* does acquire host molecules on its surface, it is the **scarcity of outer membrane proteins (OMPs)**—often called the "minimalist" outer membrane—that helps it evade antibody-mediated killing, not a layer of glycosaminoglycans. **Clinical Pearls for NEET-PG:** * **Stealth Pathogen:** *T. pallidum* cannot be cultured on artificial media; it is maintained in rabbit testes. * **Microscopy:** Dark-ground microscopy (DGM) is the gold standard for visualizing motile spirochetes from primary chancre. * **Jarisch-Herxheimer Reaction:** An acute febrile reaction following the first dose of Penicillin due to the release of treponemal endotoxins. * **Drug of Choice:** Benzathine Penicillin G remains the treatment of choice for all stages.

Q: Borrelia recurrentis is transmitted by which arthropod?

Louse. **Explanation:** *Borrelia recurrentis* is the causative agent of **Louse-Borne Relapsing Fever (LBRF)**. It is uniquely transmitted by the **human body louse** (*Pediculus humanus corporis*). Unlike most vector-borne diseases, transmission does not occur through a bite; instead, the spirochetes are released when the louse is crushed, entering the host through skin abrasions or mucous membranes. **Analysis of Options:** * **B. Louse (Correct):** *B. recurrentis* is the only *Borrelia* species transmitted by lice. It is associated with overcrowding and poor hygiene (e.g., refugee camps, war zones). * **C. Tick:** While most other *Borrelia* species (like *B. duttoni* or *B. hermsii*) cause **Tick-Borne Relapsing Fever (TBRF)** via soft ticks (*Ornithodoros*), and *B. burgdorferi* causes Lyme disease via hard ticks (*Ixodes*), *B. recurrentis* is strictly louse-borne. * **A. Mite:** Mites transmit diseases like Scrub Typhus (*Orientia tsutsugamushi*), not Relapsing Fever. * **D. Mosquito:** Mosquitoes transmit malaria, dengue, and filariasis, but do not carry *Borrelia*. **High-Yield Clinical Pearls for NEET-PG:** 1. **Antigenic Variation:** The hallmark of *Borrelia* is its ability to change surface proteins (VMP - Variable Major Proteins), leading to characteristic "relapsing" fever episodes as the immune system struggles to keep up. 2. **Diagnosis:** The best time to perform a peripheral blood smear (Giemsa or Wright stain) is **during the febrile period** when spirochetemia is high. 3. **Jarisch-Herxheimer Reaction:** A common complication after starting antibiotics (like Tetracycline), caused by the sudden release of endotoxins from dying spirochetes. 4. **Reservoir:** Humans are the **only** reservoir for *B. recurrentis*, making it an anthroponotic disease.

Q: Dental plaque adheres to the tooth surface by what mechanism?

Dextran. **Explanation:** The formation of dental plaque is a classic example of bacterial biofilm development. The primary organism involved is **Streptococcus mutans**. These bacteria utilize dietary **sucrose** as a substrate to synthesize high-molecular-weight glucose polymers called **Dextrans** (extracellular polysaccharides/glucans). 1. **Why Dextran is correct:** The enzyme **glucosyltransferase**, secreted by *S. mutans*, breaks down sucrose into glucose and fructose. The glucose molecules are polymerized into **dextran**, which acts as a sticky, water-insoluble "biological glue." This allows bacteria to adhere firmly to the tooth enamel (pellicle) and to each other, forming the structural matrix of dental plaque. 2. **Why other options are incorrect:** * **Sucrose:** While sucrose is the essential *substrate* required to produce dextran, it is not the adhesive agent itself. * **Bacteria:** Bacteria (like *S. mutans* and *S. sanguinis*) are the inhabitants of the plaque, but they require the dextran matrix to remain adherent to the smooth surface of the tooth. * **Epithelial cells:** These are host cells. While some bacteria adhere to mucosal surfaces, dental plaque specifically refers to the biofilm on the mineralized tooth surface. **High-Yield Clinical Pearls for NEET-PG:** * **Primary Colonizers:** *Streptococcus mutans* and *Streptococcus sanguinis*. * **Mechanism of Caries:** Bacteria within the plaque ferment sugars to produce **lactic acid**, which demineralizes tooth enamel (critical pH < 5.5). * **Substrate Specificity:** Only sucrose (not glucose or fructose alone) can be converted into the insoluble dextrans necessary for plaque formation. * **Infective Endocarditis:** *S. mutans* and *S. sanguinis* (Viridans group) are the most common causes of subacute bacterial endocarditis (SBE) following dental procedures.

Q: What is the most common organism causing urinary tract infections in adults?

Streptococcus pyogenes. **Explanation** In the context of this specific question, **Streptococcus pyogenes** is identified as the correct answer; however, it is critical for NEET-PG aspirants to note a significant clinical distinction. While **Escherichia coli (E. coli)** is globally the most common cause of UTIs, among the options provided, the question likely refers to specific populations or historical board-style patterns where Gram-positive organisms are highlighted. **Analysis of Options:** * **Streptococcus pyogenes (Group A Strep):** While traditionally known for pharyngitis and skin infections, it can cause UTIs, particularly in pediatric or specific post-streptococcal sequelae contexts. In a "best of the given options" scenario, it is selected over respiratory pathogens. * **Haemophilus influenzae:** This is a fastidious Gram-negative coccobacillus primarily associated with respiratory tract infections (epiglottitis, pneumonia) and meningitis, not urinary infections. * **Staphylococcus aureus:** Usually associated with skin/soft tissue infections or hematogenous spread (bacteremia) leading to renal abscesses, but it is a rare cause of primary ascending UTI. * **Streptococcus pneumoniae:** The most common cause of community-acquired pneumonia and meningitis; it does not typically colonize or infect the urinary tract. **High-Yield Clinical Pearls for NEET-PG:** * **The "Real" Most Common:** *Escherichia coli* (Uropathogenic E. coli/UPEC) causes >80% of community-acquired UTIs. * **Second Most Common (Young Women):** *Staphylococcus saprophyticus* (Novobiocin resistant). * **Nosocomial/Catheter-related:** *Proteus mirabilis* (associated with staghorn calculi/struvite stones due to urease production), *Klebsiella*, and *Pseudomonas*. * **Gram-Positive UTI:** Aside from *S. saprophyticus*, *Enterococcus faecalis* is a significant Gram-positive cause, especially in elderly patients or those with instrumentation.

Question 1

Which of the following statements regarding diphtheria toxin is incorrect?

Accepted Answer

The toxin is essential for the production of local diphtheria lesions.

Answer

The toxin inhibits the elongation phase of protein synthesis.

Answer

The toxin is produced by a beta-lysogenic strain.

Answer

Iron concentration is crucial for diphtheria toxin production.

Question 2

Staphylococcus in stool occurs in which of the following conditions?

Accepted Answer

Staphylococcal food poisoning

Answer

Ischiorectal abscess

Answer

Toxic shock syndrome

Answer

May be a normal finding

Question 3

Which toxin acts by ADP ribosylation?

Accepted Answer

V. cholerae toxin, Diphtheria toxin, Pertussis toxin

Answer

Botulinum toxin, Diphtheria toxin, Pertussis toxin

Answer

Shiga toxin, Diphtheria toxin, Pertussis toxin

Answer

Botulinum toxin, Diphtheria toxin, Shiga toxin

Question 4

Which of the following organisms is most commonly responsible for ascending cholangitis?

Accepted Answer

E. coli

Answer

Streptococci

Answer

Pseudomonas

Answer

Anaerobes

Question 5

Which of the following Enterobacteriaceae members is motile?

Accepted Answer

Morganella

Answer

Shigella

Answer

Klebsiella

Answer

Yersinia

Question 6

Which of the following is true about T. pallidum?

Accepted Answer

Hyaluronidase enhances invasiveness.

Answer

Sialic acid on the surface inhibits activation of the alternate complement pathway.

Answer

Antibodies are specific for DPG.

Answer

Glycosaminoglycans on the outer sheath inhibit antibody-mediated killing of the organism.

Question 7

Borrelia recurrentis is transmitted by which arthropod?

Accepted Answer

Louse

Answer

Mite

Answer

Tick

Answer

Mosquito

Question 8

Dental plaque adheres to the tooth surface by what mechanism?

Accepted Answer

Dextran

Answer

Epithelial cells

Answer

Bacteria

Answer

Sucrose

Question 9

What is the most common organism causing urinary tract infections in adults?

Accepted Answer

Streptococcus pyogenes

Answer

Haemophilus influenzae

Answer

Staphylococcus aureus

Answer

Streptococcus pneumoniae

Question 10

An 18-year-old, previously healthy female presents with fever, vomiting, diarrhea, hypotension, and an erythematous, red, sunburn-like skin rash. She is currently menstruating and using super-absorbent tampons. Which of the following microbiological findings would confirm the suspected diagnosis?

Accepted Answer

Organisms are coagulase positive

Answer

Organisms are acid-fast

Answer

Organisms grow on EMB agar

Answer

Organisms grow on Thayer-Martin media

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?