Bacteriology Practice Questions

Q: A young man presented with a lesion in his hand which resembles a 'bull's eye'. He recalled that he was bitten by 'something' last evening when he went outdoors. If Lyme's disease is suspected, what is the causative organism responsible?

Borrelia burgdorferi. **Explanation:** The clinical presentation of a **'bull’s eye' lesion**, medically known as **Erythema Chronicum Migrans (ECM)**, is the pathognomonic hallmark of early localized **Lyme disease**. **1. Why Borrelia burgdorferi is correct:** Lyme disease is a multisystem inflammatory disease caused by the spirochete ***Borrelia burgdorferi***. It is transmitted to humans through the bite of infected **Ixodes ticks** (hard ticks). The "bull's eye" rash typically appears 3–30 days after the bite at the site of inoculation. **2. Analysis of Incorrect Options:** * **Borrelia recurrentis:** This is the causative agent of **Epidemic Relapsing Fever**, transmitted by the human body louse (*Pediculus humanus corporis*). It does not cause a bull's eye rash. * **Borrelia hermsii & Borrelia parkeri:** These species are responsible for **Endemic Relapsing Fever**, which is transmitted by soft ticks (*Ornithodoros* species). While they cause febrile episodes, they are not associated with Lyme disease or ECM. **Clinical Pearls for NEET-PG:** * **Vector:** *Ixodes scapularis* (Deer tick). * **Stages of Lyme Disease:** 1. **Stage 1 (Early Localized):** Erythema Migrans (Bull's eye rash), flu-like symptoms. 2. **Stage 2 (Early Disseminated):** Neurological (Bell’s palsy—often bilateral) and Cardiac (AV block) manifestations. 3. **Stage 3 (Late):** Chronic large joint arthritis and encephalopathy. * **Diagnosis:** Screening is done via **ELISA**, and confirmation is done via **Western Blot**. * **Treatment:** **Doxycycline** is the drug of choice (DOC). Amoxicillin is used in children <8 years and pregnant women. Ceftriaxone is used for neurological/cardiac complications.

Q: Poached egg colony on potassium tellurite agar is characteristic of which organism?

C. diphtheriae mitis. **Explanation:** The correct answer is **C. diphtheriae mitis**. *Corynebacterium diphtheriae* is classified into four biotypes—**gravis, intermedius, mitis, and belfanti**—based on colony morphology on selective media containing potassium tellurite (e.g., McLeod’s or Hoyle’s medium). Potassium tellurite inhibits most commensals and is reduced by *C. diphtheriae* to metallic tellurium, resulting in black/grey colonies. **Why C. diphtheriae mitis is correct:** The **mitis** biotype typically produces small, circular, convex, and smooth colonies that are jet black. A characteristic feature is the **"poached egg" appearance**, where the colony has a smooth, glossy surface and a slightly paler periphery. Clinically, it is associated with milder disease but is more frequently linked to obstructive croup. **Analysis of Incorrect Options:** * **A. C. diphtheriae gravis:** Produces large, irregular, dull grey-black colonies with radiating striations, known as **"daisy head" colonies**. It is the most virulent biotype. * **B. C. diphtheriae intermedius:** Produces very small, pinpoint, flat, dull black colonies (often described as **"frog egg"** or "fried egg" colonies, but smaller and flatter than mitis). * **D. C. Jeikeium:** This is a non-diphtherial corynebacterium (diphtheroid) typically associated with opportunistic infections in immunocompromised patients; it does not exhibit the poached egg morphology. **High-Yield Clinical Pearls for NEET-PG:** * **Tellurite Reduction:** All *C. diphtheriae* reduce tellurite to tellurium (black colonies). * **Tinsdale Medium:** Another selective medium where *C. diphtheriae* produces black colonies with a **brown halo**. * **Biotype Differentiation:** Only *gravis* and *intermedius* ferment starch and glycogen; *mitis* does not. * **Hemolysis:** *Mitis* is typically hemolytic, while *gravis* and *intermedius* are non-hemolytic.

Q: A 2-year-old infant presents with hemolytic uremic syndrome and thrombocytopenia. Which of the following bacteria is most likely to be isolated from a stool specimen?

E. coli O157:H7. **Explanation:** The clinical presentation of **Hemolytic Uremic Syndrome (HUS)**—characterized by the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure—following a diarrheal illness is most strongly associated with **Enterohemorrhagic *E. coli* (EHEC)**, specifically the serotype **O157:H7**. **Why the correct answer is right:** *E. coli* O157:H7 produces **Shiga-like toxins (Verotoxins)**, which enter the bloodstream and bind to **Gb3 receptors** on vascular endothelial cells, particularly in the kidneys. This leads to endothelial damage, platelet activation, and microthrombi formation, resulting in the consumption of platelets (thrombocytopenia) and mechanical destruction of RBCs (schistocytes). **Analysis of Incorrect Options:** * **Shigella:** While *Shigella dysenteriae* Type 1 produces the Shiga toxin and can cause HUS, it is much less common globally than EHEC in the context of HUS. In modern clinical vignettes, EHEC is the primary culprit. * **Salmonella:** Typically causes enteric fever or gastroenteritis. It does not produce Shiga-like toxins and is not a recognized cause of HUS. * **Aeromonas:** Can cause watery or bloody diarrhea, but it is not associated with the systemic toxemia required to trigger HUS. **NEET-PG High-Yield Pearls:** * **Culture:** *E. coli* O157:H7 does not ferment sorbitol. It is identified on **Sorbitol MacConkey (SMAC) agar** as colorless colonies. * **Transmission:** Often linked to undercooked ground beef (hamburgers), unpasteurized milk, or contaminated green leafy vegetables. * **Management:** Antibiotics are generally **avoided** in EHEC infections as they may increase toxin release and worsen the risk of HUS. * **Diagnosis:** Look for **schistocytes** (fragmented RBCs) on a peripheral blood smear.

Q: Which of the following bacteria is NOT motile?

Cl. perfringens. **Explanation:** The genus *Clostridium* consists of Gram-positive, anaerobic, spore-forming bacilli. A defining characteristic of most members of this genus is that they are motile via **peritrichous flagella**. However, there are notable exceptions that are high-yield for the NEET-PG exam. **1. Why Option C is Correct:** **Clostridium perfringens** (formerly *Cl. welchii*) is characteristically **non-motile**. Instead of using flagella for movement, it exhibits a unique "spreading" growth on agar due to its rapid doubling time, but it lacks the structural apparatus for true motility. Additionally, it is one of the few clostridia that is **capsulated** (in tissues), which is another key identifying feature. **2. Analysis of Incorrect Options:** * **Cl. histolyticum (A):** This species is motile and known for its potent proteolytic enzymes that cause tissue digestion in gas gangrene. * **Cl. tetanosporum (B):** Like the majority of Clostridia, it possesses peritrichous flagella and is motile. * **Cl. septicum (D):** This is a highly motile organism. On solid media, its intense motility often results in **swarming growth**, similar to *Proteus* species. **3. NEET-PG High-Yield Pearls:** * **Non-motile Clostridia:** The two primary non-motile species are ***Cl. perfringens*** and ***Cl. tetani* type VI** (a non-flagellated variant). * **Cl. perfringens Key Features:** Look for "Double zone of hemolysis" on blood agar and a positive "Nagler’s reaction" (detecting Alpha-toxin/lecithinase). * **Swarming Growth:** If a question mentions a Clostridium species showing swarming, think **Cl. tetani** or **Cl. septicum**. * **Drumstick Appearance:** Remember that *Cl. tetani* is motile but has characteristic terminal spores giving it a "drumstick" or "tennis racket" appearance.

Q: Thumb print appearance in a culture film smear is seen in which of the following bacteria?

Bordetella. ### Explanation The correct answer is **Bordetella pertussis**. **1. Why Bordetella is Correct:** *Bordetella pertussis*, the causative agent of Whooping Cough, exhibits a characteristic **"thumb print"** appearance (also described as "shredded wheat" appearance) on microscopic examination of a smear from a culture. This is due to the arrangement of small, ovoid, Gram-negative coccobacilli that tend to cluster together in a pattern resembling a thumbprint. **2. Analysis of Incorrect Options:** * **Bacillus anthracis:** Known for a **"Medusa head"** appearance of colonies on agar and a **"Bamboo stick"** appearance on microscopy (long chains of Gram-positive bacilli with squared ends). * **Brucella:** These are tiny, Gram-negative coccobacilli but are typically described as having a **"fine sand"** appearance due to their minute size and pleomorphism. * **Clostridium welchii (C. perfringens):** Characterized by a **"Box-car"** appearance (thick, rectangular Gram-positive bacilli) and produces a "target hemolysis" (double zone) on blood agar. **3. High-Yield Clinical Pearls for NEET-PG:** * **Culture Media:** *B. pertussis* is fastidious. The gold standard medium is **Bordet-Gengou** (potato-blood-glycerol agar), where colonies look like **"bisected pearls"** or **"mercury drops."** Another common medium is **Regan-Lowe** (charcoal agar). * **Specimen Collection:** The preferred specimen is a **nasopharyngeal swab** (using Dacron or calcium alginate, not cotton). * **Clinical Stages:** Catarrhal (most infectious), Paroxysmal (whooping cough), and Convalescent. * **Mercury Drop Colonies:** This is a frequent companion fact to the thumbprint smear in exams.

Q: Sulphur granules in a wound are diagnostic of which of the following?

Actinomyces. **Explanation:** **Actinomyces (Option B)** is the correct answer. Sulphur granules are the hallmark clinical finding of **Actinomycosis**, most commonly caused by *Actinomyces israelii*. These are not actually made of sulphur; they are yellowish, gritty microcolonies of the bacteria embedded in a matrix of calcium phosphate. Under the microscope, these granules demonstrate the **Splendore-Hoeppli phenomenon**, characterized by a central mass of tangled filamentous Gram-positive bacilli surrounded by a radiating fringe of club-shaped eosinophilic material. **Why other options are incorrect:** * **Nocardia (Option A):** While *Nocardia* is also a filamentous Gram-positive bacterium, it is **acid-fast** (unlike Actinomyces) and typically causes pulmonary or cutaneous infections without the characteristic formation of large sulphur granules in deep-seated abscesses. * **Clostridium welchii (Option C):** Now known as *C. perfringens*, this is an anaerobic spore-forming rod that causes gas gangrene. It presents with crepitus and "dishwater pus," not sulphur granules. * **Aspergillus flavum (Option D):** This is a fungus characterized by septate hyphae with acute-angle branching. While it can form "fungal balls" (aspergillomas), it does not produce sulphur granules. **High-Yield Clinical Pearls for NEET-PG:** * **Actinomyces** is a **commensal** of the oral cavity; infection is endogenous, often following dental trauma (Lumpy Jaw). * It is a **strict anaerobe** and **non-acid fast** (distinguishes it from Nocardia). * **Ray Fungus:** A common misnomer for Actinomyces due to its radiating filamentous appearance. * **Treatment:** High-dose Penicillin G is the drug of choice (Mnemonic: **A**ctinomyces = **A**naerobic + **P**enicillin; **N**ocadia = **A**erobic + **S**ulfonamides).

Q: A 24-year-old woman in her third trimester of pregnancy presents with urinary frequency and burning for the past few days. She denies fever or chills. She has mild suprapubic tenderness, and a urine dipstick is positive for WBC, protein, and a small amount of blood. Culture produces greater than 100,000 colonies of gram-negative bacilli. Which attribute of this uropathogenic organism is most strongly associated with its virulence?

P pili. **Explanation:** The clinical presentation of urinary frequency, burning, and suprapubic tenderness in a pregnant patient, combined with a culture showing >100,000 CFU/mL of gram-negative bacilli, confirms a **Urinary Tract Infection (UTI)**, most commonly caused by **Uropathogenic *Escherichia coli* (UPEC)**. **Why P pili is the correct answer:** The most critical virulence factor for UPEC in causing UTIs is its ability to adhere to the uroepithelium. **P pili (Pyelonephritis-associated pili)** contain the **PapG adhesin**, which binds specifically to **Gal-Gal (galactose-galactose) receptors** on the surface of uroepithelial cells. This attachment prevents the bacteria from being washed away during micturition and is strongly associated with upper UTI (pyelonephritis) and persistent infections. **Analysis of Incorrect Options:** * **A. Colonisation factor antigen (CFA):** These are fimbriae used for attachment by **Enterotoxigenic *E. coli* (ETEC)** to the intestinal mucosa, causing secretory diarrhea, not UTIs. * **B & C. Heat labile (LT) and Heat stable (ST) toxins:** These toxins are also characteristic of **ETEC**. They increase cAMP and cGMP levels respectively, leading to fluid secretion in the gut. They do not play a role in the pathogenesis of UTIs. **Clinical Pearls for NEET-PG:** * **Type 1 Pili:** These bind to mannose receptors and are more commonly associated with **cystitis** (bladder infections). * **Pregnancy & UTI:** Pregnancy increases the risk of progression from asymptomatic bacteriuria to pyelonephritis due to progesterone-induced ureteral dilation and mechanical compression by the gravid uterus. * **Treatment:** In pregnancy, Nitrofurantoin (avoid near term) or Amoxicillin-Clavulanate are common first-line choices; Fluoroquinolones are generally avoided.

Q: On blood agar, the characteristic double-zone of hemolysis produced by Clostridium perfringens is due to which toxin?

All of the above. **Explanation:** The characteristic **double-zone of hemolysis** produced by *Clostridium perfringens* on blood agar is a classic diagnostic feature. This phenomenon occurs due to the synergistic action of two specific toxins: 1. **Inner Zone (Complete Hemolysis):** Caused by **Theta-toxin** (Perfringolysin O), an oxygen-labile hemolysin that completely lyses red blood cells. 2. **Outer Zone (Incomplete/Partial Hemolysis):** Caused by **Alpha-toxin**. **Why "All of the above" is correct:** The Alpha-toxin of *C. perfringens* is biochemically a **Lecithinase**, which is a type of **Phospholipase C**. Therefore, Options A, B, and C are essentially synonyms for the same virulence factor responsible for the outer zone of hemolysis. In the context of NEET-PG, examiners often test your knowledge of biochemical nomenclature; knowing that Alpha-toxin = Lecithinase = Phospholipase C is crucial. * **Alpha-toxin (Option C):** The most important lethal toxin of *C. perfringens*. * **Lecithinase (Option A):** The enzymatic name based on its substrate (lecithin in cell membranes). * **Phospholipase C (Option B):** The specific chemical class of the enzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Nagler’s Reaction:** Used to detect Lecithinase activity. Opalescence is produced around colonies on Egg Yolk Agar, which is inhibited by adding antitoxin (demonstrating neutralization). * **Gas Gangrene:** Alpha-toxin is the primary mediator of myonecrosis. * **Stormy Fermentation:** *C. perfringens* produces "stormy fermentation" in litmus milk due to heavy gas production. * **Morphology:** It is a Gram-positive, boxcar-shaped bacillus that is **non-motile** (unlike most Clostridia) and rarely shows spores in clinical samples.

Q: An IV drug user is diagnosed to have infective endocarditis involving the tricuspid valve. Which of the following is the most likely causative agent?

Staphylococcus aureus. Explanation: **1. Why Staphylococcus aureus is correct:** *Staphylococcus aureus* is the most common cause of infective endocarditis (IE) in **Intravenous Drug Users (IVDUs)** [1]. In these patients, the infection typically involves the **right side of the heart**, specifically the **tricuspid valve** (approx. 60-70% of cases). The organism enters the bloodstream through skin flora during non-sterile injections and seeds the tricuspid valve [1], [3]. *S. aureus* is highly virulent, often leading to acute IE with large, friable vegetations and a high risk of septic pulmonary emboli [1]. **2. Why the other options are incorrect:** * **Streptococcus gallolyticus (formerly S. bovis):** This is strongly associated with **colorectal cancer** or polyps [1]. If isolated in IE, the next mandatory step is a colonoscopy. * **Pseudomonas aeruginosa:** While it is a known cause of IE in IVDUs (especially in certain geographic locations like Detroit or Chicago), it is significantly less common than *S. aureus*. * **Streptococcus pyogenes:** This is a common cause of skin infections and rheumatic fever but is a rare cause of acute infective endocarditis compared to *S. aureus* [1]. **3. NEET-PG High-Yield Pearls:** * **Most common valve involved in IVDU:** Tricuspid Valve (Right-sided IE). * **Most common valve involved overall (Non-IVDU):** Mitral Valve. * **Most common organism in Subacute IE:** *Streptococcus viridans* [1]. * **Most common organism in Prosthetic Valve IE (<1 year):** *Staphylococcus epidermidis* [3]. * **Clinical Sign:** IVDU patients with tricuspid IE often present with "septic pulmonary infarcts" (nodular opacities on Chest X-ray) rather than peripheral emboli like Janeway lesions [2].

Question 1

A young man presented with a lesion in his hand which resembles a 'bull's eye'. He recalled that he was bitten by 'something' last evening when he went outdoors. If Lyme's disease is suspected, what is the causative organism responsible?

Accepted Answer

Borrelia burgdorferi

Answer

Borrelia parkeri

Answer

Borrelia recurrentis

Answer

Borrelia hermsii

Question 2

Poached egg colony on potassium tellurite agar is characteristic of which organism?

Accepted Answer

C. diphtheriae mitis

Answer

C. diphtheriae gravis

Answer

C. diphtheriae intermedius

Answer

C. Jeikeium

Question 3

A 2-year-old infant presents with hemolytic uremic syndrome and thrombocytopenia. Which of the following bacteria is most likely to be isolated from a stool specimen?

Accepted Answer

E. coli O157:H7

Answer

Shigella

Answer

Salmonella

Answer

Aeromonas

Question 4

What is the most common organism causing neonatal meningitis?

Accepted Answer

E. coli

Answer

Listeria

Answer

Pseudomonas

Answer

Staph aureus

Question 5

Which of the following bacteria is NOT motile?

Accepted Answer

Cl. perfringens

Answer

Cl. histolyticum

Answer

Cl. tetanosporum

Answer

Cl. septicum

Question 6

Thumb print appearance in a culture film smear is seen in which of the following bacteria?

Accepted Answer

Bordetella

Answer

Bacillus anthracis

Answer

Brucella

Answer

Clostridium welchii

Question 7

Sulphur granules in a wound are diagnostic of which of the following?

Accepted Answer

Actinomyces

Answer

Nocardia

Answer

Clostridium welchii

Answer

Aspergillus flavum

Question 8

A 24-year-old woman in her third trimester of pregnancy presents with urinary frequency and burning for the past few days. She denies fever or chills. She has mild suprapubic tenderness, and a urine dipstick is positive for WBC, protein, and a small amount of blood. Culture produces greater than 100,000 colonies of gram-negative bacilli. Which attribute of this uropathogenic organism is most strongly associated with its virulence?

Accepted Answer

P pili

Answer

Colonisation factor antigen

Answer

Heat labile toxins

Answer

Heat stable toxins

Question 9

On blood agar, the characteristic double-zone of hemolysis produced by Clostridium perfringens is due to which toxin?

Accepted Answer

All of the above

Answer

Lecithinase

Answer

Phospholipase C

Answer

Alpha-toxin

Question 10

An IV drug user is diagnosed to have infective endocarditis involving the tricuspid valve. Which of the following is the most likely causative agent?

Accepted Answer

Staphylococcus aureus

Answer

Streptococcus gallolyticus

Answer

Pseudomonas aeruginosa

Answer

Streptococcus pyogens

Bacteriology — MCQs

Bacteriology — MCQs

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