Bacteriology Practice Questions

Q: Pertussis toxin acts by all of the following mechanisms, EXCEPT:

Increased calcium release from sarcoplasmic reticulum. **Explanation:** **Pertussis toxin (PTX)**, produced by *Bordetella pertussis*, is a classic A-B subunit exotoxin. Its primary mechanism involves the **ADP-ribosylation** of the **Gi (inhibitory) protein** alpha subunit. 1. **Why Option C is Correct:** Increased calcium release from the sarcoplasmic reticulum is a mechanism associated with toxins like **Ryanodine** or certain physiological muscle contractions, but it is **not** a feature of Pertussis toxin. PTX primarily targets signaling pathways involving cyclic nucleotides, not direct calcium sequestration in the sarcoplasmic reticulum. 2. **Why the Other Options are Incorrect:** * **Option A & D:** PTX acts by catalyzing the **ADP-ribosylation** of the **Gαi subunit** of heterotrimeric G proteins. This modification locks the Gi protein in an inactive state, preventing it from inhibiting adenylate cyclase. * **Option B:** Because the "inhibitory" G protein (Gi) is inactivated, the enzyme **adenylate cyclase** remains constitutively active. This leads to a massive **increase in intracellular cyclic AMP (cAMP)** levels, which triggers the systemic effects of the disease (e.g., lymphocytosis, insulin hypoglycemia). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism Mnemonic:** **P**ertussis toxin **P**revents Gi (Inhibitory) action, leading to increased cAMP. (Contrast this with **C**holera toxin, which **C**onstitutively activates Gs). * **Biological Effects:** PTX is also known as **Lymphocytosis-promoting factor** because it prevents lymphocytes from entering lymphoid tissue, causing a characteristic high peripheral white cell count. * **Other Toxins:** *B. pertussis* also produces an **Invasive Adenylate Cyclase toxin** (which acts like *B. anthracis* Edema Factor) and **Tracheal Cytotoxin** (which damages ciliated epithelium).

Q: Which bacterium is known to survive in alkaline media?

Vibrio. **Explanation:** The correct answer is **Vibrio cholerae**. This bacterium is a classic example of an **alkaliphile**, meaning it thrives in environments with a high pH (alkaline). **1. Why Vibrio is correct:** *Vibrio cholerae* grows optimally at a pH of **8.2 to 8.9**, though it can survive in environments with a pH as high as 9.5. This physiological trait is exploited in clinical microbiology to create **selective media** (e.g., **TCBS Agar**—Thiosulfate Citrate Bile Salts Sucrose agar) and **enrichment media** (e.g., **Alkaline Peptone Water** with pH 8.6). The alkaline environment inhibits the growth of most other intestinal commensals, allowing *Vibrio* to flourish. **2. Why other options are incorrect:** * **E. coli, Shigella, and Salmonella:** These are members of the *Enterobacteriaceae* family. While they can tolerate a range of pH levels, they generally prefer a neutral pH (7.2–7.4). Unlike *Vibrio*, they are highly sensitive to alkaline conditions and are easily inhibited by the high pH used in *Vibrio*-specific media. Furthermore, *Vibrio* is uniquely sensitive to stomach acid (acid-labile), which is why a high infectious dose is required unless the patient is taking antacids. **High-Yield Clinical Pearls for NEET-PG:** * **Enrichment Media for Vibrio:** Alkaline Peptone Water (APW) and Monsur’s Tellurite Taurocholate Gelatin Phosphate (TTGP). * **Selective Media:** TCBS (produces yellow colonies due to sucrose fermentation). * **Transport Media:** Venkatraman-Ramakrishnan (VR) medium and Cary-Blair medium. * **The "String Test":** Used to identify *Vibrio* species; colonies emulsified in 0.5% sodium deoxycholate become mucoid and form a "string."

Q: What is a cold abscess related to?

Mycobacterium tuberculosis. ### Explanation **Correct Answer: B. Mycobacterium tuberculosis** A **cold abscess** is a hallmark clinical presentation of tuberculosis, most commonly associated with **tuberculous lymphadenitis** (scrofula) or **Pott’s spine** (tuberculosis of the vertebrae). The term "cold" is used because the abscess lacks the classic signs of acute inflammation—**calor** (heat), **rubor** (redness), and **dolor** (pain). In a pyogenic infection, the body mounts a rapid neutrophilic response leading to heat and inflammation. In contrast, *M. tuberculosis* triggers a **Type IV hypersensitivity reaction** and granulomatous inflammation. The "pus" in a cold abscess is actually **caseous necrosis** (liquefied dead tissue) rather than true purulent exudate, resulting in a fluctuant swelling that is neither warm nor tender. **Why other options are incorrect:** * **Option A (Periapical pathology):** This typically leads to acute dental abscesses or chronic granulomas. These are usually associated with pain and inflammatory markers, unlike the indolent nature of a cold abscess. * **Option C (Streptococcus viridans):** This is a pyogenic bacterium. Infections caused by *S. viridans* (like subacute bacterial endocarditis or dental abscesses) trigger an acute inflammatory response characterized by warmth and pain. **NEET-PG High-Yield Pearls:** * **Most common site:** The cervical lymph nodes are the most frequent site for a tuberculous cold abscess. * **Pott’s Spine:** A cold abscess in the lumbar spine can track down the **psoas sheath** to manifest as a swelling in the inguinal region (Psoas abscess). * **Histology:** Look for **Langhans giant cells**, epithelioid cells, and central caseous necrosis. * **Differential Diagnosis:** Actinomycosis can also present with "cold" features but is typically characterized by "sulfur granules" and multiple draining sinuses.

Q: A 30-year-old male presented with an ulcerative lesion on the glans penis. Wright-Giemsa stain showed 1-2 rounded structures within macrophage vacuoles. What is the etiology?

Calymatobacterium granulomatis. ### Explanation The clinical presentation and microscopic findings are diagnostic of **Granuloma Inguinale (Donovanosis)**, caused by *Klebsiella granulomatis* (formerly known as *Calymatobacterium granulomatis*). **Why the correct answer is right:** The hallmark of Granuloma Inguinale is the presence of **Donovan bodies**. On Wright-Giemsa or Leishman stain, these appear as 1–2 µm, rounded or safety-pin shaped, bipolar-staining bodies located within the vacuoles of large mononuclear cells (macrophages). Clinically, it presents as a chronic, painless, "beefy-red" ulcerative lesion that bleeds easily on touch (friable), often without associated lymphadenopathy (though "pseudobuboes" may occur due to subcutaneous granulation). **Why the incorrect options are wrong:** * **Chlamydia trachomatis (Serotypes L1-L3):** Causes Lymphogranuloma Venereum (LGV). It typically presents with a transient, painless papule followed by painful, suppurative regional lymphadenopathy (the "Groove sign"). It does not show Donovan bodies. * **N. gonorrhoeae:** Primarily causes urethritis (purulent discharge) rather than ulcerative lesions. Gram stain would show Gram-negative intracellular diplococci. * **H. ducreyi:** Causes Chancroid. These ulcers are characteristically **painful** and "soft." Microscopic examination shows Gram-negative coccobacilli in a "school of fish" or "railroad track" appearance. **NEET-PG High-Yield Pearls:** * **Donovan Bodies:** Pathognomonic for Granuloma Inguinale; described as "safety-pin" appearance. * **Clinical Feature:** Painless, beefy-red ulcer; "Pseudobubo" (swelling in the groin that is actually granulomatous tissue, not a lymph node). * **Treatment of Choice:** Azithromycin (1g orally once a week or 500mg daily) for at least 3 weeks. * **Staining:** Best visualized with Wright, Giemsa, or Leishman stain; the organism is difficult to culture.

Q: Carrom coin appearance of pneumococcus is due to which of the following?

Autolysin. **Explanation:** The **"Carrom coin"** or **"draughtsman"** appearance is a classic morphological feature of *Streptococcus pneumoniae* (Pneumococcus) when grown on blood agar. **Why Autolysin is correct:** As *S. pneumoniae* colonies age (usually after 24 hours of incubation), they undergo a process of **self-digestion** mediated by an endogenous enzyme called **Autolysin** (specifically *N-acetylmuramoyl-L-alanine amidase*). This enzyme breaks down the peptidoglycan in the center of the colony where the bacteria are oldest. This results in the collapse or "sinking" of the central portion of the colony, while the periphery remains raised, creating a flat, indented appearance resembling a carrom coin or a draughtsman piece. **Analysis of Incorrect Options:** * **Capsular polysaccharide:** This is the primary virulence factor and is responsible for the **mucoid** appearance of Type 3 pneumococci and the positive Quellung reaction, but it does not cause the central indentation. * **C-Carbohydrate antigen:** This is a cell wall component that reacts with C-Reactive Protein (CRP). It is used for grouping but does not influence colony morphology. * **Pneumolysin:** This is a membrane-damaging toxin (cytotoxin) released during autolysis that inhibits ciliary action and immune cells; it is not responsible for the physical shape of the colony. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, lancet-shaped diplococci. * **Culture:** Alpha-hemolytic (greenish discoloration) on blood agar. * **Bile Solubility Test:** Pneumococci are **bile soluble** because bile salts (like sodium deoxycholate) activate the same **autolytic enzymes** mentioned above, leading to rapid lysis. * **Optochin Sensitivity:** Pneumococci are sensitive to Optochin (ethylhydrocupreine hydrochloride), distinguishing them from *S. viridans*.

Q: A urine sample from a patient suspected of a urinary tract infection (UTI) was sent to the microbiology lab. On culture, the colony shows hemolysis on blood agar, a positive indole test, and a positive MUG test. The organism also shows characteristic growth on Eosin Methylene Blue (EMB) agar. What is the most likely etiologic organism?

E. coli. **Explanation:** The clinical presentation and laboratory findings are classic for **Escherichia coli**, the most common cause of community-acquired urinary tract infections. **Why E. coli is the correct answer:** * **Culture Characteristics:** Most uropathogenic strains of *E. coli* exhibit **beta-hemolysis** on blood agar. On **EMB agar**, it produces a characteristic **metallic green sheen** due to vigorous lactose fermentation. * **Biochemical Profile:** *E. coli* is part of the IMViC group "++--" (Indole positive, Methyl Red positive, Voges-Proskauer negative, Citrate negative). * **MUG Test:** This is a highly specific rapid test for *E. coli*. The enzyme **beta-glucuronidase** (produced by 97% of *E. coli* strains) cleaves the MUG substrate to release a fluorescent compound visible under UV light. **Why other options are incorrect:** * **Salmonella typhi:** It is a non-lactose fermenter (NLF), indole negative, and typically causes enteric fever rather than primary UTIs. * **Proteus vulgaris:** While indole positive, it is famous for "swarming motility" on agar and being **urease positive** (leading to alkaline urine and struvite stones), not a metallic sheen on EMB. * **Klebsiella pneumoniae:** It produces large, mucoid colonies (lactose fermenter) but is **indole negative** and **citrate positive**. **High-Yield NEET-PG Pearls:** * **EMB Agar:** Selective and differential; the metallic green sheen is a "must-know" for *E. coli*. * **Catalase/Oxidase:** *E. coli* is Catalase positive and **Oxidase negative** (like all Enterobacteriaceae). * **P-fimbriae:** The most important virulence factor for *E. coli* causing pyelonephritis. * **MUG Test:** Remember that *E. coli* O157:H7 (EHEC) is a notable exception and is typically MUG negative.

Q: Which of the following bacteria increases intracellular cyclic AMP levels?

Vibrio cholerae. **Explanation:** The correct answer is **Vibrio cholerae**. This bacterium produces the **Cholera Toxin (Choleragen)**, an A-B subunit exotoxin. The mechanism involves the ADP-ribosylation of the **Gs (stimulatory) protein**, which permanently activates **Adenylate Cyclase**. This leads to a massive increase in intracellular **cyclic AMP (cAMP)** levels in intestinal mucosal cells. Elevated cAMP causes the hypersecretion of water and electrolytes (Cl⁻, Na⁺, K⁺, and HCO₃⁻) into the intestinal lumen, resulting in characteristic "rice-water stools." **Analysis of Incorrect Options:** * **Staphylococcus aureus:** While it produces various toxins (like TSST-1 or Enterotoxins), they act as superantigens or cause pore formation; they do not primarily function by increasing cAMP. * **E. coli, heat-stable toxin (ST):** This is a common distractor. The **Heat-Stable (ST)** toxin increases **cyclic GMP (cGMP)** by activating Guanylate Cyclase. It is the **Heat-Labile (LT)** toxin of E. coli that increases cAMP (similar to Cholera toxin). * **Salmonella:** Its pathogenesis primarily involves mucosal invasion and a Type III secretion system, not a cAMP-mediated exotoxin mechanism. **NEET-PG Clinical Pearls:** * **Mnemonic for cAMP-increasing toxins:** **"cAMP"** – **C**holera (V. cholerae), **A**nthrax (Edema factor), **M**ontezuma’s revenge (ETEC - LT toxin), **P**ertussis (B. pertussis - via Gi inhibition). * **cGMP-increasing toxins:** E. coli (ST toxin) and Yersinia enterocolitica. * **Mechanism of Pertussis Toxin:** It ADP-ribosylates the **Gi (inhibitory) protein**, preventing the inhibition of Adenylate Cyclase, thus also raising cAMP.

Q: A patient with a urinary tract infection presents with urine pH greater than 8. What is the probable causative organism?

Proteus. ### Explanation **Correct Answer: B. Proteus** The key to this question lies in the **urease enzyme**. *Proteus* species (most commonly *P. mirabilis*) are potent producers of urease. This enzyme hydrolyzes urea into ammonia and carbon dioxide ($NH_2CONH_2 + H_2O \rightarrow 2NH_3 + CO_2$). The production of ammonia increases the concentration of hydroxyl ions, leading to significant **alkalinization of the urine (pH > 7.5 or 8)**. This alkaline environment promotes the precipitation of magnesium, ammonium, and phosphate, leading to the formation of **Struvite stones** (Triple phosphate/Staghorn calculi). **Why other options are incorrect:** * **A. Escherichia coli:** This is the most common cause of UTI overall. However, *E. coli* is typically **urease-negative** and does not significantly raise urine pH; in fact, urine is often acidic in these infections. * **C. Klebsiella:** While some strains of *Klebsiella* can produce urease, they are much less potent than *Proteus*. *Proteus* is the classic "textbook" association for highly alkaline urine and staghorn calculi. * **D. Candida:** Fungal UTIs are usually seen in immunocompromised or catheterized patients. They do not typically alter urine pH through urease production. **High-Yield Clinical Pearls for NEET-PG:** * **Culture Characteristic:** *Proteus* exhibits **"Swarming growth"** on blood agar due to its high motility (peritrichous flagella). This can be inhibited by using MacConkey agar (bile salts) or increasing agar concentration (6%). * **Biochemical Test:** *Proteus* is **Phenylalanine Deaminase (PPA) positive** and **Urease positive**. * **Dienes Phenomenon:** Used to differentiate two different strains of *Proteus* (they will not swarm into each other, forming a visible line). * **Weil-Felix Reaction:** Uses *Proteus* antigens (OX-19, OX-2, OX-K) to detect Rickettsial antibodies.

Q: A male patient presents with symptoms of urethritis. Examination reveals pus cells without any identified organism. What is the most likely cause?

Chlamydia trachomatis. **Explanation:** The clinical presentation of urethritis with "pus cells but no identified organism" on routine microscopy is the classic definition of **Non-Gonococcal Urethritis (NGU)**. **1. Why Chlamydia trachomatis is correct:** *Chlamydia trachomatis* (Serotypes D-K) is the most common cause of NGU worldwide. It is an **obligate intracellular bacterium**, meaning it does not stain with Gram stain and cannot be visualized under a light microscope using standard methods. Because it lacks a traditional peptidoglycan cell wall and remains intracellular, a Gram stain will show numerous polymorphonuclear leukocytes (pus cells) but will be "abacterial" (no organisms seen). **2. Why the other options are incorrect:** * **Haemophilus ducreyi:** Causes **Chancroid** (painful soft sores). It is a Gram-negative coccobacillus often seen in a "school of fish" appearance. * **Treponema pallidum:** Causes **Syphilis** (painless chancre). While it cannot be seen on Gram stain, it typically presents with a genital ulcer rather than isolated urethritis and is visualized via Dark Ground Microscopy (DGM). * **Mycobacterium tuberculosis:** While it can cause genitourinary TB (leading to "sterile pyuria"), it typically involves the bladder and kidneys and presents with systemic symptoms or chronic urinary complaints rather than acute urethritis. **Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Nucleic Acid Amplification Test (NAAT) is the investigation of choice for *Chlamydia*. * **Treatment:** Azithromycin (1g stat) or Doxycycline (100mg BD for 7 days). * **Sterile Pyuria:** Always consider *Chlamydia* or *Ureaplasma* when pus cells are present but routine cultures are negative. * **Co-infection:** Patients with Gonorrhea are frequently co-infected with Chlamydia; hence, dual therapy is often recommended.

Q: Which bacterium is a common contaminant in home-canned vegetables and smoked fish?

Clostridium botulinum. **Explanation:** The correct answer is **Clostridium botulinum**. This is a Gram-positive, anaerobic, spore-forming bacillus. The underlying medical concept is its ability to produce highly resilient **endospores** that survive sub-boiling temperatures. In **home-canned vegetables** (low-acid environments) and **vacuum-packed smoked fish**, the anaerobic conditions allow these spores to germinate and release the potent **botulinum neurotoxin**. This toxin causes "floppy baby syndrome" in infants and symmetric descending paralysis in adults by inhibiting acetylcholine release at the neuromuscular junction. **Why other options are incorrect:** * **Staphylococcus aureus:** Typically associated with protein-rich foods (custards, mayonnaise, processed meats) handled by carriers. It causes rapid-onset vomiting (1–6 hours) due to a preformed enterotoxin, but it is not specifically linked to canning processes. * **Salmonella:** Primarily associated with poultry, eggs, and unpasteurized milk. It causes inflammatory diarrhea (enteric fever or gastroenteritis) rather than intoxication from preserved goods. * **Bacillus cereus:** Classically associated with **reheated fried rice**. It produces two types of toxins: emetic (heat-stable) and diarrheal (heat-labile). **High-Yield NEET-PG Pearls:** * **Botulinum Toxin:** The most poisonous biological substance known. It is **heat-labile** (destroyed by boiling for 10 minutes), whereas the spores are heat-stable. * **Infant Botulism:** Associated with **honey** consumption (ingestion of spores, not preformed toxin). * **Clinical Triad:** Bulbar palsy (diplopia, dysphagia, dysarthria), descending paralysis, and clear sensorium. * **Diagnosis:** Demonstrated by the mouse neutralization test (gold standard).

Question 1

Pertussis toxin acts by all of the following mechanisms, EXCEPT:

Accepted Answer

Increased calcium release from sarcoplasmic reticulum

Answer

ADP ribosylation of proteins associated with receptors

Answer

Increase cyclic AMP

Answer

Acts through G alpha subunit

Question 2

Which bacterium is known to survive in alkaline media?

Accepted Answer

Vibrio

Answer

E. coli

Answer

Shigella

Answer

Salmonella

Question 3

What is a cold abscess related to?

Accepted Answer

Mycobacterium tuberculosis

Answer

Periapical pathology

Answer

Streptococcus viridans

Answer

None of the above

Question 4

A 30-year-old male presented with an ulcerative lesion on the glans penis. Wright-Giemsa stain showed 1-2 rounded structures within macrophage vacuoles. What is the etiology?

Accepted Answer

Calymatobacterium granulomatis

Answer

Chlamydia trachomatis

Answer

N. gonorrhoea

Answer

H. ducreyi

Question 5

Carrom coin appearance of pneumococcus is due to which of the following?

Accepted Answer

Autolysin

Answer

Capsular polysaccharide

Answer

C- Carbohydrate antigen

Answer

Pneumolysin

Question 6

A urine sample from a patient suspected of a urinary tract infection (UTI) was sent to the microbiology lab. On culture, the colony shows hemolysis on blood agar, a positive indole test, and a positive MUG test. The organism also shows characteristic growth on Eosin Methylene Blue (EMB) agar. What is the most likely etiologic organism?

Accepted Answer

E. coli

Answer

Salmonella typhi

Answer

Proteus vulgaris

Answer

Klebsiella pneumoniae

Question 7

Which of the following bacteria increases intracellular cyclic AMP levels?

Accepted Answer

Vibrio cholerae

Answer

Staphylococcus aureus

Answer

E. coli, heat-stable toxin

Answer

Salmonella

Question 8

A patient with a urinary tract infection presents with urine pH greater than 8. What is the probable causative organism?

Accepted Answer

Proteus

Answer

Escherichia coli

Answer

Klebsiella

Answer

Candida

Question 9

A male patient presents with symptoms of urethritis. Examination reveals pus cells without any identified organism. What is the most likely cause?

Accepted Answer

Chlamydia trachomatis

Answer

Haemophilus ducreyi

Answer

Treponema pallidum

Answer

Mycobacterium tuberculosis

Question 10

Which bacterium is a common contaminant in home-canned vegetables and smoked fish?

Accepted Answer

Clostridium botulinum

Answer

Staphylococcus

Answer

Salmonella

Answer

Bacillus cereus

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?