Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1951: Peptidoglycans are found in large quantities in the cell wall of which of the following microorganisms?

A. Virus
B. Gram positive bacteria (Correct Answer)
C. Gram negative bacteria
D. All of the above

Explanation: **Explanation:** The cell wall of bacteria is primarily composed of **peptidoglycan** (also known as murein), a polymer consisting of sugars and amino acids that provides structural rigidity and protects the cell from osmotic lysis. **1. Why Gram-positive bacteria is correct:** Gram-positive bacteria possess a very thick, multi-layered peptidoglycan shell that constitutes about **50% to 90%** of the cell wall's dry weight. This thick layer traps the Crystal Violet-Iodine complex during Gram staining, preventing it from being washed away by alcohol, which results in these bacteria appearing purple. **2. Why other options are incorrect:** * **Gram-negative bacteria:** While they do contain peptidoglycan, the layer is significantly **thinner** (only 1–2 layers thick), making up only about **5% to 10%** of the cell wall. Their structure is dominated by an outer membrane containing Lipopolysaccharide (LPS). * **Viruses:** These are non-cellular entities. They consist of genetic material (DNA or RNA) encased in a protein coat (capsid) and sometimes a lipid envelope. They **lack a cell wall** and peptidoglycan entirely. **Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Beta-lactam antibiotics (e.g., Penicillins, Cephalosporins) work by inhibiting **transpeptidase enzymes** (Penicillin-Binding Proteins), preventing the cross-linking of peptidoglycan. * **Lysozyme:** This enzyme, found in human tears and saliva, kills bacteria by cleaving the glycan backbone of peptidoglycan. * **Exception:** *Mycoplasma* species are the only bacteria that naturally **lack a cell wall** (and thus peptidoglycan), making them inherently resistant to beta-lactams. * **Teichoic Acid:** This is a major surface antigen found **only** in the cell walls of Gram-positive bacteria, embedded within the thick peptidoglycan layer.

Question 1952: Which of the following statements about Vibrio cholerae is false?

A. Produces indole and reduces nitrates.
B. Synthesizes neuraminidase.
C. Dies rapidly at low temperatures. (Correct Answer)
D. All of the above.

Explanation: ### Explanation The correct answer is **C. Dies rapidly at low temperatures.** #### 1. Why Option C is False (The Correct Answer) *Vibrio cholerae* is remarkably resistant to cold. It does not die rapidly at low temperatures; instead, it can survive for several days to weeks in ice or refrigerated food. In aquatic environments, it can even enter a "viable but non-culturable" (VBNC) state to survive harsh conditions. However, it is highly sensitive to **heat** (killed at 55°C in 30 minutes) and **acidity** (gastric acid is a major host defense). #### 2. Why Other Options are True * **Option A:** *Vibrio cholerae* is biochemically active. It is **indole positive** and has the ability to **reduce nitrates to nitrites**. These properties are utilized in the "Cholera Red Reaction," where adding sulfuric acid to a 24-hour peptone water culture produces a red color (due to the formation of nitroso-indole). * **Option B:** *Vibrio* species produce the enzyme **neuraminidase** (sialidase). This enzyme degrades gangliosides on the intestinal surface to increase the number of **GM1 receptors**, which are the specific binding sites for the Cholera Toxin (B-subunit). #### 3. High-Yield Clinical Pearls for NEET-PG * **Morphology:** Comma-shaped, Gram-negative bacilli; shows **darting motility** (liquid paraffin/hanging drop mount). * **Culture:** Grows on **TCBS agar** (Thiosulfate Citrate Bile Salts Sucrose) producing **yellow colonies** due to sucrose fermentation. * **String Test:** Positive (colonies emulsified in sodium deoxycholate become mucoid/stringy). * **Halotolerance:** *V. cholerae* is non-halophilic (can grow without NaCl), unlike other Vibrios which are halophilic. * **Toxin:** Cholera toxin (Choleragen) acts by increasing **cAMP**, leading to the classic "rice-water stools."

Question 1953: Which of the following statements regarding E. coli is true?

A. ETEC is invasive.
B. EPEC acts via cAMP.
C. Pilli are present in the uropathogenic type. (Correct Answer)
D. ETEC causes HUC.

Explanation: ### Explanation **Correct Option: C. Pilli are present in the uropathogenic type.** Uropathogenic *E. coli* (UPEC) is the most common cause of urinary tract infections. Its primary virulence factor is the presence of specialized **fimbriae (pili)**. Specifically, **P-pili** (pyelonephritis-associated pili) bind to the P-blood group antigen on uroepithelial cells, allowing the bacteria to resist being washed away by urine flow. **Type 1 pili** are also crucial for bladder colonization (cystitis). **Analysis of Incorrect Options:** * **A. ETEC is invasive:** This is false. **Enterotoxigenic *E. coli* (ETEC)** is non-invasive. It colonizes the small intestine via colonization factor antigens (CFAs) and produces toxins but does not penetrate the mucosa. *Enteroinvasive E. coli* (EIEC) is the invasive type. * **B. EPEC acts via cAMP:** This is false. **Enteropathogenic *E. coli* (EPEC)** acts through **"Attaching and Effacing" (A/E) lesions**, causing the destruction of microvilli. It is the **Heat-Labile (LT) toxin of ETEC** that acts by increasing cAMP. * **D. ETEC causes HUS:** This is false. **Hemolytic Uremic Syndrome (HUS)** is caused by **Enterohemorrhagic *E. coli* (EHEC)**, specifically the O157:H7 strain, via the production of Shiga-like toxins (Verotoxins). **High-Yield Clinical Pearls for NEET-PG:** * **ETEC:** Most common cause of **Traveler’s Diarrhea**. Toxins: LT (increases cAMP) and ST (increases cGMP). * **EHEC:** Does not ferment sorbitol (unlike other *E. coli*); causes bloody diarrhea without fever. * **EIEC:** Clinically indistinguishable from Shigellosis (dysentery with inflammatory cells in stool). * **Culture:** *E. coli* produces flat, **rose-pink colonies on MacConkey agar** (Lactose Fermenter) and a characteristic **metallic sheen on EMB agar**.

Question 1954: All of the following are true regarding Legionella except?

A. Legionella can be grown on complex media
B. L. pneumophila serogroup 1 is the most common serogroup isolated from humans
C. Legionella are communicable from infected patients to others (Correct Answer)
D. L. pneumophila is not effectively killed by polymorphonuclear leukocytes

Explanation: **Explanation:** The correct answer is **C**. *Legionella* species are unique because, despite causing severe pneumonia (Legionnaires' disease), there is **no person-to-person transmission**. Infection occurs exclusively through the inhalation or aspiration of contaminated aerosols from environmental water sources (e.g., cooling towers, AC systems, showers, or nebulizers). **Analysis of Options:** * **Option A (True):** *Legionella* are fastidious and cannot grow on standard agar. They require **complex media** supplemented with **L-cysteine and iron**, such as Buffered Charcoal Yeast Extract (BCYE) agar. * **Option B (True):** While there are over 60 species, *L. pneumophila* causes 90% of human infections, and **Serogroup 1** is the most frequent isolate worldwide. * **Option D (True):** *Legionella* is a facultative **intracellular pathogen**. It inhibits phagosome-lysosome fusion, allowing it to survive and multiply within alveolar macrophages and polymorphonuclear leukocytes (PMNs) rather than being killed by them. **High-Yield Clinical Pearls for NEET-PG:** * **Habitat:** Natural and artificial water systems; they often survive within free-living amoebae (*Acanthamoeba*). * **Clinical Features:** Presents as severe pneumonia with "extrapulmonary symptoms" like **hyponatremia**, diarrhea, and confusion. * **Diagnosis:** **Urinary Antigen Test** (detects Serogroup 1) is the rapid test of choice. Direct Fluorescent Antibody (DFA) staining is also used. * **Treatment:** Macrolides (Azithromycin) or Fluoroquinolones (Levofloxacin). They are intrinsically resistant to Beta-lactams.

Question 1955: Which of the following statements regarding Cholera is inappropriate?

A. Incubation period is typically 1-2 days.
B. Characteristic symptom is rice-watery diarrhea.
C. Azithromycin is the treatment of choice in adults. (Correct Answer)
D. It is a notifiable disease locally, nationally, and internationally.

Explanation: ### Explanation **1. Why Option C is the correct (inappropriate) statement:** While antibiotics can shorten the duration of diarrhea and reduce vibrio excretion, the **treatment of choice** for Cholera is **aggressive fluid and electrolyte replacement** (Oral Rehydration Salts or IV Ringer’s Lactate). If an antibiotic is indicated for adults, **Doxycycline** (single 300mg dose) remains the drug of choice according to the WHO and standard textbooks. Azithromycin is preferred for children and pregnant women, but not as the primary empirical choice for all adults. **2. Analysis of other options:** * **Option A (Incubation Period):** Correct. The incubation period of *Vibrio cholerae* is very short, typically ranging from **1 to 2 days** (range: few hours to 5 days), leading to rapid outbreaks. * **Option B (Rice-watery diarrhea):** Correct. This is the hallmark clinical sign. The stool is non-bilious, non-bloody, and contains flecks of mucus, epithelial cells, and vibrios, resembling water in which rice has been washed. * **Option D (Notifiable Disease):** Correct. Cholera is a **notifiable disease** at all levels due to its epidemic potential and is one of the diseases covered under the International Health Regulations (IHR). **3. High-Yield Clinical Pearls for NEET-PG:** * **Toxin:** Cholera toxin (Choleragen) is an **AB subunit toxin** that increases **cAMP**, leading to the hypersecretion of water and chlorides. * **Microscopy:** Shows characteristic **"darting motility"** (inhibited by specific antisera in the Pfeiffer phenomenon). * **Culture:** **TCBS (Thiosulfate Citrate Bile Salts Sucrose) agar** is the selective medium where it forms yellow colonies. * **Transport Media:** Venkatraman-Ramakrishnan (VR) medium or Cary-Blair medium. * **Epidemiology:** The current (7th) pandemic is caused by the **El Tor** biotype.

Question 1956: All of the following non-mycobacterial microorganisms display acid fastness, EXCEPT:

A. Nocardia
B. Fusobacterium nucleatum (Correct Answer)
C. Rhodococcus
D. Isospora

Explanation: **Explanation:** Acid-fastness is a physical property of certain microorganisms that allows them to resist decolorization by acids during staining procedures. This property is primarily due to the presence of **mycolic acids** (long-chain fatty acids) in their cell walls. **Why Fusobacterium nucleatum is the correct answer:** *Fusobacterium nucleatum* is a Gram-negative, anaerobic, non-spore-forming bacterium typically found in the oral cavity. It lacks mycolic acids in its cell wall and is **not acid-fast**. It is a common cause of periodontal disease and Lemierre-like syndromes. **Analysis of incorrect options (Acid-fast organisms):** * **Nocardia:** These are aerobic Actinomycetes that are **weakly acid-fast** (partially acid-fast). They require a weaker decolorizer (1% sulfuric acid) instead of the standard 20% used for *M. tuberculosis*. * **Rhodococcus:** Similar to Nocardia, *Rhodococcus equi* contains mycolic acids in its cell wall and can demonstrate partial acid-fastness, especially in clinical samples. * **Isospora (Cystoisospora belli):** This is a protozoan parasite. Its oocysts are **acid-fast**, which is a key diagnostic feature in stool examinations using the Modified Ziehl-Neelsen stain. **High-Yield Clinical Pearls for NEET-PG:** * **Modified Ziehl-Neelsen (Kinyoun) Stain** is used for "weakly" acid-fast organisms. * **Mnemonic for Acid-fast organisms (MY-Legs-Are-Not-Cold):** * **MY**: **My**cobacteria * **L**: **L**egionella micdadei (only this species) * **A**: **A**ctinomycetes (Nocardia, Rhodococcus, Gordonia, Tsukamurella) * **N**: **N**oshema (Microsporidia) * **C**: **C**occidian Parasites (Cryptosporidium, Cyclospora, Isospora) * **Sperm heads** and **Bacterial spores** also exhibit acid-fastness.

Question 1957: Which of the following bacteria is known to form preformed enterotoxins?

A. Clostridium tetani
B. E. coli
C. Shigella sonnei
D. Clostridium perfringens (Correct Answer)

Explanation: **Explanation:** The question focuses on the mechanism of food poisoning, specifically the production of **preformed enterotoxins**. **1. Why Clostridium perfringens is correct:** *Clostridium perfringens* (Type A) is a classic cause of food poisoning associated with reheated meat dishes. While it typically produces toxins *in vivo* (after ingestion of spores), certain strains are known to produce **preformed enterotoxins** in the food itself under specific conditions. These toxins act directly on the intestinal epithelium, leading to a relatively short incubation period (8–16 hours) characterized by watery diarrhea and abdominal cramps. **2. Analysis of Incorrect Options:** * **Clostridium tetani (A):** Produces **Tetanospasmin**, a potent neurotoxin that acts on the CNS to cause spastic paralysis. It is not an enterotoxin and does not cause food poisoning. * **E. coli (B):** Most diarrheagenic *E. coli* (like ETEC or EHEC) produce toxins **colonization-dependently** (within the gut) after ingestion of the bacteria. They do not typically form toxins in the food before consumption. * **Shigella sonnei (C):** Causes bacillary dysentery via **invasion** of the intestinal mucosa and production of Shiga toxin *in vivo*. It is not associated with preformed toxins in food. **3. NEET-PG High-Yield Pearls:** * **Rapid Onset (<6 hours):** Think of preformed toxins by *Staphylococcus aureus* (heat-stable) or *Bacillus cereus* (emetic type). * **Intermediate Onset (8–16 hours):** Think of *Clostridium perfringens*. * **Mechanism:** *C. perfringens* enterotoxin (CPE) is a pore-forming toxin that binds to **claudin receptors** in the tight junctions of the gut epithelium. * **Diagnosis:** Usually clinical; confirmed by detecting $>10^5$ organisms per gram of contaminated food.

Question 1958: Yaws is caused by which spirochete?

A. Treponema pallidum
B. Treponema endemicum
C. Treponema pertenue (Correct Answer)
D. Treponema carateum

Explanation: **Explanation:** The correct answer is **Treponema pertenue** (Option C). This question tests the classification of pathogenic spirochetes, specifically the genus *Treponema*, which are morphologically identical but cause distinct clinical syndromes known as **Treponematoses**. **1. Why Treponema pertenue is correct:** *Treponema pallidum* subspecies *pertenue* is the causative agent of **Yaws**. It is a non-venereal infection primarily affecting children in humid, tropical regions. It is characterized by skin lesions (frambesia), bone involvement, and joint destruction. It is transmitted via direct skin-to-skin contact. **2. Analysis of Incorrect Options:** * **A. Treponema pallidum (subsp. pallidum):** Causes **Venereal Syphilis**. It is sexually transmitted or congenital and involves systemic dissemination (primary, secondary, and tertiary stages). * **B. Treponema endemicum:** Causes **Endemic Syphilis (Bejel)**. It is found in arid regions (Middle East/Africa) and is transmitted via shared utensils or mouth-to-mouth contact. * **D. Treponema carateum:** Causes **Pinta**. This is the mildest form, restricted to the skin (causing pigmentary changes), and is found in Central and South America. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** All treponemes are thin, spiral-shaped, and cannot be grown on artificial media. They are visualized using **Dark-field microscopy**. * **Serology:** All four treponematoses give **positive results** on standard syphilis tests (VDRL/RPR and FTA-ABS) because they are antigenically similar. * **Drug of Choice:** **Penicillin G** remains the gold standard treatment for all treponemal infections. * **Mnemonic:** Remember **"P-P-P"** (Pinta - *T. carateum* - Pigmentation) and **"Y-P"** (Yaws - *T. pertenue*).

Question 1959: Which disease is caused by Bartonella quintana?

A. Trench fever (Correct Answer)
B. Scrub typhus
C. Endemic typhus
D. Epidemic typhus

Explanation: **Explanation:** **Bartonella quintana** is a Gram-negative, fastidious bacterium primarily transmitted by the **human body louse** (*Pediculus humanus corporis*). It is the causative agent of **Trench fever** (Option A), a disease historically significant during World War I. It is characterized by a relapsing fever (often in 5-day cycles, hence the name "quintana"), headache, and severe pretibial (shin) pain. In modern clinical practice, it is also associated with bacillary angiomatosis and culture-negative endocarditis, particularly in homeless populations or those with poor hygiene. **Analysis of Incorrect Options:** * **B. Scrub typhus:** Caused by *Orientia tsutsugamushi* and transmitted by the bite of larval mites (chiggers). It typically presents with a characteristic black eschar at the bite site. * **C. Endemic typhus (Murine typhus):** Caused by *Rickettsia typhi* and transmitted to humans by rat fleas (*Xenopsylla cheopis*). * **D. Epidemic typhus:** Caused by *Rickettsia prowazekii*. While it shares the same vector as Trench fever (the human body louse), the causative organism is different. **High-Yield Clinical Pearls for NEET-PG:** * **Vector Match:** Both *Bartonella quintana* and *Rickettsia prowazekii* are transmitted by the **human body louse**. * **Bartonella species:** Remember *B. henselae* causes **Cat Scratch Disease**, while *B. bacilliformis* causes **Carrion’s disease** (Oroya fever/Verruga peruana) transmitted by the Sandfly (*Lutzomyia*). * **Bacillary Angiomatosis:** In HIV/immunocompromised patients, both *B. quintana* and *B. henselae* can cause these vascular proliferative lesions.

Question 1960: Which of the following is the most severe form of diphtheria?

A. Nasal
B. Cutaneous
C. Nasopharyngeal
D. Laryngeal (Correct Answer)

Explanation: **Explanation:** The correct answer is **Laryngeal Diphtheria**. In diphtheria, the severity of the disease is directly proportional to the extent of the pseudomembrane and the resulting systemic absorption of the exotoxin. **1. Why Laryngeal Diphtheria is the most severe:** Laryngeal involvement is considered the most dangerous form because the thick, grayish-white **pseudomembrane** can cause acute **upper airway obstruction**. In children, the narrow lumen of the larynx makes them highly susceptible to "suffocation," often necessitating an emergency tracheostomy. Furthermore, this site allows for significant toxin absorption, leading to systemic complications like myocarditis and polyneuritis. **2. Analysis of Incorrect Options:** * **Nasal Diphtheria:** This is the **mildest form**. It usually presents with a serosanguinous discharge. Because the toxin is poorly absorbed through the nasal mucosa, systemic symptoms are minimal. * **Cutaneous Diphtheria:** Usually presents as a non-healing ulcer with a grey membrane. It is generally indolent and often acts as a reservoir for infection rather than causing acute systemic toxicity. * **Nasopharyngeal (Faucial) Diphtheria:** While common and serious (associated with "Bull neck" lymphadenopathy), it is the mechanical obstruction of the airway in the laryngeal form that poses the most immediate threat to life. **Clinical Pearls for NEET-PG:** * **Causative Agent:** *Corynebacterium diphtheriae* (Gram-positive, club-shaped bacilli). * **Virulence Factor:** Diphtheria toxin (encoded by the **tox gene** via lysogenic induction by a **beta-phage**). It inhibits protein synthesis by inactivating **EF-2**. * **Culture Media:** Löffler's serum slope (rapid growth) and Potassium Tellurite agar (black colonies). * **Elek’s Gel Precipitation Test:** Used to detect the toxigenicity of the strain.

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free