Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 191: All of the following are acid-fast bacteria except:

A. Cryptosporidia
B. Mycoplasma (Correct Answer)
C. Mycobacteria
D. Nocardia

Explanation: **Explanation:** The concept of **Acid-fastness** refers to the physical property of certain bacteria and parasites to resist decolorization by acids during staining procedures (like the Ziehl-Neelsen stain). This property is primarily due to the presence of high concentrations of **mycolic acids** (long-chain fatty acids) in their cell walls. **Why Mycoplasma is the correct answer:** **Mycoplasma** is the only option that is **not acid-fast**. In fact, Mycoplasma species are unique among bacteria because they **completely lack a cell wall**. Since acid-fastness depends on cell wall composition (specifically mycolic acid), a cell-wall-deficient organism cannot be acid-fast. **Analysis of incorrect options:** * **Mycobacteria:** These are the classic acid-fast bacilli (AFB). They have a very high lipid content (60%) in their cell walls, making them strongly acid-fast (resisting 20% $H_2SO_4$). * **Nocardia:** These are filamentous bacteria that are **weakly acid-fast**. They require a modified Ziehl-Neelsen stain using a weaker decolorizer (1% $H_2SO_4$). * **Cryptosporidia:** This is a protozoan parasite. Its oocysts are **acid-fast** and are typically identified in stool samples using the Kinyoun (cold) acid-fast stain. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Acid-Fast Organisms:** **"A**ll **C**andy **I**s **N**ot **G**ood **M**y **S**on" (**A**scaris eggs, **C**ryptosporidium/Cyclospora/Cystoisospora, **I**sospora, **N**ocardia, **G**ordonia, **M**ycobacteria, **S**permatic head/Legionella **m**icdadei). * **Decolorizer concentrations:** * *M. tuberculosis*: 20% $H_2SO_4$ * *M. leprae*: 5% $H_2SO_4$ * *Nocardia*: 1% $H_2SO_4$ * *Oocysts/Sperm*: 0.25–0.5% $H_2SO_4$ * Mycoplasma is inherently resistant to beta-lactam antibiotics (like Penicillin) because it lacks a cell wall target.

Question 192: A 20-year-old sexually active woman presents with swelling of the vaginal labia. Examination reveals sharply defined subcutaneous bleeding nodules that appear to erode through the skin. What is the most likely etiologic agent in this case?

A. Treponema pallidum
B. Herpes virus
C. Calymmatobacterium granulomatis
D. Neisseria gonorrhoeae (Correct Answer)

Explanation: ### Explanation The clinical presentation described—sharply defined subcutaneous bleeding nodules that erode through the skin—is characteristic of **Disseminated Gonococcal Infection (DGI)**. While *Neisseria gonorrhoeae* typically causes localized mucosal inflammation (cervicitis or urethritis), it can manifest as systemic skin lesions. These lesions often begin as erythematous macules that progress to hemorrhagic papules or "bleeding nodules" and pustules, frequently located near joints or on the extremities/genitalia. **Why the other options are incorrect:** * **Treponema pallidum:** Causes Syphilis. The primary lesion (chancre) is a painless, clean-based ulcer with indurated edges, not bleeding subcutaneous nodules. * **Herpes virus (HSV-2):** Typically presents as clusters of painful, fluid-filled vesicles on an erythematous base that rupture to form shallow, exquisitely tender ulcers. * **Calymmatobacterium granulomatis (now *Klebsiella granulomatis*):** Causes Granuloma Inguinale (Donovanosis). While it presents with beefy-red, painless, expanding granulomatous ulcers that bleed easily on contact, the initial presentation is usually a papule or ulcer rather than "subcutaneous nodules eroding through the skin." **NEET-PG High-Yield Pearls:** * **DGI Triad:** Tenosynovitis, Dermatitis (hemorrhagic/pustular lesions), and Polyarthralgia. * **Culture:** *N. gonorrhoeae* is fastidious; it requires **Thayer-Martin medium** (VPN: Vancomycin, Polymyxin, Nystatin) and 5-10% $CO_2$. * **Microscopy:** Gram-negative kidney-shaped diplococci found within polymorphonuclear leukocytes (intracellular). * **Treatment:** Ceftriaxone is the drug of choice for gonococcal infections. Always screen for co-infection with *Chlamydia trachomatis*.

Question 193: The pathogenicity of major pseudomonads is based on opportunism. Which of the following is an exception?

A. Pneumonia
B. Otitis media
C. Melioidosis (Correct Answer)
D. Skin infection

Explanation: **Explanation:** The core concept of this question lies in the distinction between **opportunistic pathogens** (which cause disease primarily in immunocompromised hosts or through breached barriers) and **primary pathogens** (which can cause disease in healthy individuals). **Why Melioidosis is the correct answer:** Melioidosis is caused by ***Burkholderia pseudomallei*** (formerly classified under the genus *Pseudomonas*). Unlike *Pseudomonas aeruginosa*, which is a classic opportunist, *B. pseudomallei* is a **primary pathogen**. It is highly virulent and can cause severe systemic disease, including localized nodules, pneumonia, or fatal septicemia, even in previously healthy individuals. It is often referred to as the "Vietnamese Time Bomb" due to its ability to remain latent for years. **Why the other options are incorrect:** * **Pneumonia, Otitis media, and Skin infections:** These are classic clinical manifestations of ***Pseudomonas aeruginosa***. *P. aeruginosa* is the quintessential opportunistic pathogen. It rarely causes disease in healthy tissue but thrives in patients with cystic fibrosis (pneumonia), "swimmer’s ear" (otitis externa), or severe burns (skin infections). Its pathogenicity depends on the host's weakened defenses. **High-Yield Clinical Pearls for NEET-PG:** * **Melioidosis Diagnosis:** Characterized by "safety-pin" appearance on Gram stain (bipolar staining) and a characteristic **musty/earthy odor** on culture. * **Culture Appearance:** *B. pseudomallei* produces wrinkled, metallic-looking colonies on Ashdown’s medium. * **Drug of Choice:** Ceftazidime is the gold standard for the intensive phase of Melioidosis treatment. * **Glanders Disease:** Caused by *Burkholderia mallei*, which is non-motile (unlike *B. pseudomallei*).

Question 194: An abattoir worker developed a pustule which later progressed to a necrotic ulcer. Which of the following stains is useful for demonstration of the organism from a smear made from the pustule?

A. Polychromic methylene blue (Correct Answer)
B. Chalkofluor white
C. Giemsa
D. Modified Kinyoun stain

Explanation: **Explanation:** The clinical presentation of an **abattoir worker** (occupational exposure) with a pustule progressing to a necrotic ulcer (malignant pustule) is classic for **Cutaneous Anthrax**, caused by *Bacillus anthracis*. **1. Why Polychromic Methylene Blue is correct:** *Bacillus anthracis* is a capsulated organism. The capsule is composed of **poly-D-glutamic acid**. **Polychromic methylene blue (M’Fadyean reaction)** is the specific stain used to demonstrate this capsule. When stained, the bacilli appear blue, surrounded by a **disintegrated pink/purple capsular material**. This reaction is diagnostic for *B. anthracis* in smears from tissues or blood. **2. Why other options are incorrect:** * **Calcofluor white:** A fluorescent stain used primarily for the rapid detection of **fungi** and certain parasites; it binds to cellulose and chitin. * **Giemsa:** A common hematological stain used for malaria, *Chlamydia*, and *Rickettsia*, but it does not specifically highlight the anthrax capsule for diagnostic confirmation. * **Modified Kinyoun stain:** A cold acid-fast stain used for organisms like *Nocardia*, *Cryptosporidium*, and *Cyclospora*. *B. anthracis* is not acid-fast. **Clinical Pearls for NEET-PG:** * **M’Fadyean Reaction:** Specifically identifies the capsule of *B. anthracis*. * **Morphology:** Large, Gram-positive, non-motile bacilli with "Bamboo stick" appearance and central, non-bulging spores. * **Culture:** Shows characteristic **"Medusa head"** colonies on agar. * **Virulence Factors:** Encoded on plasmids **pXO1** (Toxins: Edema factor, Lethal factor, Protective antigen) and **pXO2** (Capsule).

Question 195: In Trachoma, Halberstaeder-Prowazek (HP) bodies can be demonstrated in conjunctival discharge by staining with:

A. Gram stain
B. Carbol fuchsin
C. Giemsa stain (Correct Answer)
D. Fontana stain

Explanation: **Explanation:** **1. Why Giemsa stain is correct:** *Chlamydia trachomatis*, the causative agent of Trachoma, is an obligate intracellular bacterium. It undergoes a unique life cycle involving **Elementary Bodies (EB)** and **Reticulate Bodies (RB)**. In conjunctival scrapings, these RBs aggregate within the cytoplasm of epithelial cells to form large **intracytoplasmic inclusion bodies** known as **Halberstaeder-Prowazek (HP) bodies**. Because *Chlamydia* lacks a traditional peptidoglycan cell wall and is very small, it does not stain well with Gram stain. **Giemsa stain** is the gold standard for morphology, staining the inclusion bodies a characteristic **purple/blue** against the pale blue cytoplasm of the host cell. **2. Why other options are incorrect:** * **Gram stain (A):** *Chlamydia* are technically Gram-negative but stain very poorly or not at all due to their unique cell wall structure. * **Carbol fuchsin (B):** This is a primary stain used in Acid-Fast staining (Ziehl-Neelsen) for *Mycobacteria*. It is not used for visualizing intracellular inclusions. * **Fontana stain (C):** This is a silver impregnation stain used specifically for visualizing **Spirochetes** (like *Treponema pallidum*). **3. High-Yield Clinical Pearls for NEET-PG:** * **HP Bodies:** Pathognomonic for Trachoma; seen in conjunctival scrapings (not discharge). * **Other Stains for Chlamydia:** Iodine stain (stains glycogen-rich inclusions of *C. trachomatis* brown), Castaneda, and Machiavello stains. * **Levinthal-Cole-Lillie (LCL) bodies:** Inclusion bodies seen in *Psittacosis* (*C. psittaci*). * **Drug of Choice:** Oral **Azithromycin** (SAFE strategy: Surgery, Antibiotics, Facial cleanliness, Environmental improvement).

Question 196: The 'daisy head' appearance of Corynebacterium diphtheriae colonies on blood tellurite agar is characteristic of which biotype?

A. Gravis biotype (Correct Answer)
B. Intermedius biotype
C. Mitis biotype
D. None of the above

Explanation: **Explanation:** *Corynebacterium diphtheriae* is classified into four biotypes based on colony morphology on Potassium Tellurite agar and biochemical properties. The correct answer is **Gravis biotype**. **1. Why Gravis is correct:** The **Gravis** biotype produces large, irregular, gunmetal-grey to black colonies. These colonies exhibit a characteristic **"daisy head" appearance** due to their radial striations and crenated edges. Biochemically, Gravis is the only biotype that ferments starch and glycogen, and it is typically associated with the most severe clinical disease. **2. Why the other options are incorrect:** * **Intermedius biotype:** These colonies are very small, smooth, and pinpoint (often described as **"frog egg"** appearance). They are non-hemolytic and do not ferment starch. * **Mitis biotype:** These colonies are medium-sized, smooth, convex, and black. They are often described as **"poached egg"** colonies and are typically hemolytic. * **Belfanti biotype:** (Not listed, but relevant) Similar to Mitis but nitrate-reductase negative. **3. High-Yield Clinical Pearls for NEET-PG:** * **Selective Media:** Potassium Tellurite agar (McLeod’s/Hoyle’s medium) inhibits most oropharyngeal flora; *C. diphtheriae* reduces tellurite to metallic tellurium, turning colonies black. * **Enrichment Media:** Loeffler’s Serum Slope (LSS) is used for rapid growth (6–8 hours) and enhances the development of **metachromatic (volutin) granules**. * **Staining:** Albert’s stain is used to visualize granules, which appear bluish-black against a green bacillary body (**Chinese letter arrangement**). * **Toxin Detection:** The **Elek’s Gel Precipitation Test** is the gold standard for demonstrating toxigenicity (detecting the *tox* gene carried by the Beta-phage).

Question 197: Which organism most commonly causes infective endocarditis?

A. Streptococcus mitis
B. Streptococcus viridans (Correct Answer)
C. Staphylococcus aureus
D. Staphylococcus pyogenes

Explanation: **Explanation:** Infective Endocarditis (IE) is an infection of the endocardial surface of the heart, typically involving the valves. The causative agent varies depending on the clinical context (e.g., acute vs. subacute, prosthetic vs. native valve). **Why Streptococcus viridans is correct:** *Streptococcus viridans* (a group including *S. mitis, S. sanguinis, S. mutans*) is the **most common cause of Subacute Bacterial Endocarditis (SBE)** and overall the most frequent cause of native valve endocarditis in non-intravenous drug users. These organisms are commensals of the oral cavity. Following dental procedures or even vigorous brushing, they enter the bloodstream (transient bacteremia) and adhere to previously damaged heart valves (e.g., rheumatic heart disease or congenital defects) via **dextrans**. **Analysis of Incorrect Options:** * **Streptococcus mitis (Option A):** While *S. mitis* is a member of the *Viridans* group, "Streptococcus viridans" is the broader, more standard collective term used in examinations to denote the most common cause. * **Staphylococcus aureus (Option C):** This is the most common cause of **Acute Infective Endocarditis** and the leading cause in **Intravenous Drug Users (IVDUs)**, typically affecting the tricuspid valve. It can infect previously healthy valves. * **Staphylococcus pyogenes (Option D):** Also known as Group A Streptococcus, it is primarily associated with Rheumatic Fever (an immunological sequel) rather than being a common direct cause of infective endocarditis. **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause overall:** *Streptococcus viridans*. * **Most common cause in IVDUs:** *Staphylococcus aureus*. * **Most common cause in Prosthetic Valve (Early, <1 year):** *Staphylococcus epidermidis*. * **Culture-Negative Endocarditis:** Most commonly due to prior antibiotic use or **HACEK** group organisms. * **Streptococcus bovis (S. gallolyticus):** If isolated in IE, always screen the patient for **Colorectal Carcinoma**.

Question 198: What is the most common causative agent of osteomyelitis in patients with sickle cell disease?

A. Salmonella species (Correct Answer)
B. Escherichia coli
C. Klebsiella species
D. Enterococcus species

Explanation: **Explanation:** The correct answer is **Salmonella species**. While *Staphylococcus aureus* is the most common cause of osteomyelitis in the general population, **Salmonella** is the most common causative agent specifically in patients with **Sickle Cell Disease (SCD)**. **Why Salmonella?** Patients with SCD are uniquely predisposed to Salmonella infections due to several factors: 1. **Autosplenectomy:** Chronic splenic infarction leads to functional asplenia, impairing the clearance of encapsulated organisms. 2. **Bowel Infarction:** Micro-infarctions in the intestinal mucosa allow *Salmonella* (normal gut flora in some or acquired via ingestion) to enter the bloodstream. 3. **Expanded Bone Marrow:** Chronic hemolysis leads to expanded, sluggish marrow circulation and areas of bone necrosis (infarcts), which act as a nidus for bacterial seeding during transient bacteremia. **Analysis of Incorrect Options:** * **B, C, & D (E. coli, Klebsiella, Enterococcus):** While these Gram-negative and Gram-positive organisms can occasionally cause osteomyelitis (especially in the elderly or post-surgery), they do not show the specific, high-frequency association with Sickle Cell Disease that *Salmonella* does. **High-Yield Clinical Pearls for NEET-PG:** * **General Population:** *Staphylococcus aureus* remains the #1 cause of osteomyelitis overall. * **Sickle Cell Disease:** *Salmonella* is the most common (often tested), though *S. aureus* is a close second. * **IV Drug Users:** Increased incidence of *Pseudomonas aeruginosa* (often involving the spine or sternoclavicular joints). * **Neonates:** *Group B Streptococcus* and *E. coli* are common. * **Diagnostic Tip:** On imaging, look for the "hand-foot syndrome" (dactylitis) in young SCD patients, which can be the first clinical presentation of bone involvement.

Question 199: Staphylococcus aureus causes which of the following conditions?

A. Erythrasma
B. Chancroid
C. Acne vulgaris
D. Bullous impetigo (Correct Answer)

Explanation: **Explanation:** **Bullous Impetigo** is the correct answer because it is a localized form of Staphylococcal Scalded Skin Syndrome (SSSS). It is caused by specific strains of ***Staphylococcus aureus*** (Phage Group II, types 71 and 55) that produce **Exfoliative Toxins (A and B)**. These toxins act as serine proteases that specifically target and cleave **Desmoglein-1**, a cell-adhesion molecule in the *stratum granulosum* of the epidermis. This leads to subcorneal cleavage, resulting in the characteristic thin-walled, fluid-filled bullae. **Analysis of Incorrect Options:** * **A. Erythrasma:** Caused by ***Corynebacterium minutissimum***. It presents as reddish-brown patches in intertriginous areas and is classically diagnosed by a coral-red fluorescence under Wood’s lamp (due to porphyrin production). * **B. Chancroid:** A sexually transmitted infection caused by the Gram-negative coccobacillus ***Haemophilus ducreyi***. It presents as painful genital ulcers with inguinal lymphadenopathy (buboes). * **C. Acne vulgaris:** Primarily associated with ***Cutibacterium acnes*** (formerly *Propionibacterium acnes*), an anaerobic bacterium that colonizes the pilosebaceous unit. **NEET-PG High-Yield Pearls:** * **Non-bullous Impetigo:** The most common form of impetigo; can be caused by both *S. aureus* (most common) and *Streptococcus pyogenes*. It is characterized by "honey-colored crusts." * **Toxic Shock Syndrome (TSS):** Another *S. aureus* condition mediated by the superantigen **TSST-1**. * **Protein A:** A key virulence factor of *S. aureus* that binds to the Fc portion of IgG, preventing opsonization and phagocytosis.

Question 200: Which of the following is NOT found in meningococci?

A. Capsule
B. Lipopolysaccharide
C. Plasmid (Correct Answer)
D. Pili

Explanation: **Explanation:** The correct answer is **C. Plasmid**. *Neisseria meningitidis* (meningococcus) is a Gram-negative diplococcus characterized by its fastidious nature. Unlike many other pathogenic bacteria, **plasmids are extremely rare** in meningococci. While they are frequently found in *Neisseria gonorrhoeae* (carrying antibiotic resistance genes like R-plasmids), meningococci typically carry their genetic determinants, including virulence factors and antibiotic resistance, on their **chromosomal DNA**. **Why the other options are incorrect:** * **Capsule (A):** This is the most important virulence factor of *N. meningitidis*. It is a polysaccharide capsule that inhibits phagocytosis and is the basis for serogrouping (A, B, C, W-135, X, and Y). * **Lipopolysaccharide (B):** As a Gram-negative organism, it possesses LPS in its outer membrane. In *Neisseria*, this is often referred to as **LOS (Lipooligosaccharide)** because it lacks the long polysaccharide O-antigen. It is a potent endotoxin responsible for the vascular damage and shock seen in meningococcemia. * **Pili (D):** These are essential for the initial attachment (adhesion) of the bacteria to the non-ciliated columnar epithelium of the nasopharynx. **High-Yield Clinical Pearls for NEET-PG:** * **Specimen of choice:** CSF (for meningitis) and Blood (for septicemia). * **Culture:** Grows best on **Thayer-Martin Medium** (selective) or Chocolate agar at 5-10% $CO_2$. * **Oxidase Test:** All *Neisseria* species are **Oxidase positive**. * **Sugar Fermentation:** Meningococci ferment both **Glucose and Maltose** (Mnemonic: **M**eningococci = **M**altose; **G**onococci = **G**lucose only). * **Prophylaxis:** Rifampicin is the drug of choice for close contacts.

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

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