Bacteriology — MCQs
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Which of the following microorganisms can be grown in cell culture media?
What is the commonest method for detecting diphtheria carriers?
A man presented with diarrhea three days after consuming uncooked meat. Stool examination revealed comma-shaped organisms with red blood cells and white blood cells. What is the causative organism?
Swimming pool conjunctivitis is caused by which of the following microorganisms?
What is the approximate number of genes in the complete genome sequence of M. tuberculosis?
"Genital elephantiasis" is caused by which of the following?
Chlamydia are isolated by which of the following methods?
Which is the most common complication of this rash?
Which of the following is coagulase-negative Staphylococcus?
Which of the following media does not support the growth of H. influenzae?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 91: Which of the following microorganisms can be grown in cell culture media?
- A. Chlamydia (Correct Answer)
- B. Ureaplasma urealyticum
- C. Pseudomonas aeruginosa
- D. Tropheryma whipplei
Explanation: **Explanation:** **Correct Option: A. Chlamydia** The fundamental concept here is the distinction between **obligate intracellular** and extracellular organisms. *Chlamydia* species (e.g., *C. trachomatis*, *C. pneumoniae*) lack the metabolic pathways to synthesize their own ATP, making them "energy parasites." Because they can only replicate within a living host cell, they cannot be grown on artificial, cell-free agar. Instead, they require **cell culture systems** (such as McCoy, HeLa, or Vero cells), yolk sacs of embryonated eggs, or animal inoculation for growth. **Incorrect Options:** * **B. Ureaplasma urealyticum:** While it lacks a cell wall (like Mycoplasma), it is not an obligate intracellular pathogen. It can be grown on specialized **cell-free artificial media** (e.g., PPLO agar/Shepard’s 10B broth) supplemented with urea. * **C. Pseudomonas aeruginosa:** This is a classic extracellular, non-fastidious bacterium that grows easily on routine laboratory media like **Nutrient agar** and **MacConkey agar** (producing characteristic bluish-green pigment). * **D. Tropheryma whipplei:** Although historically difficult to culture, it is now known to be a fastidious bacterium that can be grown in specialized cell-free media, though it is primarily diagnosed via PAS stain or PCR. **High-Yield NEET-PG Pearls:** * **Obligate Intracellular Pathogens:** Remember the mnemonic **"Stay inside (cells) when it's Really Cold"** — **R**ickettsia and **C**hlamydia. These *always* require cell cultures. * **Cell Culture for Chlamydia:** **McCoy cells** treated with cycloheximide are the gold standard for isolation. * **Diagnosis:** In modern clinical practice, **NAAT (Nucleic Acid Amplification Test)** has replaced culture as the investigation of choice for *Chlamydia* due to higher sensitivity.
Question 92: What is the commonest method for detecting diphtheria carriers?
- A. Schick test (Correct Answer)
- B. Dick test
- C. Casoni's test
- D. Charles' test
Explanation: **Explanation:** The **Schick test** is the correct answer as it is the classic method used to determine the immune status of an individual against *Corynebacterium diphtheriae* and to identify susceptible individuals or **carriers** in a population. * **Mechanism:** It involves the intradermal injection of a minute amount of diphtheria toxin. If the person lacks antibodies (susceptible), a localized inflammatory reaction occurs. If the person has circulating antitoxin (immune), the toxin is neutralized, and no reaction occurs. In epidemiological surveys, those who are immune but harbor the bacilli in their throat/nasopharynx are identified as **carriers**. **Analysis of Incorrect Options:** * **Dick Test:** Used to determine susceptibility to **Scarlet Fever** (caused by *Streptococcus pyogenes*). It involves the intradermal injection of erythrogenic toxin. * **Casoni’s Test:** An immediate hypersensitivity skin test used for the diagnosis of **Hydatid disease** (*Echinococcus granulosus*). * **Charles’ Test:** This is a distractor and is not a recognized diagnostic test in medical microbiology. **High-Yield Clinical Pearls for NEET-PG:** * **Culture Gold Standard:** While the Schick test identifies carriers, the definitive diagnosis of diphtheria is made via culture on **Löffler's serum slope** or **Potassium Tellurite agar** (black colonies). * **Toxin Detection:** The **Elek’s gel precipitation test** is the gold standard for detecting the toxigenicity of a *C. diphtheriae* strain. * **Morphology:** Look for "Chinese letter" patterns or cuneiform arrangements on Gram stain and **Albert’s stain** for metachromatic (volutin) granules.
Question 93: A man presented with diarrhea three days after consuming uncooked meat. Stool examination revealed comma-shaped organisms with red blood cells and white blood cells. What is the causative organism?
- A. Vibrio cholerae
- B. Shigella
- C. Campylobacter jejuni (Correct Answer)
- D. Yersinia enterocolitica
Explanation: ### Explanation The correct answer is **Campylobacter jejuni**. This case highlights the classic presentation of inflammatory diarrhea caused by this organism. **1. Why Campylobacter jejuni is correct:** * **Morphology:** *C. jejuni* is characteristically described as a **comma-shaped** (or S-shaped/seagull-wing) Gram-negative rod. * **Clinical Presentation:** It is a leading cause of bacterial gastroenteritis worldwide, often transmitted via **undercooked poultry/meat** or unpasteurized milk. * **Stool Findings:** Unlike *Vibrio*, *Campylobacter* causes **inflammatory diarrhea**. The presence of **RBCs and WBCs** (fecal leukocytes) in the stool indicates mucosal invasion and inflammation, which is a hallmark of this pathogen. **2. Why other options are incorrect:** * **Vibrio cholerae:** While also comma-shaped, it causes **non-inflammatory, watery "rice-water" stools**. It does not invade the mucosa, so RBCs and WBCs are typically absent. * **Shigella:** Causes bacillary dysentery (RBCs/WBCs present), but the organism is a **straight rod**, not comma-shaped. * **Yersinia enterocolitica:** Can cause inflammatory diarrhea and pseudoappendicitis, but it is a **coccobacillus** and is more commonly associated with contaminated pork or cold storage (psychrophilic). **3. High-Yield Clinical Pearls for NEET-PG:** * **Culture:** Requires special media (e.g., **Skirrow’s medium**) and is **microaerophilic**, growing best at **42°C** (heat-tolerant). * **Motility:** Exhibits characteristic **"darting motility."** * **Complications:** Strongly associated with **Guillain-Barré Syndrome** (due to molecular mimicry with gangliosides) and Reactive Arthritis. * **Treatment:** Erythromycin or Azithromycin (though most cases are self-limiting).
Question 94: Swimming pool conjunctivitis is caused by which of the following microorganisms?
- A. Pseudomonas
- B. Coxsackievirus
- C. Chlamydia (Correct Answer)
- D. Mycoplasma
Explanation: **Explanation:** The correct answer is **Chlamydia**. Specifically, "Swimming pool conjunctivitis" refers to **Inclusion Conjunctivitis** caused by **Chlamydia trachomatis (Serotypes D-K)**. **Why Chlamydia is correct:** This condition occurs when the eyes are exposed to infected genital secretions, often in inadequately chlorinated swimming pools. It presents as an acute follicular conjunctivitis with mucopurulent discharge and preauricular lymphadenopathy. On cytology, it is characterized by the presence of **Halberstaedter-Prowazek (HP) inclusion bodies** (intracytoplasmic) near the nucleus of conjunctival epithelial cells. **Why the other options are incorrect:** * **Pseudomonas:** While *Pseudomonas aeruginosa* is a common cause of contact lens-associated keratitis and "swimmer’s ear" (otitis externa), it is not the classic cause of swimming pool conjunctivitis. * **Coxsackievirus:** This is a common cause of **Acute Hemorrhagic Conjunctivitis (AHC)**, often associated with Enterovirus 70. It presents with subconjunctival hemorrhages rather than the follicular pattern seen in Chlamydia. * **Mycoplasma:** These organisms are primarily associated with respiratory and urogenital infections (like atypical pneumonia or urethritis) but are not standard causes of infectious conjunctivitis. **High-Yield Clinical Pearls for NEET-PG:** * **Trachoma vs. Inclusion Conjunctivitis:** Trachoma is caused by Serotypes **A, B, Ba, and C** (leading cause of preventable blindness), while Inclusion Conjunctivitis is caused by **D-K**. * **Drug of Choice:** Oral **Azithromycin** (single dose) or Doxycycline. * **Adenovirus:** Note that Adenovirus (Types 3, 7) causes **Pharyngoconjunctival Fever**, which is also associated with swimming pools but typically presents with a triad of fever, pharyngitis, and conjunctivitis.
Question 95: What is the approximate number of genes in the complete genome sequence of M. tuberculosis?
- A. 4100 genes (Correct Answer)
- B. 3100 genes
- C. 2100 genes
- D. 1100 genes
Explanation: **Explanation:** The genome of *Mycobacterium tuberculosis* (H37Rv strain) was first fully sequenced in 1998. It consists of a single circular chromosome of approximately **4.4 million base pairs (Mbp)**. This large genome size reflects the metabolic complexity of the organism, which must survive within the hostile environment of host macrophages. 1. **Why A is correct:** The *M. tuberculosis* genome contains approximately **4,000 to 4,100 protein-coding genes**. A significant portion of these genes (about 5%) is dedicated to encoding enzymes for lipid metabolism, which is essential for synthesizing its complex, waxy cell wall (containing mycolic acids). 2. **Why B, C, and D are incorrect:** These options underestimate the genetic complexity of the tubercle bacillus. For comparison, *Escherichia coli* has about 4,300 genes, while smaller intracellular pathogens like *Mycoplasma pneumoniae* have only about 700 genes. Options B, C, and D do not account for the extensive regulatory and metabolic machinery *M. tuberculosis* requires for latency and persistence. **High-Yield Clinical Pearls for NEET-PG:** * **GC Content:** The genome is notably **GC-rich (65.6%)**, which contributes to its stability. * **PE/PPE Families:** About 10% of the genome encodes two large families of glycine-rich proteins (PE and PPE), which are thought to be involved in antigenic variation and immune evasion. * **Drug Resistance:** Unlike many other bacteria, *M. tuberculosis* lacks plasmids; therefore, antibiotic resistance is primarily due to **chromosomal mutations** rather than horizontal gene transfer. * **Slow Growth:** The large genome and complex cell wall synthesis contribute to its slow generation time (15–20 hours).
Question 96: "Genital elephantiasis" is caused by which of the following?
- A. Lymphogranuloma venereum (Correct Answer)
- B. Chancroid
- C. Syphilis
- D. Rickettsia
Explanation: **Explanation:** **Lymphogranuloma venereum (LGV)** is a sexually transmitted infection caused by **Chlamydia trachomatis serovars L1, L2, and L3**. The pathogenesis involves the spread of the organism from the primary site of infection to the regional lymph nodes, leading to chronic inflammation and lymphatic obstruction. This obstruction results in chronic lymphedema of the external genitalia, a condition clinically known as **"Esthiomene"** or **Genital Elephantiasis**. **Analysis of Options:** * **A. Lymphogranuloma venereum (Correct):** The hallmark of LGV is the "Bubonic stage," characterized by painful inguinal lymphadenopathy (Groove sign). If untreated, the resulting lymphatic scarring leads to genital elephantiasis and rectal strictures. * **B. Chancroid:** Caused by *Haemophilus ducreyi*, it presents with painful, soft ulcers and painful inflammatory buboes. While it causes tissue destruction, it does not typically lead to chronic lymphatic obstruction or elephantiasis. * **C. Syphilis:** Caused by *Treponema pallidum*. Primary syphilis presents with a painless chancre, and tertiary syphilis involves gummas and cardiovascular/neuro-complications, but not genital elephantiasis. * **D. Rickettsia:** These are obligate intracellular bacteria causing systemic vasculitis (e.g., Typhus, Rocky Mountain Spotted Fever). They are not associated with sexually transmitted genital lesions or lymphatic obstruction. **High-Yield Clinical Pearls for NEET-PG:** * **Groove Sign:** Pathognomonic for LGV; it is the indentation produced by the inguinal ligament between the inflamed superficial and deep inguinal lymph nodes. * **Frei’s Test:** A delayed hypersensitivity skin test previously used for LGV diagnosis (now largely replaced by NAAT). * **Drug of Choice:** Doxycycline (100 mg twice daily for 21 days) is the preferred treatment for LGV.
Question 97: Chlamydia are isolated by which of the following methods?
- A. Enzyme immune assay
- B. Yolk sac inoculation (Correct Answer)
- C. Direct immunofluorescence
- D. Microimmunofluorescence
Explanation: **Explanation:** **Why Yolk Sac Inoculation is Correct:** *Chlamydia* are **obligate intracellular bacteria**; they lack the metabolic machinery to produce their own ATP and therefore cannot be grown on artificial, cell-free agar (like Blood Agar or MacConkey). To isolate them, they must be cultured in living systems. Historically, **yolk sac inoculation** of 6–8 day old embryonated hen’s eggs was the gold standard for isolation. While modern laboratories now prefer **cell cultures** (e.g., McCoy, HeLa-229, or BHK-21 cells treated with cycloheximide), yolk sac inoculation remains a classic, definitive method for isolation mentioned in standard textbooks and frequently tested in exams. **Analysis of Incorrect Options:** * **A. Enzyme Immuno Assay (EIA):** This is a **serological/immunological method** used to detect chlamydial antigens or antibodies. It is a diagnostic test, but it does not "isolate" (grow) the live organism. * **C. Direct Immunofluorescence (DFA):** This uses fluorescent-labeled monoclonal antibodies to detect **Elementary Bodies (EBs)** directly in clinical smears. It is a rapid diagnostic tool, not a method of isolation. * **D. Microimmunofluorescence (MIF):** This is the "gold standard" **serological test** for typing chlamydial antibodies (IgM/IgG) and differentiating species (*C. trachomatis, C. pneumoniae, C. psittaci*). It does not involve isolation of the pathogen. **High-Yield Clinical Pearls for NEET-PG:** * **Staining:** *Chlamydia* are Gram-negative but are better visualized using **Giemsa, Castaneda, or Gimenez stains**. * **Inclusion Bodies:** *C. trachomatis* forms **Halberstaedter-Prowazek (HP) inclusions** which contain glycogen and stain with **Iodine** (unlike *C. psittaci*). * **Drug of Choice:** Azithromycin (single dose) or Doxycycline (7 days) are preferred for uncomplicated genital infections. * **Lycogen Content:** Only *C. trachomatis* inclusions stain with iodine because they contain glycogen.
Question 98: Which is the most common complication of this rash?
- A. Otitis media
- B. Bronchopneumonia
- C. Secondary skin infections (Correct Answer)
- D. Reye syndrome
Explanation: ***Secondary skin infections*** - **Chickenpox vesicles** are intensely pruritic, leading to **scratching** and subsequent **bacterial superinfection** with organisms like *Staphylococcus aureus* or *Streptococcus pyogenes*. - This is the **most frequent complication** of varicella, occurring in up to 5-10% of cases, especially in children. *Otitis media* - While **otitis media** can occur as a complication of varicella, it is **less common** than secondary bacterial skin infections. - More frequently associated with **respiratory viral infections** rather than the characteristic vesicular rash of chickenpox. *Bronchopneumonia* - **Varicella pneumonia** is a serious but **rare complication**, occurring in less than 1% of cases, primarily in adults and immunocompromised patients. - **Secondary skin infections** remain far more common than respiratory complications in typical pediatric varicella cases. *Reye syndrome* - **Reye syndrome** is a rare but serious complication associated with **aspirin use** during varicella or influenza infections. - This complication has become **extremely uncommon** due to widespread avoidance of aspirin in children with viral illnesses.
Question 99: Which of the following is coagulase-negative Staphylococcus?
- A. Staphylococcus aureus
- B. Staphylococcus epidermidis (Correct Answer)
- C. Staphylococcus intermedius
- D. Staphylococcus hyicus
Explanation: **Explanation:** The genus *Staphylococcus* is primarily classified into two groups based on the production of the enzyme **coagulase**, which converts fibrinogen to fibrin. This enzyme serves as a major virulence factor by protecting the bacteria from phagocytosis. **1. Why the Correct Answer is Right:** * **Staphylococcus epidermidis (Option B):** It is the most clinically significant member of the **Coagulase-Negative Staphylococci (CoNS)**. It is a normal commensal of the skin but acts as an opportunistic pathogen, particularly associated with infections of prosthetic devices (heart valves, joints) and intravenous catheters due to its ability to produce **biofilms (slime layer)**. **2. Why the Other Options are Wrong:** * **Staphylococcus aureus (Option A):** This is the prototypical **coagulase-positive** staphylococcus and the most pathogenic species in humans. * **Staphylococcus intermedius (Option C) & Staphylococcus hyicus (Option D):** While less common in human medicine, these are important veterinary pathogens that are also **coagulase-positive**. *S. intermedius* is often associated with dog bites, and *S. hyicus* causes exudative epidermitis in pigs. **High-Yield Clinical Pearls for NEET-PG:** * **Novobiocin Sensitivity:** Among CoNS, *S. epidermidis* is **Novobiocin sensitive**, whereas *S. saprophyticus* (a common cause of UTI in young women) is **Novobiocin resistant**. * **Culture Characteristic:** *S. aureus* typically produces golden-yellow colonies (due to staphyloxanthin), while *S. epidermidis* produces white, non-hemolytic colonies. * **Screening:** The **Tube Coagulase Test** detects free coagulase, while the **Slide Coagulase Test** (Clumping factor) detects bound coagulase.
Question 100: Which of the following media does not support the growth of H. influenzae?
- A. Chocolate agar
- B. Fields agar
- C. Nutrient agar (Correct Answer)
- D. Blood agar with S. aureus streak line
Explanation: **Explanation:** *Haemophilus influenzae* is a fastidious organism that requires two specific growth factors found in blood: **Factor X (Hemin)** and **Factor V (NAD)**. 1. **Why Nutrient Agar is the correct answer:** Nutrient agar is a basic medium that lacks both Factor X and Factor V. Since *H. influenzae* cannot synthesize these essential nutrients, it fails to grow on this medium. 2. **Analysis of Incorrect Options:** * **Chocolate Agar:** This is the gold standard medium for *H. influenzae*. Heating blood to 80°C (to make chocolate agar) lyses the RBCs, releasing Factor X and Factor V into the medium while simultaneously inactivating NADases that would otherwise destroy Factor V. * **Fildes Agar:** This is a peptic digest of blood. The digestion process releases both X and V factors from the red cells, making it an enriched medium that supports excellent growth of *H. influenzae*. * **Blood Agar with *S. aureus* streak:** While *H. influenzae* cannot grow on sheep blood agar alone (as Factor V is hidden inside RBCs), *Staphylococcus aureus* synthesizes excess Factor V. When streaked together, *H. influenzae* grows as tiny colonies near the *S. aureus* streak—a phenomenon known as **Satellitism**. **High-Yield Clinical Pearls for NEET-PG:** * **Factor Requirements:** *H. influenzae* (X+V), *H. ducreyi* (X only), *H. parainfluenzae* (V only). * **Satellitism:** Used as a presumptive diagnostic test for *H. influenzae*. * **Culture Media:** Levinthal’s medium is another enriched medium used for its growth. * **Capsule:** The Type b (Hib) capsule consists of **Polyribosyl Ribitol Phosphate (PRP)**, which is the target for the Hib vaccine.
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Staphylococci
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