Bacteriology — MCQs

A 50-year-old man presented with flu-like symptoms. He was treated with penicillin and sent home. A week later, he returned feeling worse, complaining of fever, night sweats, persistent cough, weight loss, and lethargy. He is a rancher with a history of congenital heart disease and rheumatic fever. The physician suspected subacute bacterial endocarditis. Blood cultures were negative, but serum analysis showed positive antibodies against a specific bacterium. Treatment with doxycycline was initiated, leading to improvement. What is the most likely causative organism?

Q1273Easy

Who discovered Vibrio cholerae?

Q1274Medium

All of the following are true regarding Streptococcus pneumoniae except:

Q1275Easy

What is the commonest cause of non-gonococcal urethritis?

Q1276Easy

Which of the following is NOT a method of transmission of Legionella?

Q1277Easy

Which of the following organisms is a Gram-negative diplococcus?

Q1278Easy

In Gram-stained smears, gonococci are typically observed within which type of cells?

Q1279Easy

Which of the following structures is found only in Gram-negative microorganisms?

Q1280Easy

Which of the following diseases does not have a carrier stage?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1271: Which of the following statements about Rickettsiae are true?

A. Multiply only within living cells
B. Produce typhus fever of epidemic type only
C. Transmitted by arthropod vectors
D. Respond to tetracycline therapy (Correct Answer)

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** Rickettsiae are obligate intracellular bacteria that possess a cell wall similar to Gram-negative bacteria. They are highly sensitive to antibiotics that inhibit protein synthesis. **Tetracyclines (specifically Doxycycline)** are the drug of choice for all Rickettsial infections, regardless of the patient's age. Chloramphenicol is considered a second-line alternative. **2. Analysis of Incorrect Options:** * **Option A (Multiply only within living cells):** While Rickettsiae are obligate intracellular parasites, there is a notable exception: ***Coxiella burnetii*** (formerly classified with Rickettsiae) and ***Rickettsia quintana*** (now *Bartonella quintana*) can survive outside cells or grow on cell-free media. Therefore, the statement "only" makes it technically incorrect in a strict microbiological context. * **Option B (Produce typhus fever of epidemic type only):** Rickettsiae cause a wide spectrum of diseases, including **Endemic typhus** (*R. typhi*), **Spotted fevers** (e.g., Rocky Mountain Spotted Fever by *R. rickettsii*), and **Scrub typhus** (*Orientia tsutsugamushi*). * **Option C (Transmitted by arthropod vectors):** Most Rickettsiae are transmitted by vectors (lice, ticks, mites, fleas). However, **Q fever (*Coxiella burnetii*)** is a significant exception as it is primarily transmitted via inhalation of infected aerosols, not by an arthropod vector. **3. High-Yield Clinical Pearls for NEET-PG:** * **Weil-Felix Reaction:** A heterophile agglutination test using *Proteus* antigens (OX-19, OX-2, OX-K) used for presumptive diagnosis. Note: It is **negative** in Q fever. * **Drug of Choice:** Always **Doxycycline**, even in children (the benefit outweighs the risk of tooth staining in life-threatening Rickettsial diseases). * **Triad of symptoms:** Fever, headache, and rash (except in Q fever, where rash is absent). * **Gold Standard Diagnosis:** Indirect Immunofluorescence Assay (IFA).

Question 1272: A 50-year-old man presented with flu-like symptoms. He was treated with penicillin and sent home. A week later, he returned feeling worse, complaining of fever, night sweats, persistent cough, weight loss, and lethargy. He is a rancher with a history of congenital heart disease and rheumatic fever. The physician suspected subacute bacterial endocarditis. Blood cultures were negative, but serum analysis showed positive antibodies against a specific bacterium. Treatment with doxycycline was initiated, leading to improvement. What is the most likely causative organism?

A. Chlamydophila psittaci
B. Chlamydophila pneumoniae
C. Chlamydia trachomatis
D. Coxiella burnetii (Correct Answer)

Explanation: **Explanation:** The clinical presentation points toward **Q fever**, caused by **_Coxiella burnetii_**. The patient’s occupation as a **rancher** is a classic high-yield clue, as _C. burnetii_ is a zoonotic pathogen found in cattle, sheep, and goats. While it typically causes a self-limiting flu-like illness (Acute Q fever), it can progress to **Chronic Q fever**, which most commonly manifests as **Culture-Negative Endocarditis**, especially in patients with pre-existing valvular heart disease or congenital heart defects. **Why the other options are incorrect:** * **_Chlamydophila psittaci_:** Causes Psittacosis (ornithosis). While it presents with fever and cough, the history would typically involve exposure to **birds/parrots**, not cattle or ranching. * **_Chlamydophila pneumoniae_:** A common cause of atypical pneumonia and bronchitis. It does not typically cause endocarditis or correlate with ranching exposure. * **_Chlamydia trachomatis_:** Primarily causes urogenital infections, lymphogranuloma venereum (LGV), or trachoma; it is not a cause of endocarditis. **NEET-PG High-Yield Pearls:** * **Culture-Negative Endocarditis:** _Coxiella burnetii_ is the most common cause worldwide. Other causes include _HACEK_ organisms, _Bartonella_, and _Brucella_. * **Diagnosis:** Since it is an obligate intracellular bacterium, it does not grow on routine blood cultures. Diagnosis relies on **Serology (IFA)**. * **Phase Variation:** In acute infection, antibodies to **Phase II antigen** are higher. In chronic infection (endocarditis), antibodies to **Phase I antigen** are significantly elevated (IgG > 1:800). * **Treatment:** Doxycycline is the drug of choice. For chronic endocarditis, a combination of Doxycycline and Hydroxychloroquine is used for 18+ months.

Question 1273: Who discovered Vibrio cholerae?

A. Koch (Correct Answer)
B. Mekintoff
C. John Snow
D. Virchow

Explanation: **Explanation:** **Correct Answer: A. Robert Koch** Robert Koch, the father of modern bacteriology, is credited with the discovery of *Vibrio cholerae*. In **1883**, while leading a German Cholera Commission in Egypt and later India, he successfully isolated the bacterium in pure culture from the intestinal mucosal linings of cholera victims. He famously described its morphology as "comma-shaped." **Analysis of Incorrect Options:** * **B. Elie Metchnikoff:** He is best known for the discovery of **phagocytosis** and his pioneering work in immunology (cellular immunity), not for the discovery of the cholera pathogen. * **C. John Snow:** Known as the "Father of Modern Epidemiology," Snow famously traced the 1854 London cholera outbreak to the **Broad Street pump**. While he correctly deduced that cholera was waterborne, he did not isolate or identify the specific bacterium. * **D. Rudolf Virchow:** Known as the "Father of Modern Pathology," he is famous for the cell theory (*Omnis cellula e cellula*) and his work on leukemia and thrombosis, but he was not involved in the discovery of *V. cholerae*. **High-Yield Clinical Pearls for NEET-PG:** * **Filippo Pacini:** Although Koch is the standard answer for "discovery," Pacini actually first observed the organism in 1854; however, his work was ignored until Koch's definitive isolation. * **Morphology:** *Vibrio cholerae* are Gram-negative, curved (comma-shaped) rods that show characteristic **"darting motility"** on hanging drop preparation due to a single polar flagellum. * **Culture Media:** **TCBS (Thiosulfate Citrate Bile Salts Sucrose)** agar is the selective medium of choice, where it produces large yellow colonies due to sucrose fermentation. * **Enrichment Media:** Venkatraman-Ramakrishnan (VR) medium and Alkaline Peptone Water (APW).

Question 1274: All of the following are true regarding Streptococcus pneumoniae except:

A. Bile soluble
B. Produces a draughtsman-like appearance on blood agar
C. Ferments inulin
D. Resistant to 2-ethyl hydrocupreine (Correct Answer)

Explanation: **Explanation:** The correct answer is **D (Resistant to 2-ethyl hydrocupreine)** because *Streptococcus pneumoniae* is characteristically **sensitive** to Optochin (the chemical name for which is 2-ethyl hydrocupreine hydrochloride). This sensitivity is a primary laboratory test used to differentiate *S. pneumoniae* from other alpha-hemolytic streptococci (Viridans group), which are typically resistant. **Analysis of Options:** * **A. Bile soluble:** *S. pneumoniae* possesses autolytic enzymes (amidases) that are activated by bile salts (e.g., sodium deoxycholate), leading to the lysis of the colony. This is a gold-standard biochemical test for identification. * **B. Draughtsman-like appearance:** On blood agar, older colonies undergo autolysis in the center, resulting in a flattened surface with raised edges, resembling a "draughtsman" or "checkerboard" piece. * **C. Ferments inulin:** Unlike most other streptococci, *S. pneumoniae* has the ability to ferment inulin, producing acid (detected by a color change in the medium). **NEET-PG High-Yield Pearls:** * **Morphology:** Gram-positive, lancet-shaped diplococci. * **Capsule:** The most important virulence factor; identified by the **Quellung Reaction** (capsular swelling). * **Hemolysis:** Shows **Alpha-hemolysis** (partial/green) under aerobic conditions. * **Vaccines:** Two types are available—the 23-valent polysaccharide vaccine (T-cell independent) and the 13-valent conjugate vaccine (T-cell dependent, better for children <2 years). * **Clinical:** Most common cause of Community-Acquired Pneumonia (CAP) and bacterial meningitis in adults.

Question 1275: What is the commonest cause of non-gonococcal urethritis?

A. Chlamydia (Correct Answer)
B. Ureaplasma urealyticum
C. E. coli
D. Proteus

Explanation: **Explanation:** **Non-gonococcal urethritis (NGU)** refers to an inflammation of the urethra not caused by *Neisseria gonorrhoeae*. It is the most common sexually transmitted syndrome in men. **Why Chlamydia is Correct:** *Chlamydia trachomatis* (Serotypes D-K) is the **single most common cause** of NGU worldwide, accounting for approximately 30–50% of cases. It is an obligate intracellular bacterium. Clinically, it presents with a longer incubation period (7–21 days) and a more mucoid, less purulent discharge compared to the "creamy" discharge of gonorrhea. **Why the Other Options are Incorrect:** * **B. Ureaplasma urealyticum:** This is the second most common cause of NGU (approx. 10–20%). While significant, it is statistically less frequent than Chlamydia. * **C. E. coli & D. Proteus:** These are common causes of **Urinary Tract Infections (UTIs)** and cystitis, typically via ascending infection. They are rarely implicated in sexually transmitted urethritis unless there is associated instrumentation or anatomical abnormalities. **High-Yield Clinical Pearls for NEET-PG:** * **Co-infection:** Up to 30% of patients with Gonorrhea have a co-infection with *Chlamydia*. This is why the standard treatment (Ceftriaxone + Azithromycin/Doxycycline) covers both. * **Diagnosis:** The gold standard for *Chlamydia* is **NAAT (Nucleic Acid Amplification Test)** using a first-void urine sample or urethral swab. * **Complications:** If untreated, NGU can lead to epididymitis in men and Pelvic Inflammatory Disease (PID) or **Reiter’s Syndrome** (Reactive Arthritis: "Can't see, can't pee, can't climb a tree"). * **Other causes of NGU:** *Mycoplasma genitalium* (emerging cause), *Trichomonas vaginalis*, and HSV.

Question 1276: Which of the following is NOT a method of transmission of Legionella?

A. Direct contact with infected patients (Correct Answer)
B. Aerosolized droplets from air conditioning systems
C. Consumption of contaminated food
D. Contaminated respiratory support equipment

Explanation: **Explanation:** The correct answer is **A. Direct contact with infected patients**. *Legionella pneumophila* is unique because, despite causing severe pneumonia (Legionnaires' disease), it is **not transmitted from person to person**. There is no documented evidence of horizontal transmission between humans; instead, humans are "accidental hosts" who acquire the infection directly from environmental sources. **Why the other options are incorrect:** * **B. Aerosolized droplets from air conditioning systems:** This is the most common mode of transmission. *Legionella* thrives in man-made water systems (cooling towers, showers, mist machines). When these systems aerosolize contaminated water, the bacteria are inhaled deep into the alveoli. * **C. Consumption of contaminated food/water:** While inhalation is the primary route, **aspiration** of contaminated water or ice (often during swallowing) is a recognized transmission route, particularly in hospital settings or among patients with dysphagia. * **D. Contaminated respiratory support equipment:** Nosocomial outbreaks are frequently linked to the use of tap water in nebulizers, humidifiers, or the improper disinfection of respiratory devices. **High-Yield Clinical Pearls for NEET-PG:** * **Microbiology:** Gram-negative bacilli that require **BCYE (Buffered Charcoal Yeast Extract) agar** for growth (requires L-cysteine and Iron). * **Diagnosis:** The **Urinary Antigen Test** is the rapid test of choice (detects Serogroup 1). * **Clinical Presentation:** Characterized by "atypical pneumonia" associated with **hyponatremia**, high fever, and gastrointestinal symptoms (diarrhea). * **Drug of Choice:** Macrolides (Azithromycin) or Fluoroquinolones (Levofloxacin).

Question 1277: Which of the following organisms is a Gram-negative diplococcus?

A. Corynebacterium Diphtheriae
B. Neisseria Meningitidis (Correct Answer)
C. Streptococcus pneumoniae
D. Staphylococcus

Explanation: **Explanation:** The identification of bacteria based on Gram stain morphology is a fundamental concept in microbiology and a frequent high-yield topic for NEET-PG. **1. Why Neisseria meningitidis is correct:** *Neisseria meningitidis* (Meningococcus) is a classic **Gram-negative diplococcus**. Under the microscope, they appear as pairs of kidney-bean or coffee-bean-shaped cells with adjacent sides flattened. They are typically found intracellularly within polymorphonuclear leukocytes (PMNs) during acute infections. **2. Analysis of Incorrect Options:** * **Corynebacterium diphtheriae:** These are **Gram-positive bacilli** (rods). They exhibit a characteristic "Chinese letter" or cuneiform arrangement due to incomplete separation during cell division. * **Streptococcus pneumoniae:** While these are also **diplococci**, they are **Gram-positive**. They are described as "lanceolate" or flame-shaped and are usually surrounded by a prominent capsule. * **Staphylococcus:** These are **Gram-positive cocci** that typically arrange themselves in **clusters** (grape-like), not pairs. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Rule of Two":** There are two primary Gram-negative diplococci of medical importance: *Neisseria meningitidis* and *Neisseria gonorrhoeae*. (Note: *Moraxella catarrhalis* is the third). * **Biochemical Key:** All *Neisseria* species are **Oxidase positive** and **Catalase positive**. * **Sugar Fermentation:** *N. meningitidis* ferments both **M**altose and **G**lucose (**M**eningitidis = **M**altose + **G**lucose), whereas *N. gonorrhoeae* ferments only **G**lucose. * **Culture:** They are fastidious and grow best on **Thayer-Martin Medium** (a selective Chocolate agar).

Question 1278: In Gram-stained smears, gonococci are typically observed within which type of cells?

A. Lymphocytes
B. Neutrophils (Correct Answer)
C. Mast cells
D. Macrophages

Explanation: **Explanation:** **Neisseria gonorrhoeae** (gonococcus) is a Gram-negative diplococcus that causes the sexually transmitted infection, gonorrhea. The hallmark of a gonorrheal infection is the presence of a purulent discharge, which is primarily composed of an inflammatory exudate. **Why Neutrophils are correct:** Gonococci are **facultative intracellular pathogens**. In a Gram stain of urethral or cervical discharge, the classic finding is the presence of Gram-negative "kidney-bean" shaped diplococci located **within polymorphonuclear leukocytes (Neutrophils)**. Neutrophils are the first responders to acute bacterial infections; they phagocytose the bacteria, but gonococci have evolved mechanisms to survive and remain visible within the phagosome, leading to the characteristic "intracellular diplococci" appearance. **Why other options are incorrect:** * **Lymphocytes:** These are mononuclear cells involved in chronic infections and viral immunity (adaptive immunity), not the acute pyogenic response seen in gonorrhea. * **Mast cells:** These are involved in Type I hypersensitivity (allergic) reactions and the release of histamine; they do not typically phagocytose bacteria. * **Macrophages:** While macrophages can phagocytose *Neisseria*, they are more characteristic of chronic granulomatous inflammation. In the acute, diagnostic stage of gonorrhea, neutrophils are the predominant cell type seen on a smear. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Gold Standard:** While Gram stain is highly sensitive in symptomatic males (95%), **Culture on Thayer-Martin medium** (or Chocolate agar) remains the gold standard, especially for females. * **Oxidase & Catalase:** All *Neisseria* species are positive for both. * **Sugar Fermentation:** Gonococci ferment **only Glucose** (G for Gonococci), whereas Meningococci ferment both Glucose and Maltose (M for Meningococci). * **Pili:** The most important virulence factor for attachment to mucosal surfaces.

Question 1279: Which of the following structures is found only in Gram-negative microorganisms?

A. Cell envelope
B. Exotoxin
C. Peptidoglycan
D. Periplasmic space (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Periplasmic space** The **periplasmic space** is a distinct anatomical compartment found **only in Gram-negative bacteria**. It is the region located between the inner cytoplasmic membrane and the outer membrane. This space contains the thin peptidoglycan layer and a gel-like matrix (periplasm) rich in transport proteins, hydrolytic enzymes (like alkaline phosphatase), and important drug-resistance enzymes such as **beta-lactamases**. Gram-positive bacteria lack an outer membrane; therefore, they do not possess a true periplasmic space. **Analysis of Incorrect Options:** * **A. Cell envelope:** This is a general term referring to the multilayered structure enclosing the cytoplasm. Both Gram-positive (cell membrane + thick peptidoglycan) and Gram-negative (cell membrane + thin peptidoglycan + outer membrane) bacteria possess a cell envelope. * **B. Exotoxin:** Exotoxins are proteins secreted by **both** Gram-positive (e.g., *Clostridium tetani*, *Staphylococcus aureus*) and Gram-negative bacteria (e.g., *Vibrio cholerae*, *E. coli*). Note: Endotoxins (LPS) are exclusive to Gram-negatives. * **C. Peptidoglycan:** This is a fundamental component of the cell wall in **almost all bacteria** (except *Mycoplasma*). While Gram-positive bacteria have a much thicker layer, it is present in both types. **High-Yield NEET-PG Pearls:** * **Lipopolysaccharide (LPS):** Found only in the outer membrane of Gram-negative bacteria; the **Lipid A** component is responsible for endotoxic activity (septic shock). * **Teichoic Acids:** Found **only** in Gram-positive cell walls; they act as surface antigens. * **Porins:** Transmembrane proteins found in the outer membrane of Gram-negative bacteria that facilitate the diffusion of hydrophilic molecules. * **Periplasmic Enzymes:** The presence of beta-lactamases in the periplasm is a key mechanism of resistance against penicillins in Gram-negative organisms.

Question 1280: Which of the following diseases does not have a carrier stage?

A. Diphtheria
B. Typhoid
C. Hepatitis B
D. Whooping cough (Correct Answer)

Explanation: **Explanation:** In epidemiology, a **carrier** is an individual who harbors a specific infectious agent without having clinical disease and serves as a potential source of infection. The absence of a carrier state is a hallmark of certain respiratory infections, most notably **Whooping Cough (Pertussis)**. **1. Why Whooping Cough (Option D) is correct:** *Bordetella pertussis* does not have a chronic carrier state. The bacteria are highly fastidious and survive only for short periods outside the human host or in the absence of active infection. Transmission occurs primarily through respiratory droplets from symptomatic cases (especially during the catarrhal stage). Once a patient recovers, the organism is cleared from the respiratory tract. **2. Why the other options are incorrect:** * **Diphtheria (Option A):** *Corynebacterium diphtheriae* frequently establishes a carrier state in the nasopharynx or skin. Both temporary and chronic carriers exist, and they are the primary reservoir for maintaining the disease in the community. * **Typhoid (Option B):** *Salmonella Typhi* is famous for its chronic carrier state (e.g., "Typhoid Mary"). The bacteria persist in the **gallbladder** (associated with gallstones) or urinary tract for more than a year in about 2-5% of cases. * **Hepatitis B (Option C):** HBV has a well-defined chronic carrier state (defined by HBsAg persistence for >6 months). These carriers are at high risk for cirrhosis and hepatocellular carcinoma. **NEET-PG High-Yield Pearls:** * **Diseases with NO carrier state:** Whooping cough, Measles, and Smallpox. This characteristic makes a disease a potential candidate for eradication. * **Typhoid Carrier Site:** The gallbladder is the most common site for chronic carriage. * **Diphtheria Carrier Treatment:** Erythromycin is used to eradicate the carrier state.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

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Neisseria and Moraxella

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Corynebacterium and Listeria

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Bacillus and Clostridium

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Enterobacteriaceae

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Vibrio, Aeromonas, and Plesiomonas

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Pseudomonas and Related Bacteria

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Haemophilus and HACEK Group

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Bordetella and Brucella

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Mycobacteria

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Spirochetes

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