Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 661: Mycobacterium leprae can be grown on:

A. Culture media
B. Foot pad of mouse or Armadillo (Correct Answer)
C. Liver of guinea pig
D. Kidney of rabbit

Explanation: ### Explanation **1. Why the Correct Answer is Right:** *Mycobacterium leprae* is an **obligate intracellular bacterium** that has never been grown on artificial (in vitro) culture media. This is because it lacks several genes required for independent metabolism. It has a unique preference for lower body temperatures (around 30–33°C). * **Foot pad of mouse:** Introduced by Shepard (1960), this provides the necessary cool temperature for the bacilli to multiply, though the infection remains localized. * **Nine-banded Armadillo:** Introduced by Kirchheimer and Storrs (1971), armadillos have a low core body temperature and are highly susceptible, developing **generalized lepromatous leprosy**. They serve as a major source for harvesting large quantities of *M. leprae* for research and lepromin antigen production. **2. Why Incorrect Options are Wrong:** * **Option A (Culture media):** Unlike *M. tuberculosis* (which grows on LJ media), *M. leprae* is non-culturable on any cell-free media. * **Options C & D (Liver of guinea pig/Kidney of rabbit):** These animals are not susceptible to *M. leprae*. While guinea pigs are classically used for *M. tuberculosis* inoculation (causing systemic disease), they do not support the growth of the leprosy bacillus. **3. NEET-PG High-Yield Pearls:** * **Doubling Time:** *M. leprae* is the slowest-growing human pathogen, with a generation time of approximately **12–14 days**. * **Viability:** It is stained by **5% Sulfuric acid** (Modified Ziehl-Neelsen) because it is less acid-fast than *M. tuberculosis* (which uses 20% $H_2SO_4$). * **Cultivation Breakthrough:** Recently, *M. lepraemurium* (causing rat leprosy) was grown on specialized media, but *M. leprae* remains unculturable. * **Target Cells:** It has a specific tropism for **Schwann cells**, leading to characteristic nerve thickening and demyelination.

Question 662: Which of the following bacteria has an acid-fast structure?

A. Vibrio
B. Nocardia (Correct Answer)
C. E. coli
D. Bacillus anthracis

Explanation: ### Explanation **Correct Answer: B. Nocardia** **Why it is correct:** Acid-fastness is a physical property of certain bacteria that allows them to resist decolorization by acids during staining procedures. This is due to the presence of high concentrations of **mycolic acids** (long-chain fatty acids) in their cell walls. While *Mycobacterium tuberculosis* is the most famous acid-fast organism, **Nocardia** species are characterized as **weakly acid-fast** (or partially acid-fast). They require a weaker decolorizer (1% sulfuric acid) instead of the standard 20% sulfuric acid used in the Ziehl-Neelsen stain for Mycobacteria. **Why the other options are incorrect:** * **A. Vibrio:** These are Gram-negative, comma-shaped bacteria. Their cell walls lack mycolic acids, making them non-acid-fast. * **C. E. coli:** A classic Gram-negative rod. It has a standard peptidoglycan layer and an outer membrane but no acid-fast properties. * **D. Bacillus anthracis:** This is a Gram-positive, spore-forming rod. While its spores may show some resistance to staining, the vegetative cell itself is not acid-fast. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Acid-Fast Organisms (MY SUN):** **M**ycobacterium, **Y**ersinia enterocolitica (only some strains/envelope), **S**permatic head/Spore (bacterial), **U**p (Oocysts of Cryptosporidium, Isospora, Cyclospora), **N**ocardia. * **Nocardia Morphology:** It appears as Gram-positive, branching filamentous rods (often confused with Actinomyces, which is *not* acid-fast). * **Clinical Presentation:** Nocardia typically causes opportunistic infections in immunocompromised patients, presenting as pneumonia, brain abscesses, or cutaneous lesions (Mycetoma).

Question 663: In staphylococci, plasmid encoding beta-lactamase production is transmitted by which mechanism?

A. Conjugation
B. Transduction (Correct Answer)
C. Transposition
D. Transformation

Explanation: ### Explanation **Correct Option: B. Transduction** In *Staphylococcus aureus*, the transmission of antibiotic resistance, specifically the **plasmid-mediated production of beta-lactamase (penicillinase)**, occurs primarily through **transduction**. This process involves the transfer of genetic material from a donor bacterium to a recipient via a **bacteriophage** (a virus that infects bacteria). In staphylococci, this is the most common natural mechanism for horizontal gene transfer of plasmids. **Analysis of Incorrect Options:** * **A. Conjugation:** While conjugation (cell-to-cell contact via sex pili) is the most common method for spreading multi-drug resistance in **Gram-negative bacilli** (like *E. coli*), it is less common in staphylococci for beta-lactamase plasmids. * **C. Transposition:** Transposons ("jumping genes") are DNA sequences that move within a single genome (from plasmid to chromosome or vice versa). They facilitate the movement of resistance genes but are not the primary mechanism of *inter-cellular* transmission for these specific plasmids. * **D. Transformation:** This involves the uptake of "naked" DNA from the environment. While it occurs in *Streptococcus pneumoniae* and *Neisseria*, it is not a significant mechanism for plasmid transfer in *Staphylococcus*. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Resistance:** Beta-lactamase in *S. aureus* breaks the beta-lactam ring, rendering penicillin G, ampicillin, and amoxicillin ineffective. * **Treatment:** To overcome this, **penicillinase-resistant penicillins** (Oxacillin, Nafcillin) or **Beta-lactamase inhibitors** (Clavulanic acid) are used. * **MRSA Note:** Resistance in MRSA is **not** due to beta-lactamase; it is due to an altered target site (**PBP2a**) encoded by the **mecA gene** on the Staphylococcal Cassette Chromosome (SCCmec).

Question 664: All of the following statements about Streptococcus pneumoniae are true, EXCEPT:

A. Spherical gram-positive bacteria
B. 22 species are recognized
C. They reduce iron in hemoglobin to produce a green color in blood agar
D. They grow best in 40% CO2 (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. They grow best in 40% CO2** **Why Option D is the correct (False) statement:** *Streptococcus pneumoniae* (Pneumococcus) is a capnophilic organism, meaning it requires an enriched CO2 environment for optimal growth. However, the required concentration is **5–10% CO2**, not 40%. A concentration as high as 40% would be inhibitory to its growth. **Analysis of Other Options:** * **Option A (True):** Morphologically, they are Gram-positive, spherical or lanceolate-shaped cocci, typically arranged in pairs (diplococci) with the broad ends apposed. * **Option B (True):** While there are over 90 serotypes based on capsular polysaccharides, the genus *Streptococcus* contains numerous species. In the context of *S. pneumoniae* specifically, historical classifications and modern genomic studies recognize various related species within the "Mitis group." (Note: In many exams, this option refers to the broad diversity of the genus or the specific serogrouping). * **Option C (True):** *S. pneumoniae* exhibits **alpha-hemolysis** on blood agar. This is a partial hemolysis where the bacteria produce pneumolysin and hydrogen peroxide, which oxidizes hemoglobin to methemoglobin, resulting in a characteristic green discoloration around the colonies. **High-Yield Clinical Pearls for NEET-PG:** * **Quellung Reaction:** The gold standard for serotyping; involves swelling of the capsule when mixed with specific antiserum. * **Bile Solubility Test:** *S. pneumoniae* is bile soluble (autolysis occurs), which distinguishes it from *S. viridans* (bile insoluble). * **Optochin Sensitivity:** It is sensitive to Optochin (ethylhydrocupreine hydrochloride), another key differentiator from other alpha-hemolytic streptococci. * **Virulence Factor:** The **polysaccharide capsule** is the most important virulence factor; non-capsulated strains are non-pathogenic. * **Morphology:** Classically described as "Flame-shaped" or "Lanceolate" diplococci.

Question 665: Which of the following is true about Enteropathogenic E. coli?

A. Causes diarrhea in infants (Correct Answer)
B. Acts by invasion of intestinal epithelial cells
C. Adults are mostly affected
D. Affects immunocompromised hosts

Explanation: **Explanation:** **Enteropathogenic *E. coli* (EPEC)** is a major cause of infantile diarrhea, particularly in developing countries. 1. **Why Option A is correct:** EPEC is classically associated with **outbreaks of diarrhea in nurseries and daycare centers**. It primarily affects infants (children <2 years old). The hallmark of its pathogenesis is the **"Attaching and Effacing" (A/E) lesion**. It uses **Bundle-Forming Pili (BFP)** for initial attachment and the **Intimin** protein for intimate adhesion to intestinal epithelial cells, leading to the destruction of microvilli and malabsorptive diarrhea. 2. **Why other options are incorrect:** * **Option B:** EPEC is **non-invasive**. Invasion of intestinal epithelial cells is the characteristic mechanism of **Enteroinvasive *E. coli* (EIEC)**, which mimics Shigellosis. * **Option C:** Adults are rarely affected because they have usually developed acquired immunity. EPEC is a pediatric pathogen. * **Option D:** While immunocompromised hosts are at risk for many infections, EPEC is specifically defined by its predilection for the **pediatric/infant population** regardless of immune status, rather than being an opportunistic infection of the immunocompromised. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** **EPEC** = **P**ediatric/**P**ili (Bundle-forming). * **Pathogenesis:** Mediated by the **LEE (Locus of Enterocyte Effacement)** pathogenicity island. * **Diagnosis:** Characterized by "stacked-brick" appearance is NOT EPEC (that is EAEC); EPEC shows **localized adherence** on HeLa or HEp-2 cells. * **Toxins:** EPEC does **not** produce Shiga-like toxins or Enterotoxins (LT/ST).

Question 666: What is the confirmatory test for syphilis?

A. VDRL
B. FTA-ABS (Correct Answer)
C. RPR
D. None of the above

Explanation: Syphilis serology is divided into two categories: **Nonspecific (Nontreponemal)** and **Specific (Treponemal)** tests. **Why FTA-ABS is the correct answer:** The **Fluorescent Treponemal Antibody Absorption (FTA-ABS)** test is a specific treponemal test. It detects antibodies directed specifically against *Treponema pallidum*. Because it has high specificity and remains positive for life (even after treatment), it is used as a **confirmatory test** to verify a reactive screening result. **Why other options are incorrect:** * **VDRL (Venereal Disease Research Laboratory) & RPR (Rapid Plasma Reagin):** These are **nontreponemal tests**. They detect "reagin" antibodies against cardiolipin-lecithin-cholesterol antigen, not the bacterium itself. While excellent for **screening** and monitoring treatment response (as titers fall after therapy), they have high false-positive rates (e.g., in SLE, leprosy, malaria, or pregnancy). Therefore, they are not confirmatory. **High-Yield Clinical Pearls for NEET-PG:** * **Screening Sequence:** The traditional algorithm starts with a nontreponemal test (VDRL/RPR) followed by a treponemal test (FTA-ABS or TPHA) for confirmation. * **TPHA (T. pallidum Hemagglutination Assay):** Another common confirmatory treponemal test, often preferred over FTA-ABS in many labs due to ease of use. * **Neurosyphilis:** **CSF-VDRL** is the gold standard for diagnosis (highly specific, though low sensitivity). * **Prozone Phenomenon:** A false-negative VDRL result seen in secondary syphilis due to excessively high antibody titers; it is corrected by diluting the serum. * **Biological False Positive (BFP):** Conditions like infectious mononucleosis, aging, and autoimmune diseases can cause a positive VDRL but a negative FTA-ABS.

Question 667: Anthrax bacilli differs from anthracoid bacilli by being:

A. Non-capsulated
B. Strict aerobe
C. Non-motile (Correct Answer)
D. Haemolytic colonies on blood agar

Explanation: ### Explanation The differentiation between *Bacillus anthracis* (Anthrax bacilli) and *Bacillus anthracoides* (Pseudoanthrax/Anthracoid bacilli) is a classic high-yield topic in NEET-PG Microbiology. **Why "Non-motile" is correct:** *Bacillus anthracis* is characteristically **non-motile**. In contrast, almost all anthracoid bacilli (like *B. cereus* or *B. subtilis*) exhibit active motility (peritrichous flagella). This lack of motility is a key diagnostic feature used to identify the pathogen in a laboratory setting. **Analysis of Incorrect Options:** * **A. Non-capsulated:** Incorrect. *B. anthracis* is **capsulated** (polypeptide capsule made of D-glutamic acid), which is essential for its virulence. Anthracoid bacilli are generally non-capsulated. * **B. Strict aerobe:** Incorrect. Both *B. anthracis* and anthracoid bacilli are **aerobes and facultative anaerobes**. * **D. Haemolytic colonies:** Incorrect. *B. anthracis* is **non-haemolytic** (gamma-haemolysis) on blood agar. Anthracoid bacilli typically produce strong beta-haemolysis. **High-Yield Clinical Pearls for NEET-PG:** | Feature | *B. anthracis* | Anthracoid Bacilli | | :--- | :--- | :--- | | **Motility** | **Non-motile** (Statue-like) | **Motile** | | **Haemolysis** | Non-haemolytic | Usually Beta-haemolytic | | **Capsule** | Present (Polypeptide) | Absent | | **Medusa Head** | Present (on Agar) | Absent/Irregular | | **Salicin Fermentation** | Negative | Positive | | **Penicillin** | Sensitive (McFadyean's reaction) | Resistant | * **Mnemonic:** *B. anthracis* is "Lazy and Boring" (Non-motile, Non-haemolytic). * **McFadyean’s Reaction:** Uses polychrome methylene blue to visualize the **M**auve-colored capsule of *B. anthracis*. * **String of Pearls Reaction:** Seen when *B. anthracis* is grown on agar containing low concentrations of penicillin.

Question 668: Which organism is known to cause epidemics of puerperal sepsis?

A. Cytomegalovirus
B. Group A beta-hemolytic streptococci (Correct Answer)
C. Group B beta-hemolytic streptococci
D. Toxoplasma gondii

Explanation: **Explanation:** **Puerperal sepsis** is defined as an infection of the genital tract occurring at any time between the onset of rupture of membranes or labor and the 42nd day postpartum. **Why Group A beta-hemolytic streptococci (GABS) is correct:** Historically, *Streptococcus pyogenes* (Group A Strep) is the most notorious cause of **explosive epidemics** of puerperal sepsis. It is highly virulent and can be introduced into the birth canal via the hands or respiratory droplets of healthcare workers or the patient herself. While modern hygiene has reduced its frequency, it remains the primary organism associated with rapidly progressing, life-threatening outbreaks in obstetric wards. **Analysis of Incorrect Options:** * **Group B beta-hemolytic streptococci (GBS):** While *S. agalactiae* is the most common cause of neonatal sepsis and a frequent cause of sporadic postpartum infections, it typically exists as normal vaginal flora and does not usually cause the "epidemic" patterns seen with Group A. * **Cytomegalovirus (CMV):** This is a viral pathogen primarily associated with congenital infections (TORCH syndrome) rather than acute postpartum maternal sepsis. * **Toxoplasma gondii:** This is a protozoan parasite. While it can cause congenital toxoplasmosis via transplacental transmission, it is not a causative agent of puerperal sepsis. **High-Yield Clinical Pearls for NEET-PG:** * **Ignaz Semmelweis:** Known as the "Father of Infection Control," he famously discovered that handwashing could drastically reduce the mortality of puerperal sepsis caused by GABS. * **Most common overall cause:** While GABS causes epidemics, **anaerobic bacteria** (e.g., *Bacteroides*, *Peptostreptococcus*) and *E. coli* are common causes of sporadic polymicrobial puerperal infections today. * **Clinical Presentation:** Sudden high fever, pelvic pain, and foul-smelling lochia are classic signs.

Question 669: Trachoma is one of the leading causes of blindness. Which of the following best typifies the disease?

A. It is caused by Chlamydia trachomatis (Correct Answer)
B. It is best treated with systemic cephalosporins and ophthalmic tetracycline
C. It affects 400 million people in the Pacific Rim
D. It is a form of chronic uveitis

Explanation: **Explanation:** **1. Why Option A is Correct:** Trachoma is a chronic keratoconjunctivitis caused by specific serotypes of **_Chlamydia trachomatis_ (Serotypes A, B, Ba, and C)**. It is the leading infectious cause of blindness worldwide. The pathogenesis involves repeated cycles of infection leading to subepithelial follicular inflammation, conjunctival scarring (Arlt’s line), trichiasis (inward turning of eyelashes), and eventual corneal opacity. **2. Why the Other Options are Incorrect:** * **Option B:** The WHO-recommended treatment for Trachoma follows the **SAFE strategy**. The medical treatment of choice is a **single dose of oral Azithromycin** (macrolide) or topical Tetracycline. Cephalosporins are not the standard of care for Chlamydial eye infections. * **Option C:** While Trachoma is a global health issue, it is most hyperendemic in the **arid areas of Africa** (Sub-Saharan Africa), the Middle East, and parts of Asia, rather than being specifically characterized by 400 million cases in the Pacific Rim. * **Option D:** Trachoma is a disease of the **conjunctiva and cornea** (keratoconjunctivitis). Uveitis involves the middle layer of the eye (iris, ciliary body, and choroid), which is not the primary site of pathology in Trachoma. **3. NEET-PG High-Yield Pearls:** * **SAFE Strategy:** **S**urgery (for trichiasis), **A**ntibiotics (Azithromycin), **F**acial cleanliness, **E**nvironmental improvement. * **Inclusion Bodies:** Look for **Halberstaedter-Prowazek (HP) bodies** (intracytoplasmic inclusions) in conjunctival scrapings. * **Clinical Signs:** **Herbert’s pits** (scarred limbal follicles) are pathognomonic for Trachoma. * **Serotypes:** Remember **A-C** cause Trachoma; **D-K** cause inclusion conjunctivitis and NGS; **L1-L3** cause Lymphogranuloma Venereum (LGV).

Question 670: A wound shows crepitation in the subcutaneous tissue and muscle with foul-smelling discharge. Pus culture shows Cl. perfringens. All of the following features about Cl. perfringens are TRUE, EXCEPT?

A. It is the most common cause of gas gangrene.
B. Can be detected by Nagler's reaction.
C. The most important toxin is hyaluronidase. (Correct Answer)
D. Gas gangrene strains produce heat-resistant spores.

Explanation: **Explanation:** The clinical presentation of crepitation, foul-smelling discharge, and muscle involvement is classic for **Gas Gangrene (Clostridial Myonecrosis)**. **1. Why Option C is the correct answer (The False Statement):** While *Clostridium perfringens* does produce hyaluronidase (an enzyme that aids tissue spread), it is **not** the most important toxin. The primary virulence factor responsible for the pathogenesis of gas gangrene is **Alpha (α) toxin**. Alpha toxin is a **lecithinase (phospholipase C)** that degrades cell membranes, leading to massive tissue necrosis, hemolysis, and toxemia. **2. Analysis of Incorrect Options (True Statements):** * **Option A:** *Cl. perfringens* Type A is indeed the most common cause of gas gangrene worldwide (responsible for 80-90% of cases). * **Option B:** The **Nagler’s reaction** is a biochemical test used to detect Alpha toxin. When the organism is grown on egg yolk agar, the lecithinase activity creates a zone of opalescence around the colonies, which is inhibited by adding specific antitoxin. * **Option D:** Strains causing gas gangrene produce **heat-resistant spores**, which allow the bacteria to survive in soil and dust before entering a traumatic wound. (Note: Food poisoning strains of *Cl. perfringens* also produce highly heat-resistant spores). **Clinical Pearls for NEET-PG:** * **Target Hemolysis:** On blood agar, *Cl. perfringens* shows a "double zone of hemolysis" (inner zone of complete hemolysis due to Theta toxin; outer zone of incomplete hemolysis due to Alpha toxin). * **Stormy Fermentation:** In litmus milk media, it produces acid and gas, leading to a "stormy" appearance of the clot. * **Non-motile:** Unlike most Clostridia, *Cl. perfringens* is non-motile.

Practice by Chapter

Staphylococci

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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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