Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1181: Regarding Schick's test, which of the following statements is false?

A. An erythematous reaction in both arms indicates hypersensitivity.
B. A positive test means that the person is immune to diphtheria. (Correct Answer)
C. Diphtheria antitoxin is given intradermally.
D. The test is done to find out the immune status against diphtheria.

Explanation: ### Explanation **Schick’s Test** is an intradermal skin test used to determine the immune status of an individual against *Corynebacterium diphtheriae*. #### 1. Why Option B is the Correct (False) Statement A **positive test** indicates that the person **lacks immunity** (is susceptible) to diphtheria. In a positive reaction, the injected toxin causes local inflammation (erythema and swelling) because the individual does not have enough circulating antitoxin to neutralize it. Conversely, a **negative test** means the person is **immune**, as their pre-existing antibodies neutralize the toxin, resulting in no reaction. #### 2. Analysis of Other Options * **Option A (True):** If an erythematous reaction occurs in both the test arm (toxin) and the control arm (heated toxin), it indicates a **pseudo-reaction**, which signifies **hypersensitivity** to the bacterial components rather than a lack of immunity. * **Option C (True):** The test involves the intradermal injection of 0.1 ml of diphtheria toxin into one forearm and 0.1 ml of inactivated (heated) toxin into the other as a control. * **Option D (True):** The primary clinical utility of Schick’s test is to assess whether an individual is susceptible or immune to diphtheria. #### 3. High-Yield Clinical Pearls for NEET-PG * **Interpretation Summary:** * **Positive:** Reaction in test arm only (Susceptible). * **Negative:** No reaction in either arm (Immune). * **Pseudo-reaction:** Reaction in both arms, disappearing by 48 hours (Immune + Hypersensitive). * **Combined:** Reaction in both, but test arm reaction persists longer (Susceptible + Hypersensitive). * **Control:** The control arm uses **heat-inactivated toxin** (heated at 70°C for 30 mins) to distinguish between true toxin activity and hypersensitivity. * **Current Status:** Schick's test is now largely obsolete in clinical practice, replaced by direct measurement of antitoxin titers via ELISA.

Question 1182: What is the most common genetic predisposition in Neisseria infections?

A. Male gender
B. HLA B27
C. Complement deficiency (Correct Answer)
D. IgA deficiency

Explanation: **Explanation:** **1. Why Complement Deficiency is Correct:** The complement system plays a critical role in the innate immune response against Gram-negative bacteria, particularly *Neisseria* species (*N. meningitidis* and *N. gonorrhoeae*). These bacteria are uniquely susceptible to **Complement-Mediated Lysis**. Specifically, deficiencies in the **Terminal Complement Components (C5, C6, C7, C8, and C9)**, which form the **Membrane Attack Complex (MAC)**, are the most significant genetic predispositions. Without a functional MAC, the body cannot effectively punch holes in the diplococcal cell wall, leading to recurrent or disseminated infections. **2. Why Other Options are Incorrect:** * **Male Gender:** While certain infections may show gender disparity due to behavioral factors, there is no inherent genetic predisposition linked to the male sex for *Neisseria*. * **HLA B27:** This human leukocyte antigen is strongly associated with seronegative spondyloarthropathies (e.g., Ankylosing Spondylitis, Reiter’s syndrome). While Reactive Arthritis can follow a gonococcal infection, HLA B27 itself does not predispose a person to the primary infection. * **IgA Deficiency:** This is the most common primary immunodeficiency and predisposes patients to sinopulmonary and GI infections. While IgA is present on mucosal surfaces, it is the serum complement system that is the primary defense against invasive *Neisseria*. **3. High-Yield Clinical Pearls for NEET-PG:** * **C1, C2, C4 Deficiency:** Associated with immune-complex diseases like **SLE**. * **C3 Deficiency:** Leads to severe, recurrent infections with **encapsulated bacteria** (e.g., *S. pneumoniae*, *H. influenzae*). * **Properdin/Factor D Deficiency:** Also increases risk for *Neisseria* due to impairment of the alternative pathway. * **Exam Tip:** If a question mentions "recurrent meningococcal meningitis in a teenager," always look for "Terminal Complement Deficiency" in the options.

Question 1183: Which of the following is a virulence factor for meningococci?

A. Capsule (Correct Answer)
B. Pili
C. Endotoxin
D. Coagulase

Explanation: **Explanation:** **1. Why Capsule is the Correct Answer:** The **polysaccharide capsule** is the most critical virulence factor for *Neisseria meningitidis*. It is essential for systemic survival because it is **anti-phagocytic**, allowing the bacteria to evade the host's immune system and survive in the bloodstream. Furthermore, the capsule is the basis for **serogrouping** (A, B, C, Y, W-135) and is the primary component used in meningococcal vaccines (except for Serogroup B). **2. Analysis of Incorrect Options:** * **B. Pili:** While pili are important for **initial attachment** and colonization of the nasopharyngeal epithelium, they are not considered the primary virulence factor responsible for the systemic disease and mortality associated with meningococci. * **C. Endotoxin:** *N. meningitidis* actually possesses **LOS (Lipooligosaccharide)** rather than the typical LPS (Lipopolysaccharide). While LOS is responsible for the clinical manifestations of sepsis (DIC, shock, and petechiae), the capsule is the primary factor that allows the infection to establish and persist. * **D. Coagulase:** This is a characteristic virulence factor for ***Staphylococcus aureus***, used to convert fibrinogen to fibrin. It is not produced by *Neisseria* species. **3. High-Yield Clinical Pearls for NEET-PG:** * **Serogroup B:** Its capsule is poorly immunogenic because it resembles human neural cell adhesion molecules; hence, the vaccine for Group B is protein-based (recombinant), not polysaccharide-based. * **Waterhouse-Friderichsen Syndrome:** Severe meningococcemia leading to bilateral adrenal hemorrhage. * **Deficiency of Late Complement Components (C5-C9):** Patients with these deficiencies are at a significantly increased risk for recurrent *Neisseria* infections.

Question 1184: What organism is most commonly implicated in the causation of subacute bacterial endocarditis?

A. Staphylococcus albus
B. Staphylococcus Aureus
C. Salmonella typhi
D. Streptococcus Viridans (Correct Answer)

Explanation: **Explanation:** **Subacute Bacterial Endocarditis (SBE)** typically occurs on previously damaged or prosthetic heart valves and follows an indolent, slow-progressing clinical course. **Why Streptococcus Viridans is the Correct Answer:** *Streptococcus viridans* (a group including *S. sanguinis*, *S. mutans*, and *S. mitis*) is the **most common cause** of SBE. These organisms are normal commensals of the oral cavity. Following dental procedures or even vigorous brushing, they enter the bloodstream (transient bacteremia). Due to their ability to produce **dextrans**, they easily adhere to fibrin-platelet aggregates on damaged heart valves (e.g., in Rheumatic Heart Disease or Mitral Valve Prolapse), leading to vegetation formation. **Analysis of Incorrect Options:** * **Staphylococcus aureus:** This is the most common cause of **Acute Bacterial Endocarditis**. It is highly virulent and can attack previously healthy, normal heart valves, leading to rapid destruction. It is also the leading cause in intravenous drug users (IVDU). * **Staphylococcus albus (S. epidermidis):** This is the most common cause of endocarditis in patients with **prosthetic heart valves** (early onset, <1 year post-surgery) due to its ability to form biofilms on foreign bodies. * **Salmonella typhi:** While it causes systemic enteric fever, it is an extremely rare cause of endocarditis. **NEET-PG High-Yield Pearls:** * **Most common cause overall (Acute):** *S. aureus*. * **Most common cause (Subacute):** *Viridans streptococci*. * **Culture-negative endocarditis:** Most commonly due to *HACEK* organisms or *Coxiella burnetii*. * **Association with Colon Cancer:** *Streptococcus gallolyticus* (formerly *S. bovis*) endocarditis is a classic board association.

Question 1185: What is the infective dose of Salmonella typhi?

A. One bacillus
B. 10^8-10^10 bacilli
C. 10^2-10^5 bacilli (Correct Answer)
D. 1-10 bacilli

Explanation: **Explanation:** The infective dose of a pathogen refers to the minimum number of organisms required to cause disease in a susceptible host. For **Salmonella typhi**, the causative agent of Typhoid fever, the infective dose is relatively moderate, typically ranging between **10² and 10⁵ bacilli**. **1. Why Option C is Correct:** Unlike some enteric pathogens that are highly sensitive to gastric acid, *S. typhi* possesses mechanisms to survive the acidic environment of the stomach to reach the small intestine. A dose of $10^3$ to $10^5$ organisms is generally sufficient to overcome host defenses and initiate infection in healthy individuals. **2. Analysis of Incorrect Options:** * **Option A & D (1–10 bacilli):** These represent a very low infective dose. This is characteristic of **Shigella**, **EHEC (O157:H7)**, and **Giardia**, where even a few organisms can cause clinical disease because they are highly resistant to gastric acidity. * **Option B (10⁸–10¹⁰ bacilli):** This is a very high infective dose. This is typical for **Vibrio cholerae** and **ETEC**, which are highly acid-sensitive; a massive inoculum is required to ensure enough bacteria survive the stomach "acid barrier" to colonize the intestine. **3. Clinical Pearls for NEET-PG:** * **Acid Sensitivity:** Factors that decrease gastric acidity (e.g., use of Antacids, PPIs, or H2 blockers) significantly **lower** the infective dose required for *Salmonella* and *Vibrio* to cause infection. * **Transmission:** Because of the moderate infective dose, *S. typhi* is primarily transmitted through contaminated food and water (fecal-oral route). * **Comparison Table:** * **Low dose (<10²):** *Shigella*, *Entamoeba*, *Giardia*. * **Moderate dose (10²–10⁵):** *Salmonella typhi*, *Campylobacter jejuni*. * **High dose (>10⁸):** *Vibrio cholerae*.

Question 1186: What is the most common infection caused by Streptococcus pneumoniae?

A. Otitis media (Correct Answer)
B. Sore throat
C. Meningitis
D. Pneumonia

Explanation: **Explanation:** *Streptococcus pneumoniae* (Pneumococcus) is a leading cause of mucosal and invasive infections. While the organism is the most common cause of community-acquired pneumonia (CAP) and bacterial meningitis in adults, the question asks for the **most common** infection overall. **1. Why Otitis Media is correct:** Among all clinical manifestations, **Acute Otitis Media (AOM)** is numerically the most frequent infection caused by *S. pneumoniae*, particularly in the pediatric population. It is estimated that a vast majority of children experience at least one episode of pneumococcal otitis media before age 5. **2. Analysis of Incorrect Options:** * **Sore throat:** This is primarily caused by viruses or *Streptococcus pyogenes* (Group A Strep). *S. pneumoniae* is not a typical cause of pharyngitis. * **Meningitis:** While *S. pneumoniae* is the #1 cause of bacterial meningitis in adults and the elderly, it is a relatively rare occurrence compared to the high incidence of middle ear infections. * **Pneumonia:** *S. pneumoniae* is the most common cause of bacterial pneumonia; however, in terms of absolute case numbers across all age groups, otitis media occurs far more frequently. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, lancet-shaped diplococci; Alpha-hemolytic on blood agar. * **Virulence Factor:** The polysaccharide **capsule** is the most important (detected by the Quellung reaction). * **Sensitivity:** It is **Optochin sensitive** and **Bile soluble** (distinguishes it from *S. viridans*). * **Vaccines:** PCV-13 (Conjugate) is used in the routine pediatric schedule; PPSV-23 (Polysaccharide) is for adults >65 or immunocompromised. * **Mnemonic (MOPS):** *S. pneumoniae* is the #1 cause of **M**eningitis, **O**titis media, **P**neumonia, and **S**inusitis.

Question 1187: Which of the following agents causing acute infectious diarrhea is associated with enterotoxic, enteropathogenic, enteroinvasive, and enterohemorrhagic pathogenetic mechanisms?

A. Vibrio cholerae
B. E. coli (Correct Answer)
C. Shigella
D. Rotavirus

Explanation: **Explanation:** The correct answer is **E. coli**. This organism is unique because it encompasses a diverse group of strains, each possessing distinct virulence factors that allow it to cause diarrhea through multiple pathogenetic mechanisms. These are categorized into specific "pathotypes": 1. **Enterotoxigenic (ETEC):** Produces LT (heat-labile) and ST (heat-stable) toxins; causes "Traveler’s diarrhea." 2. **Enteropathogenic (EPEC):** Causes "attaching and effacing" lesions, leading to infantile diarrhea. 3. **Enteroinvasive (EIEC):** Invades the colonic epithelium, clinically resembling Shigellosis (dysentery). 4. **Enterohemorrhagic (EHEC/STEC):** Produces Shiga-like toxins (Verotoxins); associated with Hemolytic Uremic Syndrome (HUS) and hemorrhagic colitis (notably serotype O157:H7). 5. **Enteroaggregative (EAEC):** Associated with persistent diarrhea. **Why other options are incorrect:** * **Vibrio cholerae:** Primarily **enterotoxic**. It produces the Cholera toxin, which increases cAMP, leading to profuse watery "rice-water" stools without invading the mucosa. * **Shigella:** Primarily **enteroinvasive** and cytotoxic (via Shiga toxin). While it shares similarities with EIEC/EHEC, it does not possess the full spectrum of pathotypes (like EPEC or ETEC) seen in E. coli. * **Rotavirus:** A viral pathogen that causes diarrhea via **malabsorption** (destruction of enterocytes) and an enterotoxin-like protein (NSP4). **High-Yield Clinical Pearls for NEET-PG:** * **EHEC** is the only E. coli that **does not ferment Sorbitol** (detected on Sorbitol MacConkey Agar). * **EIEC** is non-motile and does not ferment lactose (resembles Shigella). * **ETEC** is the most common cause of Traveler’s diarrhea. * **EPEC** is a major cause of outbreaks in nursery settings (Infantile diarrhea).

Question 1188: All of the following statements regarding Chlamydia are true, except?

A. Gram positive (Correct Answer)
B. Trachoma
C. Causes Psittacosis
D. Causes Lymphogranuloma venerum

Explanation: ### Explanation **Chlamydia** species are unique, obligate intracellular bacteria. The correct answer is **Option A** because Chlamydia are technically **Gram-negative** organisms. However, they are poorly visualized on a standard Gram stain because their cell wall lacks a traditional peptidoglycan layer (historically referred to as the "peptidoglycan paradox"). Instead, they use cysteine-rich proteins for structural integrity. #### Analysis of Options: * **A. Gram positive (Incorrect Statement):** Chlamydia possess an inner and outer membrane similar to Gram-negative bacteria and contain Lipopolysaccharide (LPS). They do not retain crystal violet, making the statement "Gram-positive" false. * **B. Trachoma:** True. *Chlamydia trachomatis* serotypes A, B, Ba, and C are the leading infectious causes of blindness worldwide (Trachoma). * **C. Causes Psittacosis:** True. *Chlamydophila psittaci* is transmitted via bird droppings (psittacines like parrots) and causes atypical pneumonia known as Psittacosis. * **D. Causes Lymphogranuloma venereum (LGV):** True. *C. trachomatis* serotypes L1, L2, and L3 cause LGV, characterized by painless genital ulcers followed by painful inguinal lymphadenopathy (buboes). #### NEET-PG High-Yield Pearls: 1. **Biphasic Life Cycle:** They exist in two forms: **Elementary Body (EB)** which is infectious and extracellular, and **Reticulate Body (RB)** which is the metabolically active, replicating intracellular form. 2. **Staining:** Best visualized using **Giemsa, Castaneda, or Gimenez stains**. 3. **Inclusion Bodies:** *C. trachomatis* forms iodine-staining glycogen-rich inclusion bodies, whereas *C. psittaci* and *C. pneumoniae* do not. 4. **Drug of Choice:** Azithromycin (single dose) or Doxycycline (7 days).

Question 1189: Which of the following is NOT true regarding pseudomembranous colitis?

A. It is caused by Clostridium difficile.
B. The organism is a normal commensal of the gut.
C. It is due to the production of phospholipase A. (Correct Answer)
D. It is treated by vancomycin.

Explanation: **Explanation:** Pseudomembranous colitis (PMC) is a severe inflammation of the colon typically occurring after broad-spectrum antibiotic use. **Why Option C is the correct answer (The False Statement):** The pathogenesis of *Clostridium difficile* is mediated by two potent exotoxins: **Toxin A (Enterotoxin)** and **Toxin B (Cytotoxin)**. These toxins act by inactivating Rho GTPases, leading to actin depolymerization, cell death, and mucosal inflammation. **Phospholipase A** (specifically Alpha toxin) is the primary virulence factor for *Clostridium perfringens* (causing gas gangrene), not *C. difficile*. **Analysis of other options:** * **Option A:** *Clostridium difficile* is the definitive causative agent of PMC. It flourishes when the normal intestinal flora is suppressed by antibiotics (most commonly Clindamycin, Fluoroquinolones, and Cephalosporins). * **Option B:** *C. difficile* is a normal commensal of the gut in about 3–5% of healthy adults and up to 70% of healthy neonates. Disease occurs only upon overgrowth. * **Option D:** Oral **Vancomycin** or **Fidaxomicin** are the first-line treatments. Metronidazole is now reserved for non-severe cases if other options are unavailable. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** The gold standard is the **Tissue Culture Cytotoxicity Assay**, but the most common rapid test is detecting toxins in stool via **ELISA**. * **Morphology:** Characteristic "volcano lesions" (exudates of fibrin and neutrophils) form the pseudomembrane. * **Alcohol Rubs:** Hand sanitizers do not kill *C. difficile* spores; handwashing with **soap and water** is mandatory for infection control.

Question 1190: The "L-form" phenomenon is most likely to occur in which of the following organisms?

A. Mycoplasma (Correct Answer)
B. Mycobacteria
C. Proteus
D. Pasteurella

Explanation: **Explanation:** The correct answer is **Mycoplasma**. **1. Why Mycoplasma is correct:** L-forms (also known as Cell Wall Deficient or CWD forms) are bacterial variants that have lost their ability to synthesize a cell wall, typically due to exposure to antibiotics like penicillin or lysozyme. **Mycoplasma** is unique because it is the only genus of bacteria that **naturally lacks a cell wall** entirely. While other bacteria "become" L-forms under stress, Mycoplasma is considered the biological prototype of the L-form state. They contain sterols in their cell membrane (unlike other bacteria) to provide structural integrity in the absence of a wall. **2. Why the other options are incorrect:** * **Proteus:** While *Proteus* species are famous for their "swarming motility" and can be induced to form L-forms in a laboratory setting (Protoplasts/Spheroplasts), they possess a standard peptidoglycan cell wall in their natural state. * **Mycobacteria:** These possess a highly complex, lipid-rich cell wall containing mycolic acids. They do not naturally exist as L-forms. * **Pasteurella:** This is a Gram-negative coccobacillus with a standard cell wall structure. **3. High-Yield Clinical Pearls for NEET-PG:** * **Antibiotic Resistance:** Because Mycoplasma lacks a cell wall, it is **innately resistant** to all Beta-lactam antibiotics (Penicillins, Cephalosporins) which target peptidoglycan synthesis. * **Culture Appearance:** Mycoplasma colonies on specialized media (PPLO agar) show a characteristic **"Fried Egg" appearance**. * **Eaton’s Agent:** *Mycoplasma pneumoniae* is a common cause of "Atypical Pneumonia" (Walking Pneumonia). * **Staining:** They do not take up Gram stain; they are best visualized using Giemsa or Castaneda stains.

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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