Bacteriology — MCQs

A healthy 45-year-old female who underwent root canal treatment approximately three weeks ago now presents with a new heart murmur, fever, painful skin nodules, abdominal pain, and an abnormal liver function test. Which of the following organisms is most likely to cause endocarditis and is implicated in dental caries or root canal infections?

Q999Medium

Which of the following statements regarding certain strains of Streptococcus mutans is/are true?

Q1000Easy

Friedlander's bacillus is the eponym of which bacterium?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 991: What is the differentiating feature between Neisseria gonococcus and Neisseria meningitidis?

A. Lactose fermentation
B. Maltose fermentation (Correct Answer)
C. Mannitol fermentation
D. Sucrose fermentation

Explanation: The differentiation of *Neisseria* species is a high-yield topic in microbiology, primarily based on their carbohydrate utilization patterns. ### **Explanation of the Correct Answer** Both *N. gonorrhoeae* and *N. meningitidis* are Gram-negative diplococci that are oxidase and catalase positive. The definitive biochemical test to distinguish them is the **oxidation of sugars** (CTA sugars). * **Neisseria meningitidis** oxidizes both **Glucose and Maltose**. * **Neisseria gonorrhoeae** oxidizes **Glucose only**. **Mnemonic:** **M**eningitidis ferments **M**altose and Glucose; **G**onorrhoeae ferments **G**lucose only. ### **Analysis of Incorrect Options** * **A. Lactose fermentation:** This is used to differentiate enteric Gram-negative bacilli (e.g., *E. coli* is a lactose fermenter, while *Shigella* is not). Among *Neisseria* species, only *N. lactamica* ferments lactose. * **C. Mannitol fermentation:** This is a key feature used to identify *Staphylococcus aureus* (using Mannitol Salt Agar), not *Neisseria*. * **D. Sucrose fermentation:** While some commensal *Neisseria* species (like *N. sicca*) can ferment sucrose, it is not used to differentiate the two primary pathogens, as neither *N. gonorrhoeae* nor *N. meningitidis* ferment sucrose. ### **NEET-PG Clinical Pearls** * **Culture Media:** Both grow on **Thayer-Martin Medium** (Selective medium containing Vancomycin, Colistin, Nystatin, and Trimethoprim). * **Specimen Transport:** *Neisseria* are highly sensitive to cold; specimens should never be refrigerated and should be transported in **Stuart’s or Amies medium**. * **Virulence:** The presence of a **polysaccharide capsule** is unique to *N. meningitidis* (used for serotyping and vaccines), whereas *N. gonorrhoeae* is non-capsulated.

Question 992: Which of the following statements about Campylobacter is false?

A. Uncooked meat can be a source of infection.
B. Poultry is a common source of infection.
C. Campylobacter is a spore-forming bacterium. (Correct Answer)
D. Campylobacter is an enteric pathogen.

Explanation: **Explanation:** The correct answer is **C** because **Campylobacter species are non-spore-forming**, Gram-negative, curved or spiral-shaped bacilli. In medical microbiology, spore formation is a characteristic primarily reserved for specific Gram-positive rods, such as *Bacillus* and *Clostridium* species. **Analysis of Options:** * **Option A & B (Correct statements):** *Campylobacter jejuni* is a major zoonotic pathogen. The primary reservoir is the gastrointestinal tract of wild and domestic animals, especially **poultry**. Human infection typically occurs via the fecal-oral route through the consumption of **undercooked meat**, contaminated milk, or water. * **Option D (Correct statement):** *Campylobacter* is a leading **enteric pathogen** worldwide. It is a common cause of bacterial gastroenteritis, characterized by inflammatory, often bloody diarrhea (dysentery), fever, and abdominal cramps. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Described as "seagull-wing" shaped appearance. * **Motility:** Exhibits characteristic **"darting motility"** via a single polar flagellum. * **Culture Requirements:** It is **microaerophilic** (requires 5% $O_2$) and **thermophilic**, growing best at **42°C** on selective media like **Skirrow’s medium** or **Preston agar**. * **Complications:** It is the most common antecedent infection associated with **Guillain-Barré Syndrome (GBS)** due to molecular mimicry between bacterial lipooligosaccharides and human gangliosides. It is also linked to Reactive Arthritis.

Question 993: All the following are false regarding C. diphtheriae toxin EXCEPT:

A. It is an endotoxin
B. Toxin production depends on optimum concentration of iron (Correct Answer)
C. It acts by activating adenyl cyclase
D. Schick Test demonstrates circulating toxin

Explanation: ### Explanation **Correct Answer: B. Toxin production depends on optimum concentration of iron** **1. Why Option B is Correct:** The production of the diphtheria toxin is regulated by the **DtxR (Diphtheria Toxin Repressor)** protein. This repressor is iron-dependent. When iron levels are high, the DtxR-iron complex binds to the toxin gene operator and inhibits production. Toxin production is maximal only when iron concentrations are low/limiting (**optimum concentration: 0.1 mg/L**). If iron levels exceed this, toxin production is suppressed. **2. Analysis of Incorrect Options:** * **Option A:** *C. diphtheriae* toxin is a potent **exotoxin** (specifically an A-B type polypeptide), not an endotoxin. Endotoxins are typically components of the cell wall of Gram-negative bacteria (LPS). * **Option C:** The toxin acts by **inhibiting protein synthesis**. The 'A' subunit catalyzes the ADP-ribosylation of **Elongation Factor-2 (EF-2)**, inactivating it. (Activating adenyl cyclase is the mechanism of *Vibrio cholerae* and ETEC toxins). * **Option D:** The Schick Test is used to demonstrate **circulating antibodies (immunity)** against the toxin, not the toxin itself. A positive test indicates susceptibility to the disease (lack of antibodies). **3. High-Yield Clinical Pearls for NEET-PG:** * **Lysogenic Conversion:** The *tox* gene is not chromosomal; it is introduced into the bacterium by a temperate bacteriophage called **Corynephage beta**. * **Elek’s Gel Precipitation Test:** This is the standard *in vitro* test to demonstrate the toxigenicity of a *C. diphtheriae* strain. * **Target Organs:** While the bacteria remain localized in the upper respiratory tract (forming a pseudomembrane), the toxin disseminates, primarily affecting the **heart** (myocarditis) and **nerves** (polyneuropathy). * **Culture:** Use **Löffler's Serum Slope** (rapid growth) or **Potassium Tellurite Agar** (black colonies).

Question 994: Which of the following statements is not true about Vibrio cholerae?

A. It is non-halophilic.
B. It grows on simple media.
C. Man is the only natural host.
D. It cannot survive in an extracellular environment. (Correct Answer)

Explanation: **Explanation:** The correct answer is **D**, as the statement is false. *Vibrio cholerae* is a facultative intracellular pathogen, but it is primarily known for its ability to survive and multiply extensively in **extracellular environments**, particularly in brackish water and estuarine ecosystems. It often attaches to the chitinous shells of copepods (zooplankton) for survival outside the human host. **Analysis of Options:** * **Option A (Non-halophilic):** This is true. Unlike most other *Vibrio* species (like *V. parahaemolyticus*) which are halophilic (salt-loving) and require high NaCl concentrations, *V. cholerae* is non-halophilic and can grow in media with 0% NaCl, though it tolerates low salt concentrations (up to 3%). * **Option B (Simple media):** This is true. *V. cholerae* is not fastidious. It grows well on routine laboratory media like Nutrient Agar and MacConkey Agar (forming colorless colonies). * **Option C (Man is the only natural host):** This is true. While the bacteria persist in water bodies, there is no known animal reservoir; humans are the only natural hosts affected by the clinical disease. **High-Yield NEET-PG Pearls:** * **Morphology:** Gram-negative, comma-shaped (vibrion) rods with a single polar flagellum showing **"darting motility."** * **Selective Media:** **TCBS** (Thiosulfate Citrate Bile salts Sucrose) agar is the gold standard, where it produces **yellow colonies** due to sucrose fermentation. * **Enrichment Media:** Alkaline Peptone Water (APW) and Monsur’s Taurocholate Tellurite Peptone Water. * **Biochemical Key:** It is **Oxidase positive** (distinguishes it from Enterobacteriaceae) and grows at a high pH (alkaline pH 8.2–9.0).

Question 995: Which bacterium is known for producing the lecithinase test?

A. Clostridium tetani
B. Clostridium botulinum
C. Clostridium perfringens (Correct Answer)
D. Clostridium difficile

Explanation: **Explanation:** **Clostridium perfringens** is the correct answer because it produces **Alpha (α) toxin**, a potent **lecithinase** (phospholipase C). This enzyme acts on the lecithin present in host cell membranes, splitting it into phosphorylcholine and diglyceride. In the laboratory, this is demonstrated using the **Nagler Reaction**, where the organism is grown on Egg Yolk Agar (EYA). The lecithinase breaks down the lecithin in the egg yolk, resulting in a characteristic zone of opalescence around the colonies. **Analysis of Incorrect Options:** * **Clostridium tetani:** Known for producing **Tetanospasmin** (a neurotoxin causing spastic paralysis). It does not produce lecithinase and typically shows "swarming" growth on agar. * **Clostridium botulinum:** Produces **Botulinum toxin**, which inhibits acetylcholine release at the neuromuscular junction. It is not associated with the lecithinase test. * **Clostridium difficile:** Primarily produces **Toxin A (enterotoxin)** and **Toxin B (cytotoxin)**, leading to pseudomembranous colitis. It is identified via stool PCR or GDH assays, not lecithinase production. **High-Yield Clinical Pearls for NEET-PG:** * **Nagler Reaction:** This is a biochemical test used to identify *C. perfringens*. Opalescence is inhibited on the side of the plate where antitoxin is spread (neutralization test). * **Stormy Fermentation:** *C. perfringens* produces acid and heavy gas in litmus milk, leading to a "stormy" appearance. * **Double Zone of Hemolysis:** On blood agar, *C. perfringens* produces a target-like appearance: an inner zone of complete hemolysis (theta toxin) and an outer zone of incomplete hemolysis (alpha toxin). * **Clinical Association:** It is the most common cause of **Gas Gangrene** (myonecrosis) and a frequent cause of food poisoning.

Question 996: VDRL is most sensitive for the diagnosis during which stage of syphilis?

A. Primary
B. Secondary (Correct Answer)
C. Tertiary
D. Congenital

Explanation: **Explanation:** The **VDRL (Venereal Disease Research Laboratory)** test is a non-treponemal screening test that detects **reagin antibodies** (IgM and IgG against cardiolipin-cholesterol-lecithin antigen). 1. **Why Secondary Syphilis is Correct:** During the secondary stage, the bacterial load of *Treponema pallidum* is at its peak, leading to a maximal systemic immune response. Consequently, the sensitivity of VDRL reaches **100%** during this stage. It is the gold standard time for screening, though the "Prozone phenomenon" (false negative due to antibody excess) can occasionally occur here. 2. **Why Other Options are Incorrect:** * **Primary Syphilis:** VDRL has a sensitivity of only **70–80%**. It often takes 1–4 weeks after the appearance of a chancre for reagin antibodies to reach detectable levels. Dark-field microscopy is the investigation of choice here. * **Tertiary Syphilis:** Sensitivity drops to approximately **70%**. In late stages, reagin titers naturally decline (and may even become non-reactive), making treponemal-specific tests (like FTA-ABS) more reliable. * **Congenital Syphilis:** While VDRL is used for diagnosis, sensitivity varies based on maternal treatment and timing of infection. It is not the stage where the test is "most" sensitive compared to the systemic peak of the secondary stage. **High-Yield Clinical Pearls for NEET-PG:** * **Prozone Phenomenon:** Seen in secondary syphilis; if VDRL is negative despite high clinical suspicion, dilute the serum to get a positive result. * **Monitoring Treatment:** VDRL is used to monitor response to therapy (titers should fall). Treponemal tests (FTA-ABS, TPHA) remain positive for life. * **False Positives (B-L-A-S-T):** **B**arbiturates, **L**eprosy/Lupus, **A**naemia/Aging, **S**yphilis (true positive), **T**uberculosis/Malaria.

Question 997: Enterobacteriaceae is classified based on which of the following tests?

A. Mannitol fermentation
B. Catalase and oxidase reaction
C. Oxygen requirement
D. Lactose fermentation (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Lactose Fermentation** The family **Enterobacteriaceae** consists of Gram-negative, non-spore-forming bacilli that are characteristically **Oxidase negative** and **Catalase positive**. In clinical microbiology, the primary and most essential step in classifying these organisms is their ability to ferment **Lactose**. This classification is visually demonstrated on **MacConkey Agar**: * **Lactose Fermenters (LF):** Produce acid, lowering the pH and turning the colonies **pink** (e.g., *Escherichia coli, Klebsiella*). * **Non-Lactose Fermenters (NLF):** Do not ferment lactose, resulting in **pale/colorless** colonies (e.g., *Salmonella, Shigella, Proteus, Yersinia*). * **Late Lactose Fermenters (LLF):** Ferment lactose slowly due to a lack of permease (e.g., *Shigella sonnei*). --- ### Analysis of Incorrect Options: * **A. Mannitol fermentation:** While used to differentiate species within a genus (e.g., *Staphylococcus aureus* from other Staphylococci or *Shigella* species), it is not the primary classification tool for the entire Enterobacteriaceae family. * **B. Catalase and oxidase reaction:** These tests are used to identify the family itself (Enterobacteriaceae are Oxidase negative, distinguishing them from *Pseudomonas* and *Vibrio*), but they do not classify members *within* the family. * **C. Oxygen requirement:** Most Enterobacteriaceae are **facultative anaerobes**. Since this trait is shared by almost all members, it cannot be used as a basis for classification. --- ### High-Yield Clinical Pearls for NEET-PG: * **Triple Sugar Iron (TSI) Agar:** Used to further differentiate Enterobacteriaceae based on the fermentation of glucose, lactose, and sucrose, along with gas and $H_2S$ production. * **IMViC Tests:** (Indole, Methyl Red, Voges-Proskauer, Citrate) are the gold standard for biochemical differentiation of genera within this family. * **Exception:** *Plesiomonas* is the only member of Enterobacteriaceae that is **Oxidase positive**.

Question 998: A healthy 45-year-old female who underwent root canal treatment approximately three weeks ago now presents with a new heart murmur, fever, painful skin nodules, abdominal pain, and an abnormal liver function test. Which of the following organisms is most likely to cause endocarditis and is implicated in dental caries or root canal infections?

A. Alpha-hemolytic streptococci (Correct Answer)
B. Bacteroides fragilis
C. Escherichia coli
D. Lactobacillus species

Explanation: **Explanation:** The clinical presentation of fever, a new heart murmur, painful skin nodules (Osler nodes), and systemic involvement (abdominal pain/abnormal LFTs suggesting embolic phenomena) following a dental procedure is a classic description of **Subacute Bacterial Endocarditis (SBE)**. **1. Why Alpha-hemolytic streptococci is correct:** The **Viridans group streptococci (VGS)**, which are alpha-hemolytic, are normal commensals of the oral cavity. They are the most common cause of SBE on previously damaged or native heart valves. Procedures like root canals or tooth extractions cause transient bacteremia, allowing these organisms to seed the endocardium. Specifically, *Streptococcus mutans* and *S. sanguinis* are heavily implicated in dental caries and biofilm formation on heart valves via the production of **dextrans** (extracellular polysaccharides). **2. Why the other options are incorrect:** * **Bacteroides fragilis:** While an anaerobe found in the mouth, it is more commonly associated with intra-abdominal abscesses and rarely causes endocarditis. * **Escherichia coli:** A common cause of UTIs and sepsis, but it is a rare cause of endocarditis (part of the non-HACEK gram-negative group). * **Lactobacillus species:** Though found in the oral flora and associated with deep dental caries, they are extremely rare causes of endocarditis, usually seen only in severely immunocompromised patients. **Clinical Pearls for NEET-PG:** * **Most common cause of SBE:** Viridans Streptococci (Alpha-hemolytic). * **Most common cause of Acute Endocarditis (and in IV drug users):** *Staphylococcus aureus*. * **Culture-negative endocarditis:** Often caused by the **HACEK** group (*Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella*). * **Dextran production:** The key virulence factor for VGS to adhere to fibrin-platelet aggregates on heart valves.

Question 999: Which of the following statements regarding certain strains of Streptococcus mutans is/are true?

A. a) Have been proved to be the cause of human dental caries by means of control studies. (Correct Answer)
B. b) Depend for their cariogenicity on the production of insoluble polysaccharides.
C. c) Cause dental caries by the production of lactic and other acids from fermentation of sugar.
D. d) Are a cause of infective endocarditis.

Explanation: **Explanation:** **Streptococcus mutans** is a Gram-positive coccus and a primary member of the viridans group. It is widely recognized as the principal etiological agent of human dental caries. **1. Why Option A is Correct:** The causal relationship between *S. mutans* and dental caries has been rigorously established through **controlled clinical and epidemiological studies**. These studies demonstrated that the presence and concentration of *S. mutans* in dental plaque correlate directly with the initiation and progression of enamel decay, fulfilling modified Koch’s postulates for dental infections. **2. Analysis of Other Options:** * **Option B:** While *S. mutans* produces extracellular polysaccharides (glucans) via glucosyltransferase, its cariogenicity is not *solely* dependent on the **insolubility** of these polysaccharides. It depends on the ability to form a biofilm (plaque) and survive in acidic environments. * **Option C:** While *S. mutans* is indeed acidogenic (producing lactic acid), this is a general characteristic of many oral bacteria (e.g., *Lactobacillus*). The question asks for a specific statement that defines the "proven" status of *S. mutans* in clinical research. * **Option D:** While *S. mutans* can cause infective endocarditis (IE) following dental procedures, it is **Streptococcus sanguinis** and **Streptococcus mitis** that are more frequently implicated in IE within the viridans group. **High-Yield Clinical Pearls for NEET-PG:** * **Glucosyltransferase:** The enzyme used by *S. mutans* to convert sucrose into dextrans (sticky polysaccharides) that facilitate adherence to tooth enamel. * **Aciduric Nature:** *S. mutans* can thrive in low pH environments (below 5.5), which is the critical pH for enamel demineralization. * **Transmission:** It is typically transmitted vertically from mother to child (the "window of infectivity"). * **Prophylaxis:** In patients with prosthetic valves undergoing dental work, *S. mutans* bacteremia is a risk, necessitating antibiotic prophylaxis (usually Amoxicillin).

Question 1000: Friedlander's bacillus is the eponym of which bacterium?

A. Klebsiella rhinoscleromatis
B. Klebsiella pneumoniae (Correct Answer)
C. Proteus vulgaris
D. Bacillus anthracis

Explanation: **Explanation:** **Klebsiella pneumoniae** is historically known as **Friedlander's bacillus**. It was first described by Carl Friedländer in 1882 as a causative agent of lobar pneumonia. It is a Gram-negative, non-motile, encapsulated bacillus belonging to the Enterobacteriaceae family. The prominent polysaccharide capsule is responsible for its characteristic mucoid colony appearance on culture media (like MacConkey agar) and its virulence. **Analysis of Options:** * **A. Klebsiella rhinoscleromatis:** While part of the same genus, this organism specifically causes **Rhinoscleroma**, a chronic granulomatous infection of the nose and upper respiratory tract characterized by Mikulicz cells. * **C. Proteus vulgaris:** Known for its "swarming motility" on agar and production of urease, but it does not carry the Friedlander eponym. * **D. Bacillus anthracis:** Known as the **Anthrax bacillus**. It is a Gram-positive, spore-forming rod, famously described by Robert Koch. **High-Yield Clinical Pearls for NEET-PG:** * **Pneumonia Profile:** Typically affects alcoholics and diabetics. It causes "bulging fissure sign" on X-ray and produces characteristic **"Red Currant Jelly" sputum**. * **Morphology:** Large, Gram-negative, "safety-pin" appearance (due to the capsule). * **Biochemicals:** It is a **Lactose Fermenter (LF)**, Indole negative (usually), and Citrate positive. * **Drug Resistance:** A major producer of Carbapenemases (KPC), making it a significant healthcare-associated infection (HAI) pathogen.

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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