Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 761: Which of the following bacteria produces a red pigment?

A. Escherichia coli
B. Bordetella parapertussis
C. Pseudomonas aeruginosa
D. Serratia marcescens (Correct Answer)

Explanation: **Explanation:** The correct answer is **Serratia marcescens**. **1. Why Serratia marcescens is correct:** *Serratia marcescens* is a Gram-negative rod belonging to the Enterobacteriaceae family. Its most defining characteristic in laboratory diagnosis is the production of a non-diffusible, bright red/pink pigment called **prodigiosin**. This pigment is typically produced when the bacteria are grown at room temperature (20–25°C) rather than at 37°C. In clinical settings, this can sometimes be seen as a "pink film" in bathrooms or on medical equipment. **2. Analysis of Incorrect Options:** * **Escherichia coli:** A Gram-negative rod that is a lactose fermenter. On MacConkey agar, it produces pink colonies due to acid production, but it does not produce an endogenous red pigment. * **Bordetella parapertussis:** This organism is known for producing a **brown pigment** on Bordet-Gengou or Regan-Lowe medium, not red. * **Pseudomonas aeruginosa:** Famous for producing various pigments, most notably **Pyocyanin** (blue-green) and **Pyoverdin** (fluorescent yellow-green). It does not produce red pigment. **3. NEET-PG High-Yield Pearls:** * **Serratia marcescens:** Often associated with hospital-acquired infections (UTIs, pneumonia) and endocarditis in IV drug users. It is frequently multi-drug resistant. * **Other Pigment Producers:** * *Staphylococcus aureus:* Golden yellow (Staphyloxanthin). * *Pseudomonas aeruginosa:* Blue-green (Pyocyanin). * *Mycobacterium kansasii:* Yellow/Orange (Photochromogen). * *Corynebacterium erythrasma:* Coral red fluorescence under Wood’s lamp (due to porphyrins). * **Mnemonic:** Remember **"Red Serratia"** to recall the pigment association quickly during the exam.

Question 762: Verocytotoxin is found in the ______ of individuals suffering from hemolytic uremic syndrome?

A. Blood (Correct Answer)
B. Tissues
C. Faeces
D. Saliva

Explanation: **Explanation:** Hemolytic Uremic Syndrome (HUS) is a serious complication primarily caused by **Enterohemorrhagic *Escherichia coli* (EHEC)**, specifically the O157:H7 strain. The pathogenesis involves the production of **Verocytotoxin** (also known as Shiga-like toxin). 1. **Why Blood is Correct:** While the bacteria colonize the intestine, the Verocytotoxin is absorbed into the **systemic circulation (blood)**. Once in the bloodstream, the toxin binds to **Gb3 receptors** located on the vascular endothelial cells, particularly in the kidneys. This leads to endothelial damage, microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. Detecting the toxin or its effects in the blood is central to the systemic manifestation of HUS. 2. **Why Incorrect Options are Wrong:** * **Faeces:** While EHEC bacteria and toxins are present in faeces during the initial diarrheal phase, HUS is a **systemic sequela**. By the time HUS develops, the toxin has already entered the circulation to cause distal organ damage. * **Tissues:** Although the toxin binds to specific tissues (like the renal glomeruli), it is transported via the blood. "Blood" is the primary clinical medium where the circulating toxin is identified in the context of the syndrome's progression. * **Saliva:** Verocytotoxin is not secreted or diagnostic in saliva. **High-Yield Clinical Pearls for NEET-PG:** * **Triad of HUS:** Microangiopathic hemolytic anemia (schistocytes on smear), Thrombocytopenia, and Acute Renal Failure. * **Mechanism:** Verocytotoxin inhibits protein synthesis by damaging the **28S ribosomal RNA** (60S subunit). * **Antibiotic Caution:** Avoid using antibiotics for EHEC diarrhea, as they can trigger increased toxin release, potentially worsening the risk of HUS.

Question 763: Helicobacter pylori is carcinogenic due to all mechanisms EXCEPT:

A. Cag A gene
B. Vac A gene
C. Elaboration of interleukin 8
D. Causes RNA splicing (Correct Answer)

Explanation: **Explanation:** *Helicobacter pylori* is a Class I carcinogen strongly associated with gastric adenocarcinoma and MALToma. Its oncogenic potential is driven by chronic inflammation and direct cellular manipulation, but it **does not** involve RNA splicing. **Why Option D is Correct:** *H. pylori* exerts its effects at the **DNA and protein levels**, not by interfering with the spliceosome or RNA splicing machinery. Carcinogenesis is primarily driven by DNA damage from reactive oxygen species (ROS) and the activation of intracellular signaling pathways that promote cell proliferation and inhibit apoptosis. **Why the Other Options are Incorrect:** * **CagA (Cytotoxin-associated gene A):** This is the most important virulence factor. Once injected into gastric epithelial cells via a Type IV secretion system, it acts as a "bacterial oncoprotein," disrupting cell junctions and activating pro-proliferative pathways (SHP-2). * **VacA (Vacuolating cytotoxin A):** This toxin induces large cytoplasmic vacuoles and triggers apoptosis in epithelial cells. The resulting chronic tissue damage and compensatory hyperproliferation increase the risk of malignant transformation. * **Interleukin-8 (IL-8):** *H. pylori* infection induces the gastric mucosa to secrete high levels of IL-8. This potent chemoattractant recruits neutrophils, leading to chronic active gastritis. The resulting "free radical storm" causes oxidative DNA damage, a prerequisite for cancer. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Endoscopic biopsy followed by Histopathology (Warthin-Starry or Giemsa stain). * **Non-invasive Screening:** Urea Breath Test (UBT) is the test of choice for documenting eradication. * **Urease Production:** Essential for survival in acidic gastric juice by creating an alkaline ammonia cloud. * **MALToma:** *H. pylori* is the only bacterium whose eradication can lead to the regression of a malignancy (low-grade MALT lymphoma).

Question 764: Albert's stain is used for the identification of which organism?

A. Staphylococcus
B. Corynebacterium diphtheriae (Correct Answer)
C. Clostridium perfringens
D. Clostridium tetani

Explanation: **Explanation:** **Albert’s stain** is a specialized differential stain used primarily for the presumptive identification of **Corynebacterium diphtheriae**. The underlying medical concept relies on the presence of **metachromatic granules** (also known as Volutin or Babes-Ernst granules). These granules are composed of polymerized inorganic polyphosphates and serve as energy reserves. When stained with Albert’s stain (which contains Toluidine blue and Malachite green), these granules exhibit **metachromasia**—they appear bluish-black, while the bacillary body stains green. This creates a characteristic "Chinese letter" or cuneiform arrangement, which is a high-yield diagnostic feature for Diphtheria. **Analysis of Incorrect Options:** * **Staphylococcus:** These are Gram-positive cocci in clusters. They do not possess metachromatic granules and are identified using Gram stain and biochemical tests like Catalase and Coagulase. * **Clostridium perfringens:** A Gram-positive, anaerobic, spore-forming bacillus known for causing gas gangrene. It is characterized by its "box-car" shape and "double zone of hemolysis" on blood agar, not metachromatic granules. * **Clostridium tetani:** Known for its "drumstick" appearance due to terminal spores. While it is a bacillus, it does not contain the specific polyphosphate granules targeted by Albert’s stain. **NEET-PG High-Yield Pearls:** * **Composition of Albert’s Stain:** Albert A (Toluidine blue, Malachite green, Glacial acetic acid) and Albert B (Iodine in Potassium Iodide). * **Other stains for C. diphtheriae:** Neisser’s stain and Ponder’s stain also target metachromatic granules. * **Culture Media:** Löffler's Serum Slope (rapid growth) and Potassium Tellurite Agar (black colonies).

Question 765: In which part of the gastrointestinal tract is Helicobacter pylori found in the greatest number?

A. Oesophagus
B. Gastric Antrum (Correct Answer)
C. Caecum
D. Oral cavity

Explanation: **Explanation:** *Helicobacter pylori* is a microaerophilic, spiral-shaped Gram-negative rod that specifically colonizes the gastric mucosa. The **Gastric Antrum** is the site where the organism is found in the greatest density because it provides the most favorable microenvironment for its survival and proliferation. **Why Gastric Antrum is Correct:** *H. pylori* thrives in the mucus layer overlying the gastric epithelium. It produces large amounts of **urease**, which converts urea into ammonia and bicarbonate, neutralizing gastric acid and creating a protective "alkaline cloud." The antrum is the preferred site because it has a lower density of acid-secreting parietal cells compared to the gastric body (corpus), making it easier for the bacteria to maintain this pH balance. **Why Other Options are Incorrect:** * **Oesophagus:** While *H. pylori* is associated with GERD complications, it does not colonize the squamous epithelium of the esophagus; it requires the specific surface receptors of gastric-type columnar epithelium. * **Caecum:** The alkaline environment and the presence of bile salts in the lower GI tract are generally inhibitory to *H. pylori*. * **Oral Cavity:** Although *H. pylori* DNA can sometimes be detected in dental plaque, it is considered a transient site rather than a primary reservoir for colonization. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Endoscopic biopsy followed by **Rapid Urease Test (RUT)** or Histopathology (Warthin-Starry silver stain). * **Non-invasive Test of Choice:** Urea Breath Test (using C13 or C14 isotopes) is used to confirm eradication. * **Virulence Factors:** **CagA** (associated with gastric cancer) and **VacA** (vacuolating cytotoxin). * **Associations:** Type B Gastritis, Peptic Ulcer Disease (Duodenal > Gastric), Gastric Adenocarcinoma, and MALT Lymphoma.

Question 766: A patient with peptic ulcer disease was investigated endoscopically and showed chronic antral gastritis. Which of the following dyes will be able to stain the specimen?

A. PAS stain
B. Zeihl-Neelsen stain
C. Gram stain
D. Warthin-Starry stain (Correct Answer)

Explanation: **Explanation:** The clinical presentation of chronic antral gastritis in a patient with peptic ulcer disease strongly suggests infection with **_Helicobacter pylori_**. This organism is a gram-negative, spiral-shaped bacterium that resides in the gastric mucus layer. **Why Warthin-Starry stain is correct:** _H. pylori_ is notoriously difficult to visualize using standard tissue stains like Hematoxylin and Eosin (H&E). **Warthin-Starry stain**, a silver-based staining method, is considered the "gold standard" for histological identification because it stains the bacteria dark brown to black against a golden-yellow background, providing excellent contrast and sensitivity. **Analysis of Incorrect Options:** * **PAS (Periodic Acid-Schiff) stain:** Primarily used to demonstrate carbohydrates, glycogen, and mucins. While it stains the gastric mucus, it does not specifically highlight _H. pylori_. * **Ziehl-Neelsen (ZN) stain:** Used for Acid-Fast Bacilli (AFB) like _Mycobacterium tuberculosis_. _H. pylori_ is not acid-fast. * **Gram stain:** Although _H. pylori_ is gram-negative, it stains poorly in tissue sections using standard Gram techniques. Modified Giemsa is preferred over Gram stain for routine tissue screening. **NEET-PG High-Yield Pearls:** * **Other Stains for _H. pylori_:** Modified Giemsa (most common/cost-effective), Genta stain, and Steiner stain (another silver stain). * **Urease Breath Test:** The non-invasive investigation of choice for "test and treat" strategies. * **Rapid Urease Test (RUT):** The invasive (endoscopic) bedside test of choice. * **Culture:** The most specific method but difficult due to the microaerophilic requirements of the organism.

Question 767: The 'L-form' phenomenon is most likely to occur in which of the following?

A. Mycobacterium tuberculosis
B. Pseudomonas
C. Proteus
D. Mycoplasma (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Mycoplasma** **Understanding the Concept:** The term **'L-form'** (named after the Lister Institute) refers to bacteria that have lost their cell wall but retain the ability to grow and divide. This phenomenon typically occurs when bacteria are exposed to cell-wall-inhibiting agents like Penicillin or Lysozyme. However, **Mycoplasma** is unique because it is the only genus that **naturally lacks a cell wall** entirely. Because they do not possess a peptidoglycan layer, they are inherently pleomorphic (variable in shape) and resemble the L-forms of other bacteria. In the context of this question, Mycoplasma is the classic representative of the L-form morphology in clinical microbiology. **Analysis of Incorrect Options:** * **A. Mycobacterium tuberculosis:** These are acid-fast bacilli with a complex, lipid-rich cell wall (mycolic acid). While they can survive intracellularly, they do not naturally exist as L-forms. * **B. Pseudomonas:** This is a Gram-negative rod with a standard cell wall structure. It only becomes an L-form under specific laboratory stress or antibiotic pressure. * **C. Proteus:** While *Proteus* is famous for "swarming" motility and was the organism in which L-forms were first discovered by Emmy Klieneberger-Nobel, it possesses a standard cell wall. Mycoplasma is the more definitive answer as it is *permanently* and *naturally* wall-less. **High-Yield Clinical Pearls for NEET-PG:** * **Antibiotic Resistance:** Because Mycoplasma lacks a cell wall, it is **intrinsically resistant** to Beta-lactams (Penicillins/Cephalosporins). * **Cell Membrane:** Mycoplasma is the only bacterium that contains **sterols** in its cell membrane (acquired from the host), providing structural integrity in the absence of a wall. * **Culture Appearance:** Mycoplasma colonies on agar exhibit a characteristic **"Fried Egg" appearance**. * **Smallest Living Organism:** Mycoplasma is the smallest free-living organism capable of self-replication.

Question 768: Which is a venereal form of treponemal infection?

A. Yaws
B. Pinta
C. Syphilis (Correct Answer)
D. Bejel

Explanation: **Explanation:** The genus *Treponema* consists of several species and subspecies that are morphologically identical but differ in clinical presentation and mode of transmission. **1. Why Syphilis is correct:** Syphilis is caused by ***Treponema pallidum* subspecies *pallidum***. It is the only treponemal infection primarily transmitted through **sexual contact (venereal)** or vertically from mother to fetus (congenital). It is characterized by three clinical stages (primary, secondary, and tertiary) and can affect almost every organ system. **2. Why the other options are incorrect:** The other three options are collectively known as **Endemic (Non-venereal) Treponematoses**. They are typically transmitted via direct skin-to-skin contact or shared utensils, usually in childhood under poor hygienic conditions. * **Yaws (Option A):** Caused by *T. pallidum* subsp. *pertenue*. It primarily affects the skin, bones, and joints in humid tropical regions. * **Pinta (Option B):** Caused by ***T. carateum***. It is the mildest form, restricted solely to the skin (dyschromic lesions), and found in Central and South America. * **Bejel (Option C):** Also known as Endemic Syphilis, caused by *T. pallidum* subsp. *endemicum*. It is found in arid regions (Middle East/Africa) and often presents with oral mucosal lesions. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Treponemes are thin, spiral organisms that cannot be grown on culture media (non-cultivable). They are visualized using **Dark Ground Microscopy (DGM)**. * **Serology:** All treponemal infections (venereal and non-venereal) give **positive results** on both non-specific (VDRL/RPR) and specific (TPHA/FTA-ABS) tests. * **Drug of Choice:** **Penicillin G** remains the treatment of choice for all treponemal infections. * **Rabbit Inoculation:** Since they don't grow in vitro, *T. pallidum* is maintained in the testes of rabbits (**Nichol’s strain**).

Question 769: Which of the following statements about Streptococcus is true?

A. Non-motile, non-sporing cocci
B. Grows only in media containing blood or serum
C. Ferments sugar to produce acid but no gas
D. All of the above (Correct Answer)

Explanation: **Explanation:** *Streptococcus* species are a diverse group of Gram-positive cocci arranged in chains or pairs. Understanding their fundamental microbiological characteristics is essential for NEET-PG. 1. **Morphology (Option A):** Streptococci are characteristically **non-motile** and **non-sporing**. They are Gram-positive spherical or oval cells. While most are non-capsulated, certain strains (like *S. pneumoniae*) possess a prominent polysaccharide capsule. 2. **Growth Requirements (Option B):** They are fastidious organisms. Unlike Staphylococci, they have complex nutritional requirements and **grow only in media enriched with blood, serum, or carbohydrates**. On blood agar, they exhibit specific hemolytic patterns (Alpha, Beta, or Gamma), which is a primary method of classification. 3. **Biochemical Properties (Option C):** Streptococci are **catalase-negative** (distinguishing them from Staphylococci). They are facultative anaerobes that obtain energy through the fermentation of sugars. This process **produces acid but no gas**, a key biochemical marker in laboratory identification. Since all three statements accurately describe the genus *Streptococcus*, **Option D is the correct answer.** **High-Yield Clinical Pearls for NEET-PG:** * **Catalase Test:** The gold standard to differentiate *Streptococcus* (-ve) from *Staphylococcus* (+ve). * **M Protein:** The most important virulence factor of *S. pyogenes* (Group A Strep); it is anti-phagocytic and responsible for molecular mimicry in Rheumatic Fever. * **Quellung Reaction:** Used for the identification of *S. pneumoniae* (capsular swelling). * **Bacitracin Sensitivity:** Used to differentiate *S. pyogenes* (Sensitive) from other Beta-hemolytic streptococci (Resistant).

Question 770: Differentiation of Neisseria gonorrhoeae and Neisseria meningitides is by which test?

A. Glucose fermentation
B. Maltose fermentation (Correct Answer)
C. Voges-Proskauer reaction
D. Indole test

Explanation: ### Explanation The differentiation between *Neisseria gonorrhoeae* (Gonococcus) and *Neisseria meningitidis* (Meningococcus) is primarily based on their **carbohydrate utilization patterns** (oxidative fermentation). **1. Why Maltose Fermentation is Correct:** Both species are Gram-negative diplococci that ferment **Glucose**. However, they differ in their ability to ferment **Maltose**. * ***N. meningitidis*** ferments **both** Glucose and Maltose. * ***N. gonorrhoeae*** ferments **only** Glucose. * *Mnemonic:* **M**eningitidis ferments **M**altose and glucose; **G**onorrhoeae ferments **G**lucose only. **2. Analysis of Incorrect Options:** * **A. Glucose fermentation:** Both species ferment glucose; therefore, this test cannot be used to differentiate between them. * **C. Voges-Proskauer (VP) reaction:** This test detects acetoin production (used for Enterobacteriaceae like *Klebsiella*). *Neisseria* species are VP negative. * **D. Indole test:** This test identifies organisms capable of breaking down tryptophan into indole (e.g., *E. coli*). *Neisseria* species are Indole negative. --- ### High-Yield Clinical Pearls for NEET-PG: * **Oxidase & Catalase:** All medically important *Neisseria* species are **Oxidase positive** and **Catalase positive**. * **Culture Media:** *N. gonorrhoeae* is more fastidious and is typically grown on **Thayer-Martin Medium** (Selective) or Chocolate agar. * **Virulence Factor:** The primary virulence factor for *N. meningitidis* is its **polysaccharide capsule** (used for serotyping), whereas *N. gonorrhoeae* is **non-capsulated** and relies on **pili** for attachment. * **Specimen Transport:** *Neisseria* are sensitive to cold and drying; specimens should be transported in **Stuart’s or Amies medium** and never refrigerated.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

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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