Bacteriology Practice Questions

Q: Which transport medium is used for cholera?

Cary Blair medium. **Explanation:** **Vibrio cholerae**, the causative agent of cholera, is highly sensitive to dehydration and acidic environments. Therefore, specialized transport media are required to maintain the viability of the organism during transit from the field to the laboratory. **1. Why Cary-Blair Medium is Correct:** Cary-Blair medium is the **gold standard** transport medium for *Vibrio cholerae*. It is a modified version of Stuart’s medium with a higher pH (8.4). Since *Vibrio* species are halophilic and prefer alkaline conditions, this high pH inhibits the overgrowth of competing intestinal flora (like Enterobacteriaceae) while preserving the pathogen. It lacks a carbon source, preventing the rapid multiplication of bacteria that could lead to a change in pH. **2. Analysis of Incorrect Options:** * **LJ (Löwenstein–Jensen) Medium:** This is a solid, egg-based **selective/enrichment medium** used specifically for the cultivation of *Mycobacterium tuberculosis*. * **MYPA (Mannitol Yolk Polymyxin Agar) Medium:** This is a **selective and differential medium** used for the isolation of *Bacillus cereus*. * **Stewart Medium:** A classic transport medium used primarily for delicate organisms like *Neisseria gonorrhoeae*. While it can be used for stool, Cary-Blair is superior for *Vibrio* due to its specific alkaline buffering. **Clinical Pearls for NEET-PG:** * **Other Transport Media for Cholera:** VR (Venkatraman-Ramakrishnan) medium and Autoclaved Sea Water. * **Enrichment Media:** Alkaline Peptone Water (APW) and Monsur’s Taurocholate Tellurite Peptone Water. * **Selective Media:** TCBS (Thiosulfate Citrate Bile Salts Sucrose) Agar—*V. cholerae* produces characteristic **yellow colonies** due to sucrose fermentation. * **Holding Medium:** Buffered Glycerol Saline (Note: This is **not** suitable for *Vibrio* as it inhibits its growth).

Q: "Darting motility" is shown by:

Vibrio. **Explanation:** The correct answer is **Vibrio cholerae**. Motility in bacteria is determined by the presence and arrangement of flagella. *Vibrio cholerae* possesses a single polar flagellum (monotrichous) that acts like a powerful propeller, resulting in a rapid, jerky, and aggressive movement known as **"Darting Motility."** This is often compared to a "shooting star" appearance when viewed under dark-ground or phase-contrast microscopy. **Analysis of Options:** * **A. Proteus:** Exhibits **"Swarming Motility"** due to its peritrichous flagella. On solid media (like blood agar), it creates concentric ripples or waves rather than discrete colonies. * **B. Serratia:** Some species are motile via peritrichous flagella, but they are better known for producing a red pigment called **prodigiosin**. * **D. E. coli:** Shows simple **peritrichous motility** (non-directional swimming), which is much slower and less distinctive than the darting motion of Vibrio. **High-Yield Clinical Pearls for NEET-PG:** * **Vibrio cholerae:** Darting motility can be inhibited by adding specific **antisera** (the immobilization test), which is a diagnostic feature. * **Other Motility Patterns to Remember:** * **Tumbling Motility:** *Listeria monocytogenes* (at 25°C). * **Falling Leaf Motility:** *Giardia lamblia* (Parasitology). * **Corkscrew/Flexion-Extension Motility:** *Spirochetes* (e.g., *Treponema pallidum*). * **Stately Motility:** *Bacillus anthracis* (though usually non-motile, related species show this). * **Gliding Motility:** *Mycoplasma pneumoniae*.

Q: Draughtsman appearance is seen with colonies of which bacterium?

Pneumococcus. **Explanation:** **Pneumococcus (*Streptococcus pneumoniae*)** is the correct answer. The characteristic **"Draughtsman" or "Checkerboard" appearance** refers to the morphology of its colonies on blood agar after 24–48 hours of incubation. The underlying mechanism is **autolysis**. *S. pneumoniae* produces an intracellular autolytic enzyme called **LytA (amidases)**. As the culture ages, the central part of the colony undergoes autolysis, causing it to collapse or flatten, while the periphery remains raised. This results in a colony with a depressed center and raised edges, resembling the pieces used in a game of draughts (checkers). **Analysis of Incorrect Options:** * **Streptococcus (Viridans/Pyogenes):** These typically produce small, convex, or "matt/glossy" colonies. They do not possess the potent autolytic mechanism required to create the central depression seen in Pneumococci. * **Staphylococcus:** These form large, opaque, convex, "oil-paint" like colonies. *S. aureus* is known for its golden-yellow pigmentation, not a draughtsman appearance. * **Meningococcus:** *Neisseria meningitidis* forms medium-sized, convex, greyish-white, translucent, and mucoid colonies on chocolate agar. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, lancet-shaped diplococci. * **Capsule:** The main virulence factor; identified by the **Quellung Reaction** (capsular swelling). * **Bile Solubility Test:** Pneumococci are bile-soluble (autolysis is enhanced by bile salts), which differentiates them from *S. viridans*. * **Optochin Sensitivity:** Pneumococci are sensitive to Optochin (P-disk), whereas *S. viridans* is resistant. * **Hemolysis:** Shows **Alpha-hemolysis** (partial/greenish) under aerobic conditions.

Q: Which of the following bacterial toxins works by the same mechanism as Exotoxin A of Pseudomonas?

Diphtheria toxin. **Explanation:** The correct answer is **Diphtheria toxin**. Both **Pseudomonas Exotoxin A** and **Diphtheria toxin** (produced by *Corynebacterium diphtheriae*) share the exact same mechanism of action: they are **ADP-ribosyltransferases** that target **Elongation Factor-2 (EF-2)**. By attaching an ADP-ribose group to EF-2, they inhibit protein synthesis, leading to cell death. **Analysis of Options:** * **Diphtheria toxin (Correct):** Identical mechanism to Pseudomonas Exotoxin A (ADP-ribosylation of EF-2). * **Tetanus toxin:** Acts as a protease that cleaves **SNARE proteins** (specifically synaptobrevin). This prevents the release of inhibitory neurotransmitters (GABA and Glycine) from Renshaw cells, leading to spastic paralysis. * **Pertussis toxin:** Works by ADP-ribosylation of the **inhibitory G-protein (Gi)**. This leads to an increase in intracellular cAMP, interfering with immune cell signaling. * **Cholera toxin:** Works by ADP-ribosylation of the **stimulatory G-protein (Gs)**. This results in constitutive activation of adenylate cyclase, leading to massive increases in cAMP and subsequent secretory diarrhea. **High-Yield NEET-PG Pearls:** * **EF-2 Inhibitors:** Only two major toxins inhibit EF-2: Diphtheria toxin and Pseudomonas Exotoxin A. * **A-B Toxins:** All options listed (A, B, C, D) are A-B structural toxins, where the 'B' subunit binds to the cell surface and the 'A' subunit possesses the active enzymatic toxicity. * **Pseudomonas:** While Exotoxin A is its most important toxin, it also produces **Exoenzyme S**, which acts on the cytoskeleton. * **Diphtheria:** The toxin gene is encoded by a **lysogenic bacteriophage** (Beta-phage).

Q: Which toxin of Streptococcus causes hemolysis around colonies?

Streptolysin S. **Explanation:** The correct answer is **Streptolysin S**. *Streptococcus pyogenes* (Group A Strep) produces two main hemolysins that damage red blood cell membranes, leading to the characteristic zone of beta-hemolysis on blood agar. 1. **Streptolysin S (SLS):** This is a cell-bound, oxygen-stable hemolysin. Because it is not inactivated by oxygen, it is responsible for the **surface hemolysis** seen around colonies when incubated aerobically. It is non-antigenic. 2. **Streptolysin O (SLO):** This is an oxygen-labile hemolysin. It is inactivated by oxygen; therefore, it only contributes to hemolysis deep in the agar (subsurface hemolysis) where oxygen tension is low. Unlike SLS, it is highly antigenic. **Analysis of Incorrect Options:** * **A. Streptolysin O:** While it causes hemolysis, it does not cause the visible surface hemolysis around colonies under standard aerobic conditions. It is primarily used clinically for detecting recent infections (ASO titer). * **C. Streptodornase:** Also known as DNase, this enzyme liquefies thick pus by degrading DNA. it does not cause hemolysis. * **D. Hyaluronidase:** Known as the "spreading factor," it degrades hyaluronic acid in connective tissue to facilitate the spread of infection. **High-Yield Clinical Pearls for NEET-PG:** * **ASO Titer:** An Anti-Streptolysin O (ASO) titer >200 units is significant for diagnosing post-streptococcal sequelae like Rheumatic Fever. * **Dick Test:** Historically used to identify susceptibility to Scarlet Fever (caused by Erythrogenic toxin). * **CAMP Test:** Used to identify Group B Streptococci (*S. agalactiae*), which produces a factor that enhances the hemolysis of *Staphylococcus aureus*.

Q: Elek's gel precipitation test is used for diagnosing which bacterium?

Corynebacterium diphtheriae. **Explanation:** **Elek’s gel precipitation test** is a specialized *in vitro* toxigenicity test used to detect the production of exotoxin by **Corynebacterium diphtheriae**. **1. Why Corynebacterium diphtheriae is correct:** The pathogenicity of *C. diphtheriae* depends on the production of the diphtheria toxin, encoded by the *tox* gene (introduced by a lysogenic β-phage). Since not all strains are toxigenic, Elek’s test is performed to confirm virulence. In this test, a filter paper strip impregnated with antitoxin is placed on an agar plate, and the test strain is streaked perpendicular to it. If the strain is toxigenic, the toxin reacts with the antitoxin, forming visible **white precipitin lines** (forming an arrowhead pattern) at a 45-degree angle. **2. Why the other options are incorrect:** * **Corynebacterium jeikeium:** This is a multi-drug resistant opportunistic pathogen typically associated with prosthetic valve endocarditis and septicemia in immunocompromised patients; it does not produce diphtheria toxin. * **Clostridium tetani:** Diagnosis is primarily clinical. While it produces tetanospasmin, toxigenicity is usually demonstrated via *in vivo* animal inoculation (mouse lethality test), not Elek’s test. * **Clostridium perfringens:** Known for gas gangrene and food poisoning, it is identified by the **Nagler reaction** (detecting lecithinase activity) and the "stormy fermentation" of milk. **Clinical Pearls for NEET-PG:** * **Gold Standard:** While Elek’s test is classic, **PCR** for the *tox* gene is the rapid modern method. * **Culture Media:** *C. diphtheriae* grows on **Löffler's serum slope** (rapid growth) and **Potassium Tellurite Agar** (black colonies). * **Morphology:** Characterized by **Albert’s stain** showing metachromatic (volutin) granules and a "Chinese letter" arrangement.

Q: Which enterotoxin is responsible for most cases of food poisoning caused by Staphylococcus?

Type A. **Explanation:** Staphylococcal food poisoning is caused by the ingestion of preformed enterotoxins produced by *Staphylococcus aureus*. These toxins act as **superantigens**, triggering massive cytokine release and stimulating the vagus nerve, which leads to the characteristic rapid onset of projectile vomiting and abdominal cramps. * **Correct Option (D): Type A** is the most common enterotoxin associated with staphylococcal food poisoning worldwide. It is highly potent and heat-stable (withstanding boiling for 30 minutes), which allows it to persist in food even after the bacteria have been killed by cooking. * **Option (B) & (C): Type B and C** are less common causes of food poisoning. Enterotoxin B is more frequently associated with staphylococcal enterocolitis and is also studied as a potential bioweapon due to its ability to cause severe systemic inflammation. * **Option (A): Type E** is a rare cause of foodborne outbreaks compared to Type A. **High-Yield Clinical Pearls for NEET-PG:** * **Incubation Period:** Very short (1–6 hours) because the toxin is preformed in the food (e.g., creamy salads, processed meats, dairy). * **Heat Stability:** The toxin is heat-stable, whereas the organism itself is heat-labile. * **Mechanism:** Superantigen-mediated stimulation of the vomiting center via the vagus nerve. * **Diagnosis:** Primarily clinical; culture of the food is more useful than stool culture. * **TSST-1:** Do not confuse enterotoxins with Toxic Shock Syndrome Toxin-1, which causes systemic shock rather than food poisoning.

Q: A boy presents with a skin ulcer on his leg. Culture from the ulcer reveals beta hemolysis. Cultures from school children with a sore throat from some days prior also revealed beta hemolysis. What is the similarity between the organisms isolated from both sources?

C carbohydrate antigen is the same. The clinical presentation describes two different infections: **Impetigo/Pyoderma** (skin ulcer) and **Pharyngitis** (sore throat). Both are caused by *Streptococcus pyogenes*, also known as **Group A Streptococcus (GAS)**. ### Why Option C is Correct *Streptococcus pyogenes* is characterized by **Lancefield grouping**, which classifies Beta-hemolytic streptococci based on the specific **C-carbohydrate antigen** located in the cell wall. Regardless of the site of infection (skin or throat) or the specific strain, all *S. pyogenes* isolates share the **Group A** C-carbohydrate antigen. This is the fundamental similarity between the two isolates. ### Why Other Options are Incorrect * **Option A:** The *mecA* gene is associated with Methicillin-resistant *Staphylococcus aureus* (MRSA), not Streptococcus. * **Option B:** The **M protein** is the chief virulence factor of GAS. However, there are over 100 different M-protein serotypes. Generally, "pyodermal strains" (e.g., type 49) differ from "pharyngeal strains" (e.g., types 1, 3, 12). * **Option D:** The strains are usually different. Strains causing skin infections are typically "nephritogenic" (linked to PSGN), while those causing pharyngitis are "rheumatogenic" (linked to Rheumatic Fever). ### NEET-PG High-Yield Pearls * **Lancefield Grouping:** Based on C-carbohydrate (Group A to V). Note: *S. pneumoniae* and Viridans group lack this antigen. * **M Protein:** Inhibits phagocytosis by interfering with the alternate complement pathway (C3b). * **Complications:** * **Pharyngitis** can lead to both Acute Rheumatic Fever (ARF) and Post-Streptococcal Glomerulonephritis (PSGN). * **Skin infections** (Pyoderma) can lead to PSGN but **never** lead to Rheumatic Fever.

Question 1

Which transport medium is used for cholera?

Accepted Answer

Cary Blair medium

Answer

LJ medium

Answer

MYPA medium

Answer

Stewart medium

Question 2

"Darting motility" is shown by:

Accepted Answer

Vibrio

Answer

Proteus

Answer

Serratia

Answer

E. coli

Question 3

Draughtsman appearance is seen with colonies of which bacterium?

Accepted Answer

Pneumococcus

Answer

Streptococcus

Answer

Staphylococcus

Answer

Meningococcus

Question 4

Which of the following statements about Listeria monocytogenes is true?

Accepted Answer

A. Listeria monocytogenes is a food borne pathogen, B. Humans are probably accidental hosts for this microorganism, C. Case-fatality rate of listeria meningitis is reported to be 15-26%, D. Granulomatosis infantiseptica is an overwhelming listerial fetal infection with miliary micro abscesses and granulomas, and E. Adults should receive IV ampicillin at high doses (2grams every 4 hours), are all True.

Answer

A. Listeria monocytogenes is a food borne pathogen, B. Humans are probably accidental hosts for this microorganism, D. Granulomatosis infantiseptica is an overwhelming listerial fetal infection with miliary micro abscesses and granulomas, are True and C, E are False.

Answer

B. Humans are probably accidental hosts for this microorganism, C. Case-fatality rate of listeria meningitis is reported to be 15-26%, D. Granulomatosis infantiseptica is an overwhelming listerial fetal infection with miliary micro abscesses and granulomas, E. Adults should receive IV ampicillin at high doses (2grams every 4 hours), are True and A is False.

Answer

B. Humans are probably accidental hosts for this microorganism and E. Adults should receive IV ampicillin at high doses (2grams every 4 hours), are True and A, C, D are False.

Question 5

Which of the following bacterial toxins works by the same mechanism as Exotoxin A of Pseudomonas?

Accepted Answer

Diphtheria toxin

Answer

Tetanus toxin

Answer

Pertussis toxin

Answer

Cholera toxin

Question 6

Which toxin of Streptococcus causes hemolysis around colonies?

Accepted Answer

Streptolysin S

Answer

Streptolysin O

Answer

Streptodornase

Answer

Hyaluronidase

Question 7

Elek's gel precipitation test is used for diagnosing which bacterium?

Accepted Answer

Corynebacterium diphtheriae

Answer

Corynebacterium jakeium

Answer

Clostridium tetani

Answer

Clostridium perfringens

Question 8

Which of the following statements about Corynebacterium diphtheriae is false?

Accepted Answer

Toxin production is mediated by a lysogenic bacteriophage.

Answer

Organism may be identified by tests for toxigenicity.

Answer

Toxin acts by inhibiting protein synthesis.

Answer

Toxin may affect the heart and nerves.

Question 9

Which enterotoxin is responsible for most cases of food poisoning caused by Staphylococcus?

Accepted Answer

Type A

Answer

Type E

Answer

Type C

Answer

Type B

Question 10

A boy presents with a skin ulcer on his leg. Culture from the ulcer reveals beta hemolysis. Cultures from school children with a sore throat from some days prior also revealed beta hemolysis. What is the similarity between the organisms isolated from both sources?

Accepted Answer

C carbohydrate antigen is the same

Answer

Mec A gene is related to it

Answer

M protein is the same

Answer

The strains causing both conditions are the same

Bacteriology — MCQs

Bacteriology — MCQs

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