Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2141: Which of the following statements about El Tor and Classical Vibrio is true?

A. Classical is associated with more carriers
B. All of the options
C. El Tor is more severe
D. El Tor has a higher carrier rate (Correct Answer)

Explanation: ***El Tor has a higher carrier rate*** - **El Tor biotype Vibrio cholerae** is associated with a **higher carrier rate** and a greater proportion of asymptomatic infections compared to the Classical biotype. - This characteristic contributes to the **greater epidemic potential** and widespread dissemination of El Tor strains. *Classical is associated with more carriers* - The **Classical biotype** generally produces a more severe clinical disease but is associated with a **lower proportion of asymptomatic carriers**. - This means that individuals infected with Classical *Vibrio cholerae* are more likely to develop obvious symptoms, thus reducing the number of silent carriers. *All of the options* - This option is incorrect because only one of the presented statements regarding carrier rates and severity is factually accurate when comparing El Tor and Classical *Vibrio cholerae*. - The characteristics of each biotype pertaining to carrier rates and disease severity are distinct and not universally true for all statements. *El Tor is more severe* - **El Tor biotype infections** are typically **less severe** clinically, often presenting with milder symptoms or asymptomatic carriage, compared to Classical biotype infections. - The **Classical biotype** is historically associated with more cases of **severe, dehydrating cholera**, leading to rapid fluid loss and high mortality if untreated.

Question 2142: True about tetanus are all EXCEPT?

A. Spread along the nerves
B. Requires oxygen (Correct Answer)
C. Incubation period is variable
D. Tetanus bacilli cannot spread through blood

Explanation: ***Requires oxygen*** - *Clostridium tetani*, the bacteria causing tetanus, is an **anaerobic bacterium**, meaning it cannot grow in the presence of oxygen. - The spores germinate and produce toxins in **anaerobic conditions**, typically in deep, contaminated wounds. *Spread along the nerves* - The toxin produced by *Clostridium tetani* (tetanospasmin) travels via the **peripheral motor nerves** and then to the central nervous system. - This **retrograde axonal transport** is how the neurotoxin reaches the spinal cord and brainstem to exert its effects. *Incubation period is variable* - The incubation period for tetanus can vary significantly, typically ranging from **3 to 21 days**, but can be longer or shorter. - Shorter incubation periods are often associated with more severe disease and wounds closer to the central nervous system. *Tetanus bacilli cannot spread through blood* - The disease is caused by the **toxins** produced by the bacteria at the wound site, not by the systemic spread of the bacteria itself. - While the toxin is transported, primarily neuronally, the **bacterial cells do not spread** throughout the body via the bloodstream.

Question 2143: Which of the following is the first step in Gram staining?

A. Mordant- iodine treatment
B. Alcohol wash
C. Safranin
D. Crystal violet (Correct Answer)

Explanation: ***Crystal violet*** - **Crystal violet** is the **primary stain** used in Gram staining, coloring all bacterial cells purple. - It works by penetrating the **peptidoglycan layer** of both Gram-positive and Gram-negative bacteria. *Mordant- iodine treatment* - The **iodine solution** acts as a **mordant**, forming a crystal violet-iodine complex within the cell. - This step occurs *after* the primary staining with crystal violet to help retain the stain. *Alcohol wash* - The **alcohol wash** is a **decolorizing agent** applied *after* the mordant step. - It removes the crystal violet-iodine complex from Gram-negative bacteria but not from Gram-positive cells. *Safranin* - **Safranin** is the **counterstain** used in Gram staining, applied *last*. - It stains the decolorized Gram-negative cells **pink or red** to make them visible under the microscope.

Question 2144: "Endemic" typhus is caused by:

A. Rickettsia mooseri (Correct Answer)
B. Coxiella burnetii
C. Ehrlichia chafeensis
D. Rickettsia prowazekii

Explanation: ***Rickettsia mooseri (Rickettsia typhi)*** - **Endemic (murine) typhus** is caused by *Rickettsia typhi*, also known as *Rickettsia mooseri*. - This form of typhus is typically transmitted to humans through contact with **feces of infected rat fleas** (*Xenopsylla cheopis*). - Clinical features include fever, headache, and maculopapular rash. *Coxiella burnetii* - *Coxiella burnetii* is the causative agent of **Q fever**, which presents differently from typhus. - Q fever is primarily transmitted by inhaling **aerosols from infected animals**, particularly livestock (cattle, sheep, goats). - It does not cause endemic typhus. *Ehrlichia chafeensis* - *Ehrlichia chafeensis* causes **human monocytic ehrlichiosis** (HME), a tick-borne disease. - This condition is characterized by fever, headache, and myalgia, but it is distinct from typhus. - Transmitted by the **lone star tick** (*Amblyomma americanum*), not by fleas. *Rickettsia prowazekii* - *Rickettsia prowazekii* causes **epidemic (louse-borne) typhus**, which is a distinct form of typhus. - Epidemic typhus is transmitted by the **human body louse** (*Pediculus humanus corporis*). - Historically associated with overcrowded, unsanitary conditions, war, and famine.

Question 2145: True for tetanus is:

A. Due to exotoxin (Correct Answer)
B. Due to endotoxin
C. Both of the options
D. None of the options

Explanation: ***Due to exotoxin*** - Tetanus is caused by **neurotoxins** (exotoxins) produced by *Clostridium tetani*, primarily **tetanospasmin**. - These exotoxins block the release of inhibitory neurotransmitters, leading to **muscle spasms** and rigidity. *Due to endotoxin* - **Endotoxins** are components of the outer membrane of gram-negative bacteria, released upon bacterial lysis. - *Clostridium tetani* is a **gram-positive bacterium** and does not produce endotoxins. *Both of the options* - This option is incorrect because tetanus is specifically caused by **exotoxins**, not endotoxins. - The pathophysiology of tetanus is solely linked to the neurotoxic effects of **tetanospasmin**. *None of the options* - This option is incorrect because tetanus is indeed caused by an **exotoxin**. - The primary mechanism of disease is the action of **tetanospasmin** produced by the bacteria.

Question 2146: Acute mesenteric lymphadenitis is caused by?

A. Alpha Hemolytic streptococci
B. E. Coli
C. Yersinia (Correct Answer)
D. Haemophilus influenzae

Explanation: ***Yersinia*** - **_Yersinia enterocolitica_** and **_Yersinia pseudotuberculosis_** are common causative agents of acute mesenteric lymphadenitis, primarily transmitted via contaminated food or water. - Infection leads to inflammation of the **lymph nodes in the mesentery**, mimicking acute appendicitis with abdominal pain and fever. *Alpha Hemolytic streptococci* - While various streptococcal species can cause infections, **alpha-hemolytic streptococci** (e.g., *Streptococcus viridans*) are typically associated with conditions like **bacterial endocarditis** or dental infections, not primary mesenteric lymphadenitis. - They are part of the normal flora of the **oral cavity** and upper respiratory tract. *E. Coli* - **_Escherichia coli_** is a common cause of **urinary tract infections**, food poisoning, and sepsis, but it is not a primary cause of acute mesenteric lymphadenitis. - Though some strains can cause intestinal inflammation, it rarely specifically targets mesenteric lymph nodes to cause acute mesenteric lymphadenitis rather than other forms of GI infection. *Hemophilus influenza* - **_Haemophilus influenzae_** is known for causing respiratory tract infections like **otitis media**, sinusitis, and historically, meningitis and epiglottitis (before widespread vaccination). - It is not a typical pathogen associated with infections leading to **mesenteric lymphadenitis**.

Question 2147: Which of the following is NOT true regarding Chlamydia trachomatis?

A. Cause infertility
B. Gram negative
C. Transmitted sexually
D. Susceptible to penicillin (Correct Answer)

Explanation: ***Susceptible to penicillin*** - This statement is **not true** because *Chlamydia trachomatis* lack **peptidoglycan** in their cell walls, making them intrinsically resistant to penicillins and other beta-lactam antibiotics. - They are typically treated with **macrolides** (e.g., azithromycin) or **tetracyclines** (e.g., doxycycline) which interfere with bacterial protein synthesis. *Cause infertility* - This is a **true** statement; *Chlamydia trachomatis* infection can lead to **pelvic inflammatory disease (PID)** in women, causing scarring of the fallopian tubes and subsequent infertility or ectopic pregnancy. - In men, it can cause **epididymitis**, which may also impair fertility. *Gram negative* - This is a **true** statement; *Chlamydia trachomatis* are obligate **intracellular bacteria** that stain Gram-negative due to the structure of their outer membrane, although they have a unique cell wall composition. - Due to their intracellular nature and lack of traditional peptidoglycan synthesis, they do not stain distinctly with Gram stain in a manner similar to typical Gram-negative bacteria, but are still classified as Gram-negative. *Transmitted sexually* - This is a **true** statement; *Chlamydia trachomatis* is primarily transmitted through **sexual contact**, making it one of the most common sexually transmitted infections (STIs) worldwide. - It can infect the **genital tract**, rectum, and pharynx, and can also be transmitted from mother to newborn during childbirth.

Question 2148: Which of the following bacteria is classified as facultative anaerobe?

A. Bacteroides
B. Pseudomonas
C. Escherichia (Correct Answer)
D. Clostridium

Explanation: ***Escherichia*** - *Escherichia coli* (E. coli) is a classic example of a **facultative anaerobe**, meaning it can grow in the presence or absence of oxygen. - It uses **aerobic respiration** when oxygen is available and switches to **fermentation** or **anaerobic respiration** in an anaerobic environment. *Bacteroides* - *Bacteroides* species are **obligate anaerobes**, meaning they can only survive and grow in the **complete absence of oxygen**. - They are a major component of the normal human gut flora and are sensitive to oxygen exposure. *Pseudomonas* - *Pseudomonas* species, such as *Pseudomonas aeruginosa*, are **obligate aerobes**, requiring **oxygen for growth and metabolism**. - They possess enzymes like cytochrome oxidase and catalase, which are essential for aerobic respiration. *Clostridium* - *Clostridium* species, like *Clostridium tetani* and *Clostridium perfringens*, are **obligate anaerobes**. - They lack the enzymes (e.g., superoxide dismutase, catalase) necessary to detoxify reactive oxygen species, making oxygen lethal to them.

Question 2149: True about chlamydia

A. Extracellular bacteria
B. McCoy cells for isolation (Correct Answer)
C. Penicillin is drug of choice
D. Gram positive

Explanation: ***McCoy cells for isolation*** - *Chlamydia trachomatis* are **obligate intracellular bacteria** and require cell culture for laboratory isolation and propagation. - **Cycloheximide-treated McCoy cells** are the gold standard cell line for isolation of *Chlamydia trachomatis*, though other cell lines like HeLa and HEp-2 cells can also be used. - This characteristic makes their cultivation more challenging compared to extracellular bacteria, necessitating the use of viable host cells. *Extracellular bacteria* - This statement is incorrect because *Chlamydia trachomatis* is an **obligate intracellular bacterium**, meaning it can only replicate and survive within the host cell. - Unlike extracellular bacteria, it cannot be cultured on standard bacterial growth media. *Penicillin is drug of choice* - **Penicillin** is ineffective against *Chlamydia* because these bacteria lack a **peptidoglycan cell wall**, which is the primary target of penicillin. - The drugs of choice for *Chlamydia* infections are typically **macrolides** (e.g., azithromycin) or **tetracyclines** (e.g., doxycycline). *Gram positive* - Though *Chlamydia* has a cell wall, its structure is unique and lacks **peptidoglycan**, which prevents it from staining robustly with the Gram stain. - While it has characteristics of both Gram-negative and Gram-positive bacteria, it is generally considered **Gram-negative** due to its outer membrane, but often described as Gram-variable or not typically Gram-stainable.

Question 2150: True about Legionella pneumonia is -

A. Sputum shows polymorphonuclear leukocytes with organism
B. Acid fast
C. Can be isolated from lung biopsy (Correct Answer)
D. Gram positive

Explanation: ***Can be isolated from lung biopsy*** - **Legionella pneumophila** is a fastidious bacterium that can be difficult to grow on routine culture media and microscopic examination of sputum may not always reveal the organism. - In cases of severe or persistent pneumonia where a definitive diagnosis is needed, **lung biopsy** provides tissue that can be stained (e.g., with direct fluorescent antibody stain) or cultured on specialized media (like buffered charcoal yeast extract agar) for isolation of *Legionella*. *Sputum shows polymorphonuclear leukocytes with organism* - While **polymorphonuclear leukocytes (PMNs)** are typically present in sputum from bacterial pneumonia, *Legionella* is an **intracellular pathogen** and often difficult to visualize on Gram stain due to its poor staining characteristics. - The absence of readily visible organisms in sputum Gram stain despite abundant leukocytes is a classic clue for **Legionella pneumonia**. *Acid fast* - **Legionella pneumophila** is **not acid-fast**; acid-fast staining is characteristic of mycobacteria (e.g., *Mycobacterium tuberculosis*). - Special stains like **Dieterle silver stain** or **direct fluorescent antibody (DFA) stain** are used to visualize *Legionella* in tissue or samples. *Gram positive* - **Legionella pneumophila** is a **Gram-negative bacterium**, although it stains poorly with standard Gram stain. - It is typically a **rod-shaped bacterium** that requires specialized culture conditions.

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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