Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2291: A patient presents with urethral discharge. Gram stain shows intracellular gram-negative diplococci. What is the causative organism?

A. Mycoplasma genitalium
B. Treponema pallidum
C. Chlamydia trachomatis
D. Neisseria gonorrhoeae (Correct Answer)

Explanation: ***Neisseria gonorrhoeae*** - The presence of **intracellular gram-negative diplococci** in urethral discharge is a classic microscopic finding for *Neisseria gonorrhoeae*. - This organism directly invades host cells, and its unique gram staining characteristic makes it readily identifiable in clinical samples. *Mycoplasma genitalium* - This organism does not have a **cell wall** and therefore will not gram stain. It cannot be identified by Gram stain. - Diagnosis typically requires molecular methods like **PCR**. *Treponema pallidum* - This spirochete is too thin to be visualized with standard Gram stain and is typically identified using **dark-field microscopy** or serological tests. - It does not present as gram-negative diplococci. *Chlamydia trachomatis* - *Chlamydia trachomatis* is an **obligate intracellular bacterium** but does not stain well with Gram stain due to its unique cell wall structure (lacks peptidoglycan). - It is often diagnosed using **nucleic acid amplification tests (NAATs)**.

Question 2292: Which of the following is a characteristic of Klebsiella pneumoniae?

A. It causes pneumonia
B. It produces a polysaccharide capsule (Correct Answer)
C. It is a facultative anaerobe
D. It is oxidase negative

Explanation: ***It produces a polysaccharide capsule*** - *Klebsiella pneumoniae* is famously known for its prominent **polysaccharide capsule**, which is a major virulence factor. - This capsule helps the bacterium evade phagocytosis by immune cells and contributes to its characteristic **mucoid colony appearance** on agar. *It causes pneumonia* - While *Klebsiella pneumoniae* is a known cause of **pneumonia**, especially severe forms like **lobar pneumonia** with mucoid sputum, this statement describes a disease it causes, rather than a characteristic of the bacterium itself. - Many different pathogens cause pneumonia; therefore, this isn't a distinguishing characteristic of *Klebsiella pneumoniae*'s microbiology. *It is a facultative anaerobe* - *Klebsiella pneumoniae* is indeed a **facultative anaerobe**, meaning it can grow in both the presence and absence of oxygen. - However, this is a characteristic shared by many **Enterobacteriaceae** and numerous other bacterial species, making it a less specific characteristic distinguishing *Klebsiella pneumoniae* from other bacteria compared to its prominent capsule. *It is oxidase negative* - *Klebsiella pneumoniae* is an **oxidase-negative** bacterium, which is a characteristic of the entire family **Enterobacteriaceae**. - While true, this characteristic is broad and groups *Klebsiella pneumoniae* with many other pathogens like *E. coli* and *Salmonella*, making it less specific than the polysaccharide capsule.

Question 2293: Which enzyme does Vibrio cholerae toxin activate to cause profuse watery diarrhea?

A. Sodium channels
B. Adenylate cyclase (Correct Answer)
C. Potassium channels
D. Guanylate cyclase

Explanation: ***Adenylate cyclase*** - The **Vibrio cholerae toxin** (cholera toxin/choleragen) causes activation of **adenylate cyclase** through ADP-ribosylation of the Gs alpha subunit of G-proteins. - The toxin's A subunit **permanently activates the Gs protein** by preventing its GTPase activity, keeping it in an active GTP-bound state. - The constitutively active Gs protein continuously activates **adenylate cyclase**, leading to sustained elevation of intracellular **cyclic AMP (cAMP)**. - This increased cAMP causes **excessive secretion of chloride ions and water** into the intestinal lumen, resulting in profuse watery "rice-water" diarrhea characteristic of cholera. *Sodium channels* - Sodium channels are ion channels, not enzymes. - While sodium and chloride transport is affected in cholera, the toxin does not directly activate sodium channels. - The increased cAMP affects the **CFTR (cystic fibrosis transmembrane conductance regulator)** chloride channel, which indirectly affects sodium absorption. *Potassium channels* - Potassium channels are ion channels, not enzymes. - The cholera toxin does not activate potassium channels. - Potassium loss occurs as a consequence of the massive fluid secretion, not through direct channel activation. *Guanylate cyclase* - **Guanylate cyclase** is activated by different enterotoxins, notably **E. coli heat-stable enterotoxin (STa)**, leading to increased cyclic GMP (cGMP). - Increased cGMP also causes secretory diarrhea, but this is **not the mechanism of cholera toxin**. - Heat-labile E. coli toxin (LT) has a similar mechanism to cholera toxin (activating adenylate cyclase).

Question 2294: Which of the following statements is the most accurate about Chlamydia trachomatis?

A. It produces urease enzyme
B. It is a non-motile bacterium
C. It forms spores for survival
D. It cannot be seen on Gram stain (Correct Answer)

Explanation: ***It cannot be seen on Gram stain*** - *Chlamydia trachomatis* are **obligate intracellular bacteria** that are too small and lack a significant **peptidoglycan wall** to stain effectively with Gram stain. - Their unique cell wall structure makes them **Gram-negative atypical bacteria** that are usually identified by nucleic acid amplification tests (NAATs) or immunofluorescence. *It produces urease enzyme* - **Urease production** is characteristic of bacteria like *Helicobacter pylori* and *Proteus mirabilis*, which metabolize urea; *Chlamydia trachomatis* does not produce urease. - The absence of urease is a key **biochemical differentiating factor** between *Chlamydia* and other bacterial genera. *It is a non-motile bacterium* - While *Chlamydia trachomatis* is indeed **non-motile**, this statement is not the *most accurate* distinguishing characteristic as many bacteria are non-motile. - Its immotility is due to the **absence of flagella**, but its obligate intracellular lifestyle and unique cell wall are more defining features. *It forms spores for survival* - **Spore formation** is a survival mechanism primarily seen in Gram-positive bacteria like *Bacillus* and *Clostridium* genera, allowing them to resist harsh conditions. - *Chlamydia trachomatis* does not form spores; instead, it relies on its **unique developmental cycle** involving elementary bodies (infectious) and reticulate bodies (replicative) for survival and propagation.

Question 2295: Which of the following bacteria is the most likely cause of toxic shock syndrome?

A. Clostridium perfringens
B. Streptococcus pyogenes
C. Neisseria meningitidis
D. Staphylococcus aureus (Correct Answer)

Explanation: ***Staphylococcus aureus*** - This bacterium is the most common cause of **toxic shock syndrome (TSS)**, particularly in cases associated with **tampon use** or **surgical wound infections**. - It produces **toxic shock syndrome toxin-1 (TSST-1)**, a superantigen that triggers a massive, systemic inflammatory response. *Clostridium perfringens* - This bacterium is primarily associated with **gas gangrene** (clostridial myonecrosis) and **food poisoning**. - Its toxins cause tissue necrosis and gas production, which are not characteristic features of TSS. *Streptococcus pyogenes* - While *S. pyogenes* can cause a form of **toxic shock-like syndrome (STSS)**, it is distinct from TSS caused by *S. aureus*. - *S. pyogenes* is more commonly known for causing **strep throat**, **scarlet fever**, and **necrotizing fasciitis**. *Neisseria meningitidis* - This bacterium is the primary cause of **meningitis** and **meningococcemia**, a severe systemic infection. - It does not produce the specific toxins associated with TSS or STSS, and its clinical presentation is typically different, involving petechial or purpuric rash and signs of central nervous system infection.

Question 2296: Assertion: Myocarditis is seen as a complication in faucial diphtheria. Reason: It is due to the exotoxin produced by Corynebacterium diphtheriae.

A. Assertion is false, reason is true.
B. Assertion is true, reason is true but reason is not the correct explanation of the assertion.
C. Assertion is true, reason is true.
D. Assertion is true, reason is true and reason is the correct explanation of the assertion. (Correct Answer)

Explanation: ***Assertion is true, reason is true and reason is the correct explanation of the assertion*** **Analysis of Assertion:** - Myocarditis is indeed a **well-documented complication** of faucial (pharyngeal) diphtheria, occurring in 10-25% of cases - It typically appears in the **second to third week** of illness and is a major cause of mortality in diphtheria - Cardiac involvement can range from asymptomatic ECG changes to severe heart failure and cardiogenic shock **Analysis of Reason:** - The **diphtheria exotoxin** produced by *Corynebacterium diphtheriae* is directly responsible for myocardial damage - The toxin inhibits protein synthesis by **ADP-ribosylation of elongation factor-2 (EF-2)**, leading to cell death - Cardiac myocytes are particularly vulnerable to this toxin, resulting in **toxic myocarditis** **Why the reason is the correct explanation:** - The mechanism of myocarditis in diphtheria is specifically through the **cardiotoxic effect of the exotoxin**, not through immune mechanisms or bacterial invasion - This establishes a direct **cause-and-effect relationship** between the exotoxin (reason) and myocarditis (assertion) *Incorrect Options:* *Assertion is false, reason is true* - This is incorrect because the assertion is definitely true - myocarditis is a classic complication of diphtheria documented in all standard microbiology and infectious disease texts *Assertion is true, reason is true but reason is not the correct explanation of the assertion* - This is incorrect because the exotoxin IS the direct cause of myocarditis in diphtheria - the reason perfectly explains the assertion through a clear pathophysiological mechanism *Assertion is true, reason is true* - While this correctly identifies both statements as true, it fails to acknowledge the **causal relationship** - the exotoxin doesn't just happen to be produced; it is the specific mechanism causing the myocarditis

Question 2297: A lady came with yellowish discharge and stain showing gram-negative diplococci. What is the most likely causative organism of her condition?

A. Streptococcus pneumoniae
B. Enterococcus faecalis
C. Neisseria gonorrhoeae (Correct Answer)
D. Both Streptococcus and Enterococcus

Explanation: ***Neisseria gonorrhoeae*** - The presence of **yellowish discharge** and **gram-negative diplococci** on a stain is a classic presentation for **gonorrhea**, caused by *Neisseria gonorrhoeae*. - This bacterium is a common cause of **sexually transmitted infections (STIs)**, leading to conditions like cervicitis, urethritis, and pelvic inflammatory disease. *Streptococcus pneumoniae* - This organism is a **gram-positive coccus** and typically causes **respiratory infections** (e.g., pneumonia, otitis media, meningitis), not genital discharge with gram-negative diplococci. - It is not associated with STI-related yellowish genital discharge. *Enterococcus faecalis* - This is a **gram-positive coccus** and a common cause of **urinary tract infections (UTIs)** and **nosocomial infections**, but not typically associated with yellowish genital discharge showing gram-negative diplococci. - It is also not classified as a gram-negative organism. *Both Streptococcus and Enterococcus* - This option is incorrect because both *Streptococcus* and *Enterococcus* are **gram-positive organisms**. - The clinical presentation clearly describes **gram-negative diplococci**, which rules out these bacteria as the primary cause.

Question 2298: Which of the following is the most common cause of bacterial conjunctivitis?

A. Staphylococcus epidermidis
B. Staphylococcus aureus (Correct Answer)
C. Streptococcus pneumoniae
D. Pseudomonas aeruginosa

Explanation: ***Staphylococcus aureus*** - *Staphylococcus aureus* is the **most frequent bacterial isolate** in cases of bacterial conjunctivitis across all age groups. - It colonizes the skin and mucous membranes and can easily spread to the **conjunctiva**, especially with poor hand hygiene. *Staphylococcus epidermidis* - While *Staphylococcus epidermidis* is a common commensal of the **skin and conjunctiva**, it is less frequently a primary cause of acute bacterial conjunctivitis compared to *S. aureus*. - It is more commonly implicated in **post-operative endophthalmitis** or infections associated with indwelling devices. *Streptococcus pneumoniae* - *Streptococcus pneumoniae* is a significant cause of **bacterial conjunctivitis**, particularly in children, but it is not as common as *S. aureus* overall. - It can also cause **otitis media** and **pneumonia**, and its presence in the eye often reflects colonization of the upper respiratory tract. *Pseudomonas aeruginosa* - *Pseudomonas aeruginosa* is a highly virulent pathogen and a severe cause of **bacterial conjunctivitis**, especially in contact lens wearers or in cases of trauma or immunodeficiency. - However, it is not the **most common cause** of general bacterial conjunctivitis, as its prevalence is lower than that of staphylococcal species.

Question 2299: A patient presents with severe diarrhea following antibiotic use. Stool culture reveals a Gram-positive rod. What is the most likely causative agent?

A. Escherichia coli
B. Clostridioides difficile (Correct Answer)
C. Salmonella
D. Shigella

Explanation: ***Clostridioides difficile*** - **Antibiotic-associated diarrhea** caused by a **Gram-positive rod** is highly characteristic of *Clostridioides difficile* infection. - Antibiotics disrupt the normal gut flora, allowing *C. difficile* to proliferate and produce toxins that lead to **pseudomembranous colitis**. *Escherichia coli* - While *E. coli* can cause diarrhea (e.g., enterohemorrhagic *E. coli*), it is a **Gram-negative rod** and not typically associated with antibiotic-induced severe diarrhea leading to pseudomembranous colitis. - *E. coli* infections usually do not involve the same mechanism of gut flora disruption by antibiotics that allows *C. difficile* to thrive. *Salmonella* - *Salmonella* species are **Gram-negative rods** that cause foodborne illnesses, often with fever, abdominal cramps, and diarrhea. - Their infections are typically not linked to prior antibiotic use in the same manner as *C. difficile*. *Shigella* - *Shigella* species are also **Gram-negative rods** known for causing bacillary dysentery, characterized by bloody diarrhea, fever, and abdominal cramps. - They are not typically associated with antibiotic-induced diarrhea, and their Gram stain morphology differentiates them.

Question 2300: Which sexually transmitted infection is caused by the bacterium Haemophilus ducreyi?

A. Chancroid (Correct Answer)
B. Syphilis
C. Gonorrhea
D. Lymphogranuloma venereum

Explanation: ***Chancroid*** - **Chancroid** is a sexually transmitted infection (STI) caused by the gram-negative bacterium ***Haemophilus ducreyi***. - It typically presents as **painful genital ulcers with ragged borders** and often causes **inguinal lymphadenopathy**. *Syphilis* - **Syphilis** is caused by the spirochete bacterium ***Treponema pallidum***. - It is characterized by **painless chancres** in its primary stage and a variety of systemic manifestations in later stages. *Gonorrhea* - **Gonorrhea** is caused by the bacterium ***Neisseria gonorrhoeae***. - It commonly causes **urethritis** in males and cervicitis in females, often with purulent discharge, but not painful genital ulcers. *Lymphogranuloma venereum* - **Lymphogranuloma venereum** (LGV) is caused by specific serovars of ***Chlamydia trachomatis***. - It initially presents with a small, often **transient papule or ulcer**, followed by severe, often unilateral, **inguinal lymphadenopathy** (buboes).

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

Practice Questions

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