Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2331: What is the mechanism of action of diphtheria toxin?

A. Inhibiting glucose synthesis
B. Inhibiting protein synthesis (Correct Answer)
C. Promoting acetylcholine release
D. Altering cyclic GMP levels

Explanation: ***Inhibiting protein synthesis*** - Diphtheria toxin is an **A-B toxin** that enters host cells and catalyzes the **ADP-ribosylation** of **elongation factor-2 (EF-2)**. - This modification inactivates EF-2, which is crucial for the translocation step of **protein synthesis**, thereby blocking translation and leading to cell death. *Inhibiting glucose synthesis* - This is not the mechanism of action of diphtheria toxin; its primary target is the **eukaryotic protein synthesis machinery**. - While metabolic processes might be indirectly affected by cell death, directly inhibiting glucose synthesis is incorrect. *Promoting acetylcholine release* - This mechanism is associated with other toxins, such as **black widow spider venom**, which promotes the release of neurotransmitters. - Diphtheria toxin specifically targets **protein synthesis in host cells**, not neurotransmitter release. *Altering cyclic GMP levels* - Toxins like **heat-stable enterotoxins of E. coli** or **guanylin** can alter cGMP levels, affecting fluid and electrolyte balance. - Diphtheria toxin's action is distinct, involving the **inactivation of EF-2** to halt protein production.

Question 2332: On what basis are E. coli subtypes classified?

A. Virulence properties
B. Lactose fermentation
C. Maltose fermentation
D. Somatic O antigen (Correct Answer)

Explanation: ***Somatic O antigen*** - *E. coli* subtypes are commonly classified based on the **somatic O antigen**, which is part of the **lipopolysaccharide (LPS)** layer on the bacterial surface. - This antigenic classification system, along with H (flagellar) and K (capsular) antigens, helps in **epidemiological tracking** and identifying pathogenic strains. *Virulence properties* - While *E. coli* subtypes do possess different **virulence properties** (e.g., toxins, adhesins), these are characteristics of specific strains rather than the primary basis for their initial laboratory classification. - Virulence factors determine disease manifestation but are not the standard phenotypic markers used for general subtyping. *Lactose fermentation* - **Lactose fermentation** is a general characteristic of *E. coli* as a species, used to distinguish it from **non-lactose fermenting enterics** like *Salmonella* and *Shigella*. - It does not differentiate between different pathogenic or non-pathogenic *E. coli* subtypes. *Maltose fermentation* - **Maltose fermentation** is a biochemical test that can be used to differentiate certain bacteria but is not a primary or standard method for classifying *E. coli* into subtypes. - *E. coli* generally ferments maltose, but this characteristic does not provide the specificity needed for subtyping.

Question 2333: Gonococci have affinity for :

A. Columnar epithelium (Correct Answer)
B. Glandular epithelium
C. Squamous epithelium
D. Stratified squamous epithelium

Explanation: ***Columnar epithelium*** - **Gonococci** (Neisseria gonorrhoeae) have a particular tropism for **columnar epithelial cells**, which are found in areas such as the endocervix, urethra, rectum, and conjunctiva. - This affinity is due to the presence of specific surface structures on the bacteria, like **pili** and **Opa proteins**, that bind to receptors on these host cells. *Glandular epithelium* - While some glands contain columnar epithelium and can be affected (e.g., endocervical glands), **gonococci** do not exclusively target glandular tissue per se, but rather the type of epithelial cells lining them. - The primary clinical impact is on the **columnar epithelial lining** of various ducts and surfaces, not specifically the secretory cells of glands. *Stratified squamous epithelium* - **Stratified squamous epithelium**, found in areas like the vagina and ectocervix, is generally more resistant to gonococcal infection. - The multiple layers and mature surface cells of this epithelium do not offer the same binding sites for **gonococci** as columnar cells. *Squamous epithelium* - Similar to stratified squamous epithelium, typical **squamous epithelium** (which can be stratified or simple) is not the preferred target for Neisseria gonorrhoeae. - The bacterium's pathogenesis is largely dependent on its ability to adhere to and invade **columnar epithelial cells**.

Question 2334: Stalactite growth in ghee broth is due to which of the following organisms?

A. C. perfringens
B. Y. pestis
C. C. diphtheriae (Correct Answer)
D. H. influenzae

Explanation: ***C. diphtheriae*** - **Corynebacterium diphtheriae** exhibits characteristic **stalactite (or stalagmite) growth** when cultured in **ghee broth** or on **Loeffler's serum slope** (which contains ghee). - This distinctive growth pattern appears as **pendant drop-like projections** hanging from the surface, resembling stalactites in caves. - This is a classic microbiological feature used for presumptive identification of C. diphtheriae. *Y. pestis* - *Yersinia pestis*, the causative agent of plague, does not show stalactite growth. - In liquid media, Y. pestis may form a **pellicle** or show floccular growth. - On solid media, it may show **"inverted fir tree" appearance** in stab cultures, not stalactite growth. *C. perfringens* - *Clostridium perfringens* is an anaerobic bacterium causing **gas gangrene** and food poisoning. - Growth in broth is typically **turbid with gas production**, without stalactite formation. *H. influenzae* - *Haemophilus influenzae* requires **X (hemin) and V (NAD) factors** for growth. - Typically grows as **small colonies on chocolate agar**, not in ghee broth. - Does not exhibit stalactite growth patterns.

Question 2335: Which organism is most commonly associated with a fishy odour on growth?

A. Proteus species (Correct Answer)
B. Escherichia coli
C. Klebsiella species
D. Pseudomonas species

Explanation: ***Proteus species*** - **Proteus species** are classically known for their characteristic **putrid or ammonia-like odor**, NOT typically described as "fishy" - This odor is due to their production of **urease enzyme**, which breaks down urea to ammonia, creating a strong alkaline smell - The ammonia smell is particularly noticeable in urine samples infected with Proteus - **Note**: While "fishy odor" in microbiology is classically associated with **Gardnerella vaginalis** and certain anaerobes, among the options listed, this may be a legacy question with outdated terminology *Escherichia coli* - **E. coli** produces a characteristic **fecal or indolic odor** due to tryptophan metabolism - This is described as musty or fecal-smelling, distinctly different from fishy - Common cause of UTIs but with different odor profile *Klebsiella species* - **Klebsiella** produces a **sweet, bread-like or musty odor** - Their mucoid colonies (due to polysaccharide capsule) have a characteristic appearance - The odor is described as sweet or yeasty, not fishy *Pseudomonas species* - **Pseudomonas aeruginosa** has a distinctive **grape-like, fruity, or corn tortilla-like odor** - This is due to production of **2-aminoacetophenone** and other aromatic compounds - The odor is sweet/fruity and easily recognizable, not fishy - Also produces pyocyanin (blue-green pigment) as an additional identifying feature

Question 2336: Which organism is known for its ability to survive in anaerobic conditions?

A. Micrococcus
B. Clostridium (Correct Answer)
C. B. anthracis
D. Corynebacterium

Explanation: ***Clostridium*** - Species like *Clostridium perfringens* and *Clostridium tetani* are **obligate anaerobes**, meaning they can only grow in the absence of oxygen. - They produce **spores** that allow them to survive in harsh aerobic environments until anaerobic conditions are met. *Micrococcus* - **Micrococcus** species are typically **aerobic** or facultatively anaerobic bacteria. - They require oxygen for optimal growth and are not known for thriving in strictly anaerobic conditions. *B. anthracis* - **Bacillus anthracis** is a **facultative anaerobe**, meaning it can grow either with or without oxygen, but prefers aerobic conditions. - While it can survive without oxygen, it is not primarily known for its anaerobic capabilities in the same way *Clostridium* is. *Corynebacterium* - **Corynebacterium** species are generally **aerobic** or facultative aerobes. - They grow best in the presence of oxygen and are not characteristic of organisms that survive well in anaerobic environments.

Question 2337: Clinical classification of staphylococcus is based on -

A. Catalase test
B. Coagulase test (Correct Answer)
C. Mannitol fermentation
D. Optochin sensitivity

Explanation: ***Coagulase test*** - The **coagulase test** is the primary method used to differentiate pathogenic *Staphylococcus aureus* from non-pathogenic coagulase-negative staphylococci. - *S. aureus* produces **coagulase**, an enzyme that clots plasma, while other staphylococci do not. *Catalase test* - The **catalase test** differentiates staphylococci from streptococci (staphylococci are catalase-positive, streptococci are catalase-negative). - It does not help in differentiating between different *Staphylococcus* species. *Mannitol fermentation* - **Mannitol fermentation** is a characteristic used to distinguish *Staphylococcus aureus* (ferments mannitol) from some other *Staphylococcus* species (e.g., *Staphylococcus epidermidis*). - While helpful, it is not the primary test for broad classification of the genus. *Optochin sensitivity* - **Optochin sensitivity** is a test used to identify *Streptococcus pneumoniae* (sensitive) from other alpha-hemolytic streptococci. - This test is not used for the classification or differentiation of *Staphylococcus* species.

Question 2338: Species of Shigella most commonly causing arthritis

A. Shigella flexneri (Correct Answer)
B. Shigella boydii
C. Shigella dysenteriae
D. Shigella sonnei

Explanation: ***Shigella flexneri*** - This species is most commonly associated with **post-infectious reactive arthritis**, especially in individuals with the **HLA-B27 genotype**. - The arthritis typically develops days to weeks after the diarrheal illness and can affect large joints. *Shigella boydii* - While *Shigella boydii* can cause **shigellosis**, it is less frequently implicated in reactive arthritis compared to *Shigella flexneri*. - Its infections are more common in some developing regions but have a lower association with rheumatological sequelae. *Shigella dysenteriae* - *Shigella dysenteriae* causes severe forms of shigellosis, including **dysentery with potential hemolytic-uremic syndrome**, particularly serotype 1. - Although it can trigger reactive arthritis, its prevalence as a cause of this complication is lower than *Shigella flexneri*. *Shigella sonnei* - *Shigella sonnei* is the most common cause of **shigellosis in industrialized countries** and often presents with milder symptoms. - While reactive arthritis can theoretically follow any *Shigella* infection, *S. sonnei* is less commonly linked to this specific complication compared to *S. flexneri*.

Question 2339: What is the causative agent of Legionnaire's disease?

A. Klebsiella pneumoniae
B. Streptococcus pneumoniae
C. Staphylococcus aureus
D. Legionella pneumophila (Correct Answer)

Explanation: ***Legionella pneumophila*** - This bacterium is the definitive **causative agent** of Legionnaire's disease, a severe form of pneumonia. - It thrives in **warm water environments** like air conditioning systems and water pipes, and is transmitted via aerosols. *Staphylococcus aureus* - While *S. aureus* can cause various infections, including **pneumonia**, it does not cause Legionnaire's disease. - *S. aureus* is commonly associated with **skin infections**, bloodstream infections, and hospital-acquired pneumonia. *Streptococcus pneumoniae* - This bacterium is the most common cause of **community-acquired pneumonia** but is not responsible for Legionnaire's disease. - *S. pneumoniae* typically causes lobar pneumonia with characteristic clinical and radiographic findings. *Klebsiella pneumoniae* - *K. pneumoniae* is a significant cause of **hospital-acquired pneumonia**, particularly in immunocompromised individuals and alcoholics. - It is known for causing severe pneumonia with **"currant jelly" sputum**, but it is not the agent of Legionnaire's disease.

Question 2340: Pontiac fever is caused by?

A. Legionella (Correct Answer)
B. Listeria
C. Orientia
D. Leptospira

Explanation: **Legionella** *(Correct)* - **Pontiac fever** is a milder, self-limiting illness caused by species of **Legionella bacteria**, particularly **Legionella pneumophila**. - It presents with **flu-like symptoms** such as fever, myalgia, headache, and malaise, but **without pneumonia**. - Distinguished from Legionnaires' disease (the pneumonic form) by the absence of pulmonary involvement. *Listeria* (Incorrect) - **Listeria monocytogenes** causes **listeriosis**, leading to severe infections like meningitis, sepsis, and gastroenteritis, especially in immunocompromised individuals and pregnant women. - Unlike Pontiac fever, listeriosis is primarily foodborne and does not typically manifest as a mild, self-limiting respiratory syndrome. *Orientia* (Incorrect) - **Orientia tsutsugamushi** causes **scrub typhus**, a rickettsial disease characterized by fever, rash, eschar, and lymphadenopathy, usually acquired through chigger mite bites in endemic areas. - Its clinical presentation and mode of transmission are entirely distinct from Pontiac fever. *Leptospira* (Incorrect) - **Leptospira species** cause **leptospirosis**, a zoonotic disease acquired through contact with contaminated water or animal urine, leading to symptoms ranging from mild flu-like illness to severe multiorgan failure (Weil's disease). - While it can present with fever and myalgia, it is not primarily a respiratory infection and does not cause Pontiac fever.

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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