Bacteriology Practice Questions

Q: On what basis are E. coli subtypes classified?

Somatic O antigen. ***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.

Q: Gonococci have affinity for :

Columnar epithelium. ***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**.

Q: An 18-year-old girl presents with watery diarrhea. Most likely causative agent -

V. cholerae. ***V. cholerae*** - *Vibrio cholerae* is a classic cause of **acute, severe watery diarrhea** that can lead to rapid dehydration. - While other agents can cause watery diarrhea, *V. cholerae* is primarily associated with large-scale outbreaks of this symptom. *Rota virus* - While rotavirus causes **watery diarrhea**, it primarily affects **infants and young children** and is less common as the most likely cause in an 18-year-old in many settings due to widespread vaccination programs. - The diarrhea, though watery, is often accompanied by **fever and vomiting**. *Salmonella* - *Salmonella* typically causes **inflammatory diarrhea** (dysentery-like symptoms with blood/mucus in stool) or **food poisoning**, with diarrhea that may be watery but is often not as profuse or severe as cholera. - It is more commonly associated with **fever and abdominal cramps**. *Shigella* - *Shigella* causes **bacillary dysentery**, characterized by **bloody, mucoid stools**, abdominal cramps, and fever. - It is not typically associated with solely profuse watery diarrhea.

Q: The following statements are true regarding botulism except:

Infant botulism is caused by ingestion of preformed toxin.. ***Infant botulism is caused by ingestion of preformed toxin.*** - This statement is incorrect because **infant botulism** is caused by the *in vivo* production of **botulinum toxin** in the infant's intestine after *Clostridium botulinum* spores are ingested and colonize the gut. - Unlike foodborne botulism in adults, it is **not** due to consuming preformed toxin, as the infant's immature gut flora allows spore germination. *Clostridium botulinum A, B, E and F cause human disease.* - This statement is true; **neurotoxin types A, B, E, and F** are the most common causes of human botulism, with type A and B being responsible for the majority of cases worldwide. - These types produce potent **neurotoxins** that block acetylcholine release at neuromuscular junctions, leading to flaccid paralysis. *The gene for botulinum toxin is encoded by a bacteriophage.* - This statement is true. The genes for **botulinum neurotoxins** (BoNT) in *C. botulinum* are often found on **bacteriophages** or plasmids, which can transfer these virulence genes between bacteria. - This horizontal gene transfer contributes to the diversity of *C. botulinum* and the spread of toxin production capability. *Clostridium baratii may cause botulism.* - This statement is true. While *Clostridium botulinum* is the most common species, **atypical clostridia** such as *Clostridium baratii* and *Clostridium butyricum* have also been identified as rare causes of botulism, particularly infant botulism. - These species also produce **botulinum neurotoxins** (specifically type F in *C. baratii* and type E in *C. butyricum*), leading to similar clinical manifestations.

Q: What is the mechanism of action of diphtheria toxin?

Inhibiting protein synthesis. ***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.

Q: A 60-year-old farmer presents with swelling on the sole of the foot accompanied by discharging yellow granules. What is the most likely diagnosis?

Actinomycosis. ***Actinomycosis*** - The presentation of a **slowly progressive swelling** with **draining sinuses** that exude **"sulfur granules" (yellow granules)** is highly characteristic of actinomycosis. - While often associated with the cervicofacial region, actinomycosis can affect other sites such as the foot, especially after **trauma** or through direct inoculation from soil, consistent with a farmer's occupation. *Fungal mycetoma* - While fungal mycetoma also presents with **swelling, draining sinuses**, and **granules**, these granules are typically **black or white** from fungal elements, not yellow [1]. - The causative agents are true fungi, and the clinical course might differ subtly, but the **color of the granules** is a key differentiator [1]. *Actinomycetoma* - Actinomycetoma is caused by **filamentous bacteria** (like *Nocardia* or *Actinomadura*), which are a type of bacterial mycetoma. It presents similarly to fungal mycetoma with **granules**, but these are usually **white, red, or yellow-brown**, and less commonly the bright yellow "sulfur granules" seen in classical actinomycosis. - It’s important to distinguish actinomycetoma (bacterial mycetoma) from actinomycosis, which is caused by *Actinomyces* species and is characterized specifically by the classic sulfur granules. *Candidiasis* - **Candidiasis** (yeast infection) typically presents as **mucocutaneous lesions, thrush, or invasive systemic infections**, not localized swelling with discharging yellow granules on the sole of the foot. - It does not involve the formation of granules or sinuses in this manner.

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

C. diphtheriae. ***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.

Q: Most common organism grown in urine culture of a pregnant woman with asymptomatic bacteriuria?

E. coli. ***E. coli*** - **_E. coli_** is the most prevalent uropathogen due to its ability to adhere to uroepithelial cells and its common presence in the perianal flora. - It accounts for approximately **80% of all community-acquired urinary tract infections (UTIs)**, including asymptomatic bacteriuria in pregnant women. *Staph aureus* - **_Staphylococcus aureus_** is an uncommon cause of UTIs and is typically associated with **hematogenous spread** in cases of bacteremia. - It is rarely isolated in asymptomatic bacteriuria and would raise suspicion for a systemic infection if found in urine. *Pseudomonas* - **_Pseudomonas aeruginosa_** is generally associated with **nosocomial infections**, catheter-associated UTIs, or patients with structural urinary tract abnormalities or prolonged antibiotic use. - It is not a common cause of asymptomatic bacteriuria in otherwise healthy pregnant women. *Proteus* - **_Proteus_ species**, particularly **_Proteus mirabilis_**, are known for their ability to produce **urease**, which can lead to alkaline urine and the formation of struvite stones. - While they can cause UTIs, they are less common than _E. coli_ in asymptomatic bacteriuria in pregnant women.

Q: What is the generation time of Mycobacterium tuberculosis?

15-20 hours. ***15-20 hours*** - *Mycobacterium tuberculosis* is known for its **slow growth rate**, with a generation time of approximately **15-20 hours** (some sources cite up to 18-24 hours). - This unusually long generation time contributes to the **prolonged incubation period** required for culture (3-8 weeks) and makes its isolation and identification a time-consuming process. - The slow growth is due to its **complex, lipid-rich cell wall** and unique metabolic characteristics. *1-2 hours* - A generation time of 1-2 hours is typical for **rapidly growing bacteria** such as *Escherichia coli* (which has a generation time of approximately 20 minutes under optimal conditions). - *Mycobacterium tuberculosis* is characterized by its **much slower growth rate**, distinguishing it from most common bacterial pathogens. *10-15 hours* - While this is slower than typical bacteria, it is still at the **lower end** of the range for *M. tuberculosis*. - The more widely accepted generation time is **15-20 hours**, making this option slightly inaccurate. *5-10 hours* - A generation time of 5-10 hours is **too short** for *Mycobacterium tuberculosis*. - Members of the *Mycobacterium tuberculosis complex* are known for their **exceptionally long replication cycles** compared to most other bacteria.

Question 1

On what basis are E. coli subtypes classified?

Accepted Answer

Somatic O antigen

Answer

Virulence properties

Answer

Lactose fermentation

Answer

Maltose fermentation

Question 2

Gonococci have affinity for :

Accepted Answer

Columnar epithelium

Answer

Glandular epithelium

Answer

Squamous epithelium

Answer

Stratified squamous epithelium

Question 3

Which of the following statements about Vibrio cholerae is correct?

Accepted Answer

El Tor strain has more asymptomatic carriers than the classical strain.

Answer

Cholera is more common in women than men.

Answer

All statements are correct.

Answer

The classical strain of Vibrio cholerae provides immunity against El Tor strain.

Question 4

An 18-year-old girl presents with watery diarrhea. Most likely causative agent -

Accepted Answer

V. cholerae

Answer

Rotavirus

Answer

Salmonella

Answer

Shigella

Question 5

The following statements are true regarding botulism except:

Accepted Answer

Infant botulism is caused by ingestion of preformed toxin.

Answer

Clostridium botulinum A, B, E and F cause human disease.

Answer

The gene for botulinum toxin is encoded by a bacteriophage.

Answer

Clostridium baratii may cause botulism.

Question 6

What is the mechanism of action of diphtheria toxin?

Accepted Answer

Inhibiting protein synthesis

Answer

Inhibiting glucose synthesis

Answer

Promoting acetylcholine release

Answer

Altering cyclic GMP levels

Question 7

A 60-year-old farmer presents with swelling on the sole of the foot accompanied by discharging yellow granules. What is the most likely diagnosis?

Accepted Answer

Actinomycosis

Answer

Fungal mycetoma

Answer

Candidiasis

Answer

Actinomycetoma

Question 8

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

Accepted Answer

C. diphtheriae

Answer

C. perfringens

Answer

Y. pestis

Answer

H. influenzae

Question 9

Most common organism grown in urine culture of a pregnant woman with asymptomatic bacteriuria?

Accepted Answer

E. coli

Answer

Staph aureus

Answer

Pseudomonas

Answer

Proteus

Question 10

What is the generation time of Mycobacterium tuberculosis?

Accepted Answer

15-20 hours

Answer

10-15 hours

Answer

1-2 hours

Answer

5-10 hours

Bacteriology — MCQs

Bacteriology — MCQs

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