Bacteriology Practice Questions

Q: 'Spider' colonies are produced by:

Actinomyces israelii. **Explanation:** The correct answer is **Actinomyces israelii**. **Why Actinomyces israelii is correct:** *Actinomyces israelii* is a Gram-positive, anaerobic, filamentous bacterium. When grown on solid media (like Brain Heart Infusion agar) for a short duration (24–48 hours), the young colonies appear as **'Spider' colonies** due to the radiating filamentous growth. As the colonies mature (7–14 days), they develop a characteristic heaped-up, white, irregular appearance known as **'Molar Tooth' colonies**. This morphology is a classic high-yield diagnostic feature in microbiology. **Why other options are incorrect:** * **Bordetella pertussis:** Produces colonies described as **'Mercury drops'** or **'Bisected pearls'** on Bordet-Gengou medium. * **Vibrio cholerae:** On TCBS agar, it produces large, smooth, **yellow colonies** (due to sucrose fermentation). On MacConkey agar, colonies are initially colorless but may become pinkish (late lactose fermenter). * **Mycobacterium leprae:** This organism is **obligate intracellular** and **cannot be grown on artificial culture media**. It is typically studied via inoculation into the footpads of mice or nine-banded armadillos. **Clinical Pearls for NEET-PG:** 1. **Sulfur Granules:** *Actinomyces* infections (Actinomycosis) are characterized by "sulfur granules" in pus—yellowish specks that are actually masses of filamentous bacteria. 2. **Lumpy Jaw:** The most common clinical presentation is cervicofacial actinomycosis, often following dental trauma. 3. **Ray Fungus:** *Actinomyces* was historically mistaken for a fungus because of its branching filaments (hence the name "Actino-" meaning ray). 4. **Treatment:** Penicillin G is the drug of choice (long-term).

Q: Medusahead appearance on agar plates is characteristic of which bacterium?

Bacillus. **Explanation:** The "Medusa-head appearance" is a classic morphological feature of **Bacillus anthracis**, the causative agent of Anthrax. This appearance is observed on **Blood Agar** plates and is characterized by long, interlacing chains of bacilli that form wavy, filamentous projections at the margins of the colony. Under a low-power microscope, these fimbriated edges resemble the hair of the Greek mythological character Medusa. **Why the correct answer is right:** * **Bacillus anthracis:** It forms large, opaque, greyish-white, non-hemolytic colonies with a "frosted glass" appearance. The Medusa-head effect is due to the parallel arrangement of long chains of non-motile bacilli. **Why the incorrect options are wrong:** * **Hemophilus:** *H. influenzae* typically shows "Satellitism" (growth around *S. aureus*) and forms small, transparent, dew-drop colonies. * **Pneumococcus:** *Streptococcus pneumoniae* produces "draughtsman" or "checkerboard" colonies due to central autolysis of the colony. * **Yersinia:** *Yersinia pestis* is known for "Stalactite growth" in liquid media and "Ghee-like" appearance on agar, but not Medusa-head. **High-Yield Clinical Pearls for NEET-PG:** * **McFadyean’s Reaction:** Used for presumptive diagnosis of *B. anthracis* (demonstrates the capsule using polychrome methylene blue). * **String of Pearls Reaction:** Occurs when *B. anthracis* is grown on agar containing low concentrations of penicillin; the bacilli turn into spherical forms. * **Inverted Fir Tree appearance:** Seen in gelatin stab cultures due to liquefaction. * **Virulence Factors:** Encoded on plasmids **pXO1** (Toxins: Edema factor, Lethal factor, Protective antigen) and **pXO2** (Polypeptide capsule).

Q: Pontiac fever is caused by which of the following agents?

Legionella. **Explanation:** **Legionella pneumophila** is the causative agent of two distinct clinical entities, collectively known as Legionellosis: 1. **Legionnaires' Disease:** A severe form of pneumonia with multisystem involvement. 2. **Pontiac Fever:** A mild, self-limiting, influenza-like illness characterized by fever, chills, and myalgia, but **without** clinical or radiological evidence of pneumonia. It has a high attack rate and a short incubation period (1–2 days). **Why other options are incorrect:** * **Mycoplasma:** Causes "Walking Pneumonia" (Atypical pneumonia), characterized by a persistent non-productive cough and bullous myringitis. * **Rickettsia:** Known for causing spotted fevers (e.g., Rocky Mountain Spotted Fever) and typhus, typically presenting with fever, headache, and characteristic rashes. * **Salmonella:** *S. typhi* and *S. paratyphi* cause Enteric Fever (Typhoid), presenting with step-ladder fever, bradycardia, and abdominal symptoms. **High-Yield Clinical Pearls for NEET-PG:** * **Habitat:** *Legionella* thrives in man-made water systems (AC cooling towers, showers, humidifiers). * **Staining:** It is a Gram-negative bacilli but stains poorly; **Silver (Dieterle) stain** is preferred. * **Culture:** Requires **BCYE (Buffered Charcoal Yeast Extract) agar** supplemented with L-cysteine and iron. * **Diagnosis:** The **Urinary Antigen Test** is the most rapid and commonly used screening method. * **Clinical Clue:** Suspect *Legionella* in a pneumonia patient who also presents with **hyponatremia** and diarrhea.

Q: Pinta is caused by which of the following organisms?

Treponema carateum. **Explanation:** The correct answer is **Treponema carateum**. Pinta is a non-venereal treponematosis primarily found in Central and South America. It is caused by *T. carateum*, which is morphologically and serologically indistinguishable from *T. pallidum* but differs in its clinical presentation, as it affects only the skin and does not involve internal organs or the fetus. **Analysis of Options:** * **Treponema carateum (Option B):** The causative agent of **Pinta**. It presents initially as papules/plaques (pintids), followed by a secondary stage of disseminated lesions and a late stage characterized by pigmentary changes (hypopigmentation or vitiligo-like patches). * **Treponema pallidum subsp. pallidum (Option C):** The causative agent of **Venereal Syphilis**, a sexually transmitted infection with primary, secondary, and tertiary stages involving systemic organs. * **Treponema pallidum subsp. pertenue (Option A):** The causative agent of **Yaws**. It primarily affects the skin, bones, and joints in tropical regions. * **Treponema pallidum subsp. endemicum (Option D):** The causative agent of **Endemic Syphilis (Bejel)**, typically found in arid regions of Africa and the Middle East, often transmitted via shared drinking vessels. **High-Yield NEET-PG Pearls:** 1. **The "Non-Venereal Treponematoses" Rule:** All are transmitted by direct skin-to-skin contact (except Bejel, which is oral) and occur mostly in children living in poor hygienic conditions. 2. **Serology:** All these organisms trigger a positive VDRL/RPR and FTA-ABS test, making serology unable to distinguish between Pinta, Yaws, and Syphilis. 3. **Treatment:** Long-acting **Benzathine Penicillin G** remains the drug of choice for all treponemal infections. 4. **Pinta Unique Feature:** It is the "mildest" treponematosis as it is strictly cutaneous.

Q: All of the following cause meningitis in the elderly except?

Streptobacillus moniliformis. **Explanation:** The etiology of bacterial meningitis varies significantly with age. In the **elderly (typically >65 years)**, physiological changes and comorbidities increase susceptibility to specific pathogens. **Why Streptobacillus moniliformis is the correct answer:** *Streptobacillus moniliformis* is the causative agent of **Rat-bite fever**. While it causes systemic symptoms like fever, rash, and polyarthralgia, it is **not** a recognized or common cause of meningitis in any age group, including the elderly. **Analysis of Incorrect Options:** * **Pneumococci (*Streptococcus pneumoniae*):** This is the **most common** cause of bacterial meningitis in adults and the elderly. Risk increases due to waning mucosal immunity and chronic lung diseases. * **Gram-negative bacteria:** Elderly patients are more prone to Gram-negative bacilli (like *E. coli* or *Klebsiella*) meningitis, often secondary to urinary tract infections, sepsis, or neurosurgical procedures. * **Listeria (*Listeria monocytogenes*):** This is a high-yield pathogen for the elderly and immunocompromised. Due to declining cell-mediated immunity (immunosenescence), the elderly are at high risk for *Listeria* meningitis, which often requires the addition of **Ampicillin** to the empiric treatment regimen. **High-Yield Clinical Pearls for NEET-PG:** * **Neonates ( 65 years):** *S. pneumoniae* (most common), *Listeria*, and Gram-negative bacilli. * **Empiric Therapy for Elderly:** Vancomycin + 3rd Gen Cephalosporin + **Ampicillin** (to cover *Listeria*).

Q: Lepra bacilli are mainly transmitted by?

Droplets. **Explanation:** **1. Why Droplets is Correct:** The primary mode of transmission for *Mycobacterium leprae* is via **respiratory droplets** (aerosolized secretions) from the nasal mucosa of untreated patients with lepromatous leprosy. The nasal mucosa is a major reservoir, shedding millions of bacilli daily. Infection occurs when a susceptible individual inhales these droplets during prolonged, close contact. **2. Why Other Options are Incorrect:** * **Skin Contact:** While historically believed to be the main route, skin-to-skin contact is now considered an **inefficient** mode of transmission. Intact skin is an effective barrier; transmission only occurs if there is contact between broken skin and a high load of bacilli, which is rare compared to the respiratory route. * **Stool and Urine:** *M. leprae* is not an enteric or urinary pathogen. There is no evidence of fecal-oral or urogenital transmission in humans. **3. Clinical Pearls for NEET-PG:** * **Incubation Period:** Leprosy has a very long incubation period, averaging **3 to 5 years** (can range from 6 months to 20 years). * **Target Cells:** The bacilli have a predilection for **Schwann cells** (leading to nerve damage) and **macrophages**. * **Temperature Sensitivity:** *M. leprae* grows best at **30°C**, which explains its preference for cooler body parts like the nose, ears, and peripheral nerves. * **Infectivity vs. Pathogenicity:** Leprosy has **high infectivity but low pathogenicity**; most people (approx. 95%) have natural immunity and do not develop the disease even after exposure. * **Armadillos:** In certain regions (like the Southern US), the nine-banded armadillo is a known zoonotic reservoir.

Q: Gonococcus can infect all of the following structures except:

Vagina. **Explanation:** The correct answer is **B. Vagina**. The primary reason *Neisseria gonorrhoeae* does not typically infect the adult vagina is the nature of its epithelium. The adult vagina is lined by **stratified squamous epithelium**, which is thick, multi-layered, and resistant to gonococcal invasion. Furthermore, the acidic pH of the adult vagina (maintained by Döderlein’s bacilli) creates an inhospitable environment for the bacteria. In contrast, Gonococcus has a high affinity for **columnar and transitional epithelium**. **Analysis of Options:** * **Cervix (A):** This is the most common site of gonococcal infection in women. The endocervix is lined by simple columnar epithelium, which is highly susceptible. * **Bartholin Gland (C):** These glands are lined by columnar/transitional epithelium. Gonorrhea is a classic cause of acute Bartholin cyst or abscess formation. * **Fallopian Tube (D):** The tubes are lined by ciliated columnar epithelium. Ascending infection leads to salpingitis, a key component of Pelvic Inflammatory Disease (PID). **Clinical Pearls for NEET-PG:** * **Exception:** Gonococcal vaginitis *can* occur in **pre-pubertal girls** and **post-menopausal women**. In these groups, the vaginal epithelium is thin (atrophic) and the pH is neutral/alkaline due to low estrogen levels, making it vulnerable. * **Urethra:** The female urethra and Skene’s glands are also common sites of infection due to their epithelial lining. * **Gold Standard Diagnosis:** Culture on **Thayer-Martin medium** (Chocolate agar with antibiotics). * **Treatment:** Current CDC guidelines recommend **Ceftriaxone** (IM) as the first-line treatment.

Question 1

A 27-year-old man develops acute pain and swelling of the left knee 5 days after an episode of urethritis. On physical examination, the left knee is swollen, warm, and tender to the touch. No other joints are affected. Laboratory examination of fluid aspirated from the left knee joint shows numerous neutrophils. A Gram stain of the fluid shows gram-negative intracellular diplococci. No crystals are seen. Which of the following infectious agents is most likely responsible for his condition?

Accepted Answer

Neisseria gonorrhoeae

Answer

Borrelia burgdorferi

Answer

Haemophilus influenzae

Answer

Staphylococcus aureus

Question 2

'Spider' colonies are produced by:

Accepted Answer

Actinomyces israelii

Answer

Bordetella pertussis

Answer

Vibrio cholerae

Answer

Mycobacterium leprae

Question 3

Medusahead appearance on agar plates is characteristic of which bacterium?

Accepted Answer

Bacillus

Answer

Hemophilus

Answer

Pneumococcus

Answer

Yersinia

Question 4

Pontiac fever is caused by which of the following agents?

Accepted Answer

Legionella

Answer

Mycoplasma

Answer

Rickettsia

Answer

Salmonella

Question 5

Pinta is caused by which of the following organisms?

Accepted Answer

Treponema carateum

Answer

Treponema pennue

Answer

Treponema pallidum

Answer

Treponema endemicum

Question 6

What is true about gas gangrene?

Accepted Answer

The most common organism implicated is Clostridium perfringens.

Answer

Underlying skin and muscle are normal.

Answer

It is caused by tetanospasmin toxin.

Answer

Muscle rigidity and spasm are characteristic.

Question 7

All of the following cause meningitis in the elderly except?

Accepted Answer

Streptobacillus moniliformis

Answer

Pneumococci

Answer

Gram negative bacteria

Answer

Listeria

Question 8

A 47-year-old woman with a lengthy history of heartburn and dyspepsia experiences a sudden onset of abdominal pain. On physical examination, she has severe mid-epigastric pain with guarding. Bowel sounds are reduced. An abdominal plain film radiograph shows free air under the left leaf of the diaphragm. She is immediately taken to surgery, and a perforated duodenal ulcer is repaired. Which of the following organisms is most likely to have produced these findings?

Accepted Answer

Helicobacter pylori

Answer

Campylobacter jejuni

Answer

Cryptosporidium parvum

Answer

Giardia lamblia

Question 9

Lepra bacilli are mainly transmitted by?

Accepted Answer

Droplets

Answer

Skin contact

Answer

Stool

Answer

Urine

Question 10

Gonococcus can infect all of the following structures except:

Accepted Answer

Vagina

Answer

Cervix

Answer

Bartholin gland

Answer

Fallopian tube

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?