Bacteriology Practice Questions

Q: Which of the following microorganisms is described as "strictly intracellular but not cultivable"?

Mycobacterium leprae. **Explanation:** The correct answer is **Mycobacterium leprae**. **1. Why Mycobacterium leprae is correct:** *M. leprae*, the causative agent of Leprosy (Hansen’s disease), is a unique pathogen because it is an **obligate intracellular** organism that has never been successfully grown on artificial (cell-free) culture media or tissue culture. It lacks many essential genes for independent metabolism (reductive evolution). In the laboratory, it can only be cultivated in living animals, specifically the **footpads of mice** (Shepard’s method) or **nine-banded armadillos**. **2. Analysis of Incorrect Options:** * **Mycobacterium tuberculosis:** While it is a facultative intracellular pathogen, it is **cultivable** on artificial media like Lowenstein-Jensen (LJ) medium or liquid systems like BACTEC. * **Treponema pallidum:** This organism is the causative agent of Syphilis. It is indeed **non-cultivable** on artificial media; however, it is **extracellular** in nature (found in interstitial spaces), not strictly intracellular. * **Salmonella:** This is a **facultative intracellular** bacterium that grows easily on standard laboratory media like Blood Agar or MacConkey Agar. **3. NEET-PG High-Yield Pearls:** * **Generation Time:** *M. leprae* has the longest generation time among bacteria (~12–13 days), contributing to the long incubation period of leprosy. * **Staining:** It is Acid-Fast (Ziehl-Neelsen stain) but is **less acid-fast** than *M. tuberculosis*; hence, 5% sulfuric acid is used for decolorization instead of 20%. * **Target Cells:** It shows a unique tropism for **Schwann cells** (leading to nerve damage) and macrophages. * **Other Non-cultivable organisms:** *Tropheryma whipplei* and *Treponema pallidum* are also non-cultivable on artificial media, but *M. leprae* is the classic "intracellular" example.

Q: Which of the following organisms would most likely be isolated from the vagina of a normal 5-year-old girl?

Staphylococcus. The composition of vaginal flora is primarily governed by **estrogen levels**, which influence the glycogen content of the vaginal epithelium. This is a high-yield concept for NEET-PG. **Explanation of the Correct Answer:** In a **pre-pubertal girl (like a 5-year-old)**, estrogen levels are low. Consequently, the vaginal epithelium is thin and lacks glycogen. Without glycogen, acid-producing bacteria cannot thrive, resulting in a **neutral or slightly alkaline pH (around 7.0)**. In this environment, the predominant flora consists of skin and colonic organisms, most commonly **Staphylococci**, Streptococci, and enteric Gram-negative bacilli (e.g., *E. coli*). **Analysis of Incorrect Options:** * **B. Lactobacillus:** This is the dominant flora in **newborns** (due to maternal estrogen) and **post-pubertal females**. Lactobacilli ferment glycogen to lactic acid, maintaining an acidic pH (4.0–4.5). They are absent during the childhood years due to the lack of estrogen. * **A. Candida:** While *Candida* can be part of the normal flora in some adult women, it thrives in acidic environments. It is an uncommon finding in a healthy 5-year-old unless there is a history of antibiotic use or underlying immunosuppression. * **C. Neisseria:** *Neisseria species* (other than *N. gonorrhoeae*) are generally commensals of the upper respiratory tract, not the vagina. **NEET-PG High-Yield Pearls:** 1. **Newborn (0–6 weeks):** Acidic pH; dominated by *Lactobacillus* (due to maternal estrogen). 2. **Childhood (6 weeks to Puberty):** Alkaline/Neutral pH; dominated by *Staphylococci* and *Streptococci*. 3. **Puberty to Menopause:** Acidic pH; dominated by *Lactobacillus* (Doderlein’s bacilli). 4. **Post-menopause:** Returns to Neutral/Alkaline pH; *Lactobacillus* decreases, and mixed flora returns.

Q: What is true about Mycoplasma?

It may be a commensal in the throat. **Explanation:** **Mycoplasma pneumoniae** is a unique pathogen characterized by the lack of a cell wall. While it is a well-known cause of "Atypical Pneumonia," it is important to note that it can also exist as a **commensal** in the upper respiratory tract (throat) of healthy individuals or persist there for weeks following an infection. **Analysis of Options:** * **Option A (Incorrect):** While some species prefer microaerophilic conditions, $CO_2$ is not a mandatory requirement for growth. The most critical growth requirement for Mycoplasma is **exogenous sterols (cholesterol)**, which they incorporate into their cell membrane. * **Option B (Incorrect):** Mycoplasma pneumoniae produces **hydrogen peroxide ($H_2O_2$)** and superoxide radicals, which lead to the oxidative hemolysis of host red blood cells. This is a key virulence factor. * **Option D (Incorrect):** Mycoplasma is **not** an acid-fast bacillus. Because it lacks a cell wall, it does not take up Gram stain (Gram-negative appearance) and is highly pleomorphic. Acid-fastness is a characteristic of *Mycobacterium* species. **High-Yield Clinical Pearls for NEET-PG:** * **Cell Wall-less:** Naturally resistant to Beta-lactams (Penicillins/Cephalosporins). Treatment of choice: **Macrolides** (Azithromycin) or Tetracyclines. * **Culture:** Grows on **PPLO agar** (Pleuropneumonia-like organisms); exhibits a characteristic **"Fried Egg" colony** appearance. * **Diagnosis:** The **Cold Agglutinin Test** (IgM antibodies against I-antigen of RBCs) is a classic bedside screening test, though non-specific. * **Walking Pneumonia:** Patients often look clinically better than their chest X-ray suggests (patchy consolidation).

Q: An unimmunized, 2-year-old boy presents with drooling from the mouth, elevated temperature, and enlarged tonsils. During attempts at intubation, no gray-white membrane is observed but the epiglottis appears "beefy" red and edematous. Which of the following is the most likely organism?

Haemophilus influenzae. ### Explanation The clinical presentation of drooling, high fever, and a "beefy red," edematous epiglottis in an unimmunized child is a classic description of **Acute Epiglottitis**. **1. Why Haemophilus influenzae is correct:** Historically, **Haemophilus influenzae type b (Hib)** was the most common cause of acute epiglottitis in children. It is a Gram-negative coccobacillus that requires Factors X (hemin) and V (NAD) for growth. The "beefy red" appearance is due to intense inflammation of the supraglottic structures, which can lead to life-threatening airway obstruction. The lack of a gray-white membrane helps differentiate this from Diphtheria. Since the child is unimmunized, he lacked the Hib vaccine, making him highly susceptible. **2. Why the other options are incorrect:** * **A. Haemophilus haemolyticus:** This is generally considered commensal flora of the upper respiratory tract and is rarely pathogenic; it does not cause epiglottitis. * **C. Klebsiella pneumoniae:** While it causes severe pneumonia (characterized by "currant jelly" sputum), it is not a typical cause of acute epiglottitis in children. * **D. Mycoplasma pneumoniae:** This is the leading cause of "Atypical Pneumonia" (Walking Pneumonia) in older children and young adults. It typically presents with a persistent dry cough and interstitial infiltrates, not acute airway obstruction. **3. High-Yield NEET-PG Pearls:** * **X-ray Sign:** The characteristic finding on a lateral neck X-ray is the **"Thumb sign"** (swollen epiglottis). * **Clinical Triad:** The "3 Ds"—Drooling, Dysphagia, and Distress (Stridor). * **Management:** Do NOT examine the throat with a tongue depressor as it may trigger fatal laryngospasm. Secure the airway first. * **Culture:** Grows on **Chocolate Agar** (contains released X and V factors). It shows the **"Satellitism"** phenomenon around *S. aureus* on blood agar.

Q: All of the following statements about staphylococci are TRUE, EXCEPT:

Impetigo is caused only by S. aureus.. ### Explanation **1. Why Option C is the Correct Answer (The Exception):** Impetigo is a highly contagious superficial skin infection. While **Staphylococcus aureus** is the most common cause globally, it is **not the only** causative agent. **Streptococcus pyogenes (Group A Streptococcus)** is also a significant cause of impetigo, particularly the "non-bullous" variety. In many cases, both organisms may be isolated from the same lesion. Therefore, the statement that it is caused *only* by *S. aureus* is incorrect. **2. Analysis of Other Options:** * **Option A:** Toxic Shock Syndrome (TSS) was historically associated with high-absorbency tampon use in women. However, "non-menstrual TSS" occurs in both males and females due to surgical wound infections, burns, or colonized sites producing the TSST-1 toxin. * **Option B:** Approximately 20–30% of the healthy population are persistent **nasal carriers** of *S. aureus*, which serves as a major reservoir for community and hospital-acquired infections. * **Option C:** *S. epidermidis* (a Coagulase-negative Staphylococcus) is a common skin commensal. It is a leading cause of infective endocarditis in patients with prosthetic valves and is frequently isolated from endocarditis cases in **intravenous drug abusers (IVDAs)**, though *S. aureus* remains the most common overall in IVDAs. **3. NEET-PG High-Yield Pearls:** * **Impetigo Types:** Bullous impetigo is almost exclusively caused by *S. aureus* (due to exfoliative toxins), whereas non-bullous impetigo can be caused by both *S. aureus* and *S. pyogenes*. * **Catalase vs. Coagulase:** All Staphylococci are Catalase positive (distinguishes them from Streptococci). *S. aureus* is Coagulase positive, while *S. epidermidis* and *S. saprophyticus* are Coagulase-negative (CoNS). * **Novobiocin Sensitivity:** *S. epidermidis* is sensitive, while *S. saprophyticus* (common cause of UTI in young females) is resistant.

Q: All of the following Proteus species form indole EXCEPT?

P. mirabilis. **Explanation:** The genus *Proteus* belongs to the family Enterobacteriaceae. A key biochemical test used to differentiate between the species of this genus is the **Indole test**, which determines the ability of an organism to produce the enzyme tryptophanase to break down the amino acid tryptophan into indole. 1. **Why P. mirabilis is correct:** *Proteus mirabilis* is characteristically **indole-negative**. This is a high-yield distinguishing feature because *P. mirabilis* is the most common cause of human infections (especially UTIs) within this genus. 2. **Why the other options are incorrect:** * **P. vulgaris:** This species is **indole-positive**. It is often differentiated from *P. mirabilis* in the lab using this specific test. * **P. rettgeri & P. morganii:** While these have been reclassified into their own genera (*Providencia rettgeri* and *Morganella morganii*), they were historically grouped with *Proteus* and are both **indole-positive**. **NEET-PG High-Yield Pearls:** * **Swarming Growth:** *Proteus* species exhibit "swarming" (concentric waves of growth) on solid media like blood agar due to their peritrichous flagella. This can be inhibited by increasing agar concentration (6%) or adding chloral hydrate/PABA. * **Urease Production:** All *Proteus* species are strong urease producers. This raises urinary pH, leading to the formation of **Struvite stones** (Triple phosphate/Staghorn calculi). * **Dienes Phenomenon:** A method used to differentiate two strains of *Proteus*; if two different strains meet on an agar plate, a line of inhibited growth forms between them. * **Weil-Felix Reaction:** Antigens of certain *Proteus* strains (OX-19, OX-2, OX-K) cross-react with antibodies against Rickettsiae, serving as a diagnostic tool.

Q: In a case of neonatal meningitis, the etiologic bacteria is found to have properties of beta hemolysis, bacitracin resistance, and CAMP positive. Which of the following is the most likely causative agent?

S. agalactiae. **Explanation:** The clinical presentation of neonatal meningitis combined with the laboratory findings points directly to **Streptococcus agalactiae (Group B Streptococcus - GBS)**. **Why S. agalactiae is correct:** * **CAMP Test Positive:** This is the hallmark test for GBS. The organism produces the "CAMP factor," which acts synergistically with the beta-lysin of *Staphylococcus aureus* to produce an enhanced zone of hemolysis (arrowhead shape) on blood agar. * **Bacitracin Resistance:** Unlike Group A Strep, GBS is resistant to bacitracin. * **Beta-hemolysis:** GBS typically shows a narrow zone of beta-hemolysis. * **Clinical Context:** S. agalactiae is the leading cause of neonatal sepsis and meningitis, usually acquired from the maternal vaginal flora during birth. **Why other options are incorrect:** * **S. pyogenes (Group A Strep):** While it is beta-hemolytic, it is **Bacitracin sensitive** and CAMP negative. It primarily causes pharyngitis and skin infections, not neonatal meningitis. * **S. pneumoniae:** Shows **alpha-hemolysis** (greenish discoloration) and is bile soluble and optochin sensitive. It is a common cause of meningitis in adults/children, but not the primary agent in the immediate neonatal period. * **E. faecalis:** Usually shows **gamma-hemolysis** (non-hemolytic) and is characterized by growth in 6.5% NaCl and bile esculin hydrolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Hippurate Hydrolysis:** S. agalactiae is also Hippurate hydrolysis positive. * **Prevention:** Screening pregnant women at 35–37 weeks and administering intrapartum penicillin is the standard preventive measure. * **Top 3 causes of Neonatal Meningitis:** 1. *S. agalactiae*, 2. *E. coli* (K1 strain), 3. *Listeria monocytogenes*.

Question 1

Which of the following microorganisms is described as "strictly intracellular but not cultivable"?

Accepted Answer

Mycobacterium leprae

Answer

Mycobacterium tuberculosis

Answer

Treponema pallidum

Answer

Salmonella

Question 2

Which of the following organisms would most likely be isolated from the vagina of a normal 5-year-old girl?

Accepted Answer

Staphylococcus

Answer

Candida

Answer

Lactobacillus

Answer

Neisseria

Question 3

A 24-year-old woman presents with a 3-day history of fever, chills, chest pain, and cough productive of rust-colored sputum. Her past medical history includes a splenectomy 1 year ago. A chest x-ray film indicates consolidation of the right lower lobe. Blood cultures are positive for alpha-hemolytic gram-positive diplococci. Immunity to the causative organism is based on?

Accepted Answer

IgG antibodies to a surface acidic polysaccharide

Answer

Alternative complement pathway activation

Answer

Antibody to an alpha-helical coiled fimbria

Answer

IgA antibodies to C carbohydrate

Question 4

What is true about Mycoplasma?

Accepted Answer

It may be a commensal in the throat

Answer

Carbon dioxide is important for its growth

Answer

It does not cause hemolysis

Answer

It is an acid-fast bacillus

Question 5

An unimmunized, 2-year-old boy presents with drooling from the mouth, elevated temperature, and enlarged tonsils. During attempts at intubation, no gray-white membrane is observed but the epiglottis appears "beefy" red and edematous. Which of the following is the most likely organism?

Accepted Answer

Haemophilus influenzae

Answer

Haemophilus haemolyticus

Answer

Klebsiella pneumoniae

Answer

Mycoplasma pneumoniae

Question 6

In the treatment of patients who have cholera, the use of a drug that inhibits adenyl cyclase would be expected to have which of the following characteristics?

Accepted Answer

Block the action of cholera toxin

Answer

Eradicate the organism

Answer

Increase fluid secretion

Answer

Kill the patient immediately

Question 7

All of the following statements about staphylococci are TRUE, EXCEPT:

Accepted Answer

Impetigo is caused only by S. aureus.

Answer

Toxic shock syndrome may affect both males and females.

Answer

Many individuals are nasal carriers of S. aureus.

Answer

S. epidermidis is often responsible for infective endocarditis in intravenous drug abusers.

Question 8

All of the following Proteus species form indole EXCEPT?

Accepted Answer

P. mirabilis

Answer

P. vulgaris

Answer

P. rettgeri

Answer

P. morganii

Question 9

In a case of neonatal meningitis, the etiologic bacteria is found to have properties of beta hemolysis, bacitracin resistance, and CAMP positive. Which of the following is the most likely causative agent?

Accepted Answer

S. agalactiae

Answer

S. pyogenes

Answer

S. pneumoniae

Answer

E. faecalis

Question 10

All the statements about nontuberculous mycobacteria (NTM) are TRUE, EXCEPT:

Accepted Answer

Pulmonary disease is usually associated with systemic immunodeficiency.

Answer

NTM are ubiquitous in soil and water

Answer

No human-to-human transmission of NTM

Answer

Disseminated disease denotes significant immune dysfunction

Bacteriology — MCQs

Bacteriology — MCQs

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