Bacteriology Practice Questions

Q: A 70-year-old patient presents with high-grade fever, dry cough, and abdominal pain. Sputum sample collected from the patient shows Gram-negative organisms that are able to grow only on charcoal yeast extract medium. What is the most likely organism?

Legionella. ### **Explanation** The correct answer is **Legionella pneumophila**. **1. Why Legionella is correct:** The clinical presentation of high-grade fever, dry cough (atypical pneumonia), and **extrapulmonary symptoms** like abdominal pain (or diarrhea/confusion) is classic for Legionnaires' disease. The definitive microbiological clue is the organism's growth requirement: *Legionella* is a fastidious Gram-negative rod that **requires L-cysteine and iron** for growth. **Buffered Charcoal Yeast Extract (BCYE) agar** is the specific selective medium used, where the charcoal acts to neutralize toxic fatty acids and peroxides. **2. Why the other options are incorrect:** * **Haemophilus influenzae:** While it causes pneumonia, it requires **Factors X (hemin) and V (NAD)** for growth and is typically cultured on **Chocolate Agar**, not charcoal medium. * **Listeria monocytogenes:** This is a **Gram-positive** bacillus (unlike the organism in the question). It exhibits "tumbling motility" and grows well on standard media like blood agar, often showing narrow-zone beta-hemolysis. * **Moraxella catarrhalis:** This is a Gram-negative **diplococcus** (not a rod) that grows easily on blood and chocolate agar. It is a common cause of COPD exacerbations but does not require BCYE agar. **3. NEET-PG High-Yield Pearls:** * **Stain:** *Legionella* stains poorly on Gram stain; **Silver (Dieterle) stain** is preferred. * **Diagnosis:** The **Urinary Antigen Test** is the most rapid initial test (detects Serogroup 1). * **Hyponatremia:** A very common laboratory finding in *Legionella* infections (SIADH-like picture). * **Transmission:** Associated with **aerosols from water sources** (AC cooling towers, showers, nebulizers); no person-to-person transmission. * **Treatment:** Macrolides (Azithromycin) or Fluoroquinolones (Levofloxacin).

Q: In a splenectomized patient, which of the following organisms is least likely to cause infection?

Staph. aureus. **Explanation:** The spleen plays a critical role in the body’s immune defense, specifically through the filtration of blood and the production of antibodies (IgM) by splenic B-cells. Its most vital function in this context is the clearance of **encapsulated bacteria** via opsonization. **Why Staph. aureus is the correct answer:** *Staphylococcus aureus* is a Gram-positive coccus that is **not** typically encapsulated in the clinical sense that requires splenic clearance. The risk of Overwhelming Post-Splenectomy Infection (OPSI) is primarily associated with organisms that possess a polysaccharide capsule. Since the spleen is the primary site for the destruction of these opsonized encapsulated organisms, its absence does not significantly predispose a patient to *Staph. aureus* infections compared to the general population. **Why the other options are incorrect:** * **A, B, and C (Pneumococci, Klebsiella, H. influenzae):** These are all **encapsulated organisms**. * *Streptococcus pneumoniae* (Pneumococcus) is the most common cause of OPSI (50-90% of cases). * *Haemophilus influenzae* type b and *Neisseria meningitidis* are the other two "classic" encapsulated pathogens. * *Klebsiella pneumoniae* is also an encapsulated Gram-negative rod that poses an increased risk in asplenic individuals. **High-Yield Clinical Pearls for NEET-PG:** * **OPSI (Overwhelming Post-Splenectomy Infection):** Characterized by a prodrome of fever and malaise, rapidly progressing to septic shock and DIC. * **Most Common Organism:** *Streptococcus pneumoniae*. * **Vaccination Protocol:** Patients should ideally be vaccinated **2 weeks before** elective splenectomy or **2 weeks after** emergency splenectomy. * **Essential Vaccines:** Pneumococcal, Meningococcal, and *H. influenzae* type b (Hib). * **Peripheral Smear Findings:** Look for **Howell-Jolly bodies** (nuclear remnants), Pappenheimer bodies, and Heinz bodies.

Q: Which of the following microorganisms is a known cause of necrotizing enterocolitis?

Clostridium perfringens type C. **Explanation:** **Clostridium perfringens type C** is the correct answer because it is the specific causative agent of **Enteritis Necroticans** (also known as *Pigbel*). This condition is characterized by patchy, necrotizing inflammation of the small intestine, particularly the jejunum. The pathogenesis is driven by the production of the **Beta-toxin**, a potent necrotizing and lethal toxin. In individuals with low dietary protein intake or those consuming foods containing trypsin inhibitors (like sweet potatoes), the Beta-toxin is not inactivated by pancreatic trypsin, leading to severe mucosal destruction and gas gangrene of the bowel wall. **Analysis of Incorrect Options:** * **Clostridium botulinum:** Causes botulism, a paralytic illness mediated by a neurotoxin that inhibits acetylcholine release at the neuromuscular junction. It does not cause primary necrotizing bowel disease. * **Clostridium tetani:** Produces tetanospasmin, which blocks inhibitory neurotransmitters (GABA/glycine) in the spinal cord, leading to spastic paralysis and "lockjaw." * **Clostridium perfringens type A:** While it is the most common cause of gas gangrene (myonecrosis) and food poisoning (via an enterotoxin), it is not the primary agent associated with the specific clinical entity of necrotizing enteritis/Pigbel. **NEET-PG High-Yield Pearls:** * **Beta-toxin** is the hallmark of *C. perfringens* Type C; it is highly sensitive to trypsin. * **Nagler’s Reaction:** Used to identify *C. perfringens* by detecting **Alpha-toxin** (Lecithinase) activity on egg yolk agar. * **Stormy Fermentation:** A characteristic laboratory finding in litmus milk culture due to vigorous gas production by *C. perfringens*. * **Note:** While *C. perfringens* Type C causes Enteritis Necroticans, neonatal Necrotizing Enterocolitis (NEC) is often multifactorial, though *C. perfringens* and *C. butyricum* are frequently implicated.

Q: What is the causative organism of hemolytic uremic syndrome?

Shigella. **Explanation:** **Hemolytic Uremic Syndrome (HUS)** is a clinical triad consisting of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. **Why Shigella is the correct answer:** The primary causative agent among the options is **Shigella dysenteriae type 1**. It produces the potent **Shiga toxin (Stx)**. This toxin binds to the Gb3 receptors on glomerular endothelial cells, leading to endothelial damage, microthrombi formation, and subsequent renal failure. While *Enterohemorrhagic E. coli* (EHEC/O157:H7) is the most common cause of HUS globally, *Shigella dysenteriae* is a classic and frequently tested cause in the context of bacillary dysentery. **Why the other options are incorrect:** * **ETEC (Option A):** Causes traveler’s diarrhea via heat-labile (LT) and heat-stable (ST) toxins. It does not produce Shiga toxin and is not associated with HUS. * **Salmonella (Option B):** Typically causes enteric fever or gastroenteritis. While it can cause systemic illness, it does not produce the specific toxins required to trigger HUS. * **Pseudomonas (Option D):** An opportunistic pathogen causing pneumonia, UTIs, and sepsis (especially in burns or cystic fibrosis), but it is not an etiological agent for HUS. **NEET-PG High-Yield Pearls:** * **Most common cause overall:** Enterohemorrhagic *E. coli* (EHEC), specifically the O157:H7 strain (produces Shiga-like toxin/Verotoxin). * **The Toxin:** Shiga toxin inhibits protein synthesis by damaging the **28S ribosomal RNA** (60S subunit). * **Clinical Tip:** Antibiotics are generally **avoided** in EHEC-related diarrhea as they may increase toxin release and worsen the risk of HUS. * **Schistocytes:** A peripheral smear in HUS will characteristically show fragmented RBCs (schistocytes) due to mechanical destruction in microvasculature.

Q: A painful vaginal ulcer with inguinal lymphadenopathy and a "school of fish" appearance on microscopy are characteristic of which sexually transmitted infection?

Chancroid. ### Explanation **Correct Answer: D. Chancroid** **Why it is correct:** Chancroid is caused by the Gram-negative coccobacillus ***Haemophilus ducreyi*** [1]. It is characterized by the clinical triad of **painful** genital ulcers (often multiple with ragged edges), inflammatory **inguinal lymphadenopathy** (buboes), and a specific microscopic arrangement [1]. On Gram stain, the organisms align in parallel rows, described classically as a **"school of fish"** or "railroad track" appearance. **Why the other options are incorrect:** * **Syphilis (Treponema pallidum):** Presents with a **painless**, indurated ulcer (chancre) [1]. Microscopy via dark-field illumination shows corkscrew-shaped motile spirochetes, not a school of fish pattern. * **Lymphogranuloma venereum (LGV):** Caused by *Chlamydia trachomatis* (L1-L3). It features a transient, **painless** papule/ulcer followed by painful "Groove sign" lymphadenopathy [1]. It is an obligate intracellular organism. * **Granuloma inguinale (Donovanosis):** Caused by *Klebsiella granulomatis*. It presents as **painless**, beefy-red velvety ulcers that bleed on touch [1]. Microscopy reveals **Donovan bodies** (safety-pin appearance) within macrophages. **High-Yield Clinical Pearls for NEET-PG:** * **Painful vs. Painless:** Remember **"H. ducreyi makes you cry"** (Painful = Chancroid and Herpes; Painless = Syphilis, LGV, and Donovanosis). * **Buboes:** In Chancroid, lymphadenopathy is painful and may suppurate (unilateral). * **Treatment:** A single dose of **Azithromycin (1g)** or Ceftriaxone (250mg IM) is the first-line management. * **Culture:** Requires special media (e.g., Mueller-Hinton agar with vancomycin) and is difficult to grow.

Q: Which of the following is NOT a metastatic complication of gonococci infection?

Nephritis. **Explanation:** *Neisseria gonorrhoeae* primarily causes localized infections of the urogenital tract. However, in approximately 0.5% to 3% of cases, the bacteria enter the bloodstream, leading to **Disseminated Gonococcal Infection (DGI)**. **1. Why Nephritis is the Correct Answer:** Nephritis is **not** a recognized metastatic complication of gonococcal infection. While *N. gonorrhoeae* can cause ascending infections like pelvic inflammatory disease (PID) or localized urethritis, it does not typically seed the kidneys via the hematogenous route. In contrast, renal involvement is more characteristic of other bacterial infections (like *Staphylococcus aureus* or *Streptococcus pyogenes* via immune-mediated mechanisms). **2. Analysis of Incorrect Options (Metastatic Sites):** When gonococci disseminate (DGI), they tend to localize in specific extra-genital sites: * **Arthritis (Option D):** This is the most common metastatic complication. It typically presents as either a polyarthralgia-tenosynovitis syndrome or a purulent monoarthritis (septic arthritis). * **Endocarditis (Option A):** A rare but severe complication where the bacteria seed the heart valves. * **Meningitis (Option B):** Another rare manifestation where the organism crosses the blood-brain barrier. **High-Yield Clinical Pearls for NEET-PG:** * **Thayer-Martin Medium:** The selective medium of choice for isolating *N. gonorrhoeae*. * **DGI Triad:** Tenosynovitis, dermatitis (painless pustules), and polyarthralgia. * **Virulence Factor:** Pili are essential for initial attachment to mucosal surfaces. * **Deficiency Link:** Patients with **late complement component deficiencies (C5–C9)** are at a significantly higher risk for disseminated gonococcal infections.

Q: Most of the clinical cases of Listeria monocytogenes infections are attributed to which serovar?

4b. **Explanation:** *Listeria monocytogenes* is a Gram-positive, facultative intracellular bacillus known for causing foodborne illnesses, neonatal sepsis, and meningitis in immunocompromised individuals. While there are 13 known serovars based on somatic (O) and flagellar (H) antigens, the vast majority of human clinical disease is caused by only three: **1/2a, 1/2b, and 4b.** **Why 4b is the correct answer:** Serovar **4b** is the most clinically significant because it is responsible for the majority of **large-scale foodborne outbreaks** and sporadic cases of listeriosis worldwide. It possesses specific virulence factors that enhance its ability to cross the intestinal, blood-brain, and placental barriers, making it the predominant serovar in invasive disease. **Analysis of Incorrect Options:** * **Option A (1):** While serovars 1/2a and 1/2b are common causes of sporadic human listeriosis, they are less frequently associated with major epidemic outbreaks compared to 4b. * **Option B (4a):** This serovar is primarily associated with environmental sources or animal infections and is rarely isolated from human clinical cases. * **Option D (6):** Serovar 6 (and others like 5 or 7) are extremely rare in human pathology and do not contribute significantly to the clinical burden of the disease. **High-Yield Clinical Pearls for NEET-PG:** * **Tumbling Motility:** Characteristically seen at 25°C (peritrichous flagella), but absent at 37°C. * **Cold Enrichment:** It can grow at temperatures as low as 4°C (important for refrigerated food contamination like soft cheese and deli meats). * **CAMP Test Positive:** Shows "block" hemolysis (unlike the "arrowhead" of Group B Streptococcus). * **Treatment of Choice:** Ampicillin (Listeria is inherently resistant to all cephalosporins).

Q: A child with repeated skin infections presents with a liver abscess. What is the most likely causative organism?

Staphylococcus aureus. **Explanation:** The clinical presentation of a child with **repeated skin infections** followed by a **liver abscess** strongly points toward **Staphylococcus aureus**. **1. Why Staphylococcus aureus is correct:** In the pediatric population, *S. aureus* is the most common cause of liver abscesses, particularly when there is a history of recurrent pyogenic skin infections (like furuncles or impetigo). The organism typically reaches the liver via **hematogenous spread** (bacteremia) from a primary skin focus. This association is also a classic "red flag" for **Chronic Granulomatous Disease (CGD)**, an immunodeficiency where patients are susceptible to catalase-positive organisms like *S. aureus*. **2. Why the other options are incorrect:** * **Streptococcus milleri (Option A):** While part of the *S. anginosus* group known for causing abscesses, these are usually associated with direct extension from intra-abdominal infections (like appendicitis) rather than skin infections. * **Bacteroides fragilis (Option B):** This is an anaerobe typically involved in polymicrobial liver abscesses arising from portal vein drainage (e.g., following bowel perforation or inflammatory bowel disease). * **Streptococcus agalactiae (Option D):** Also known as Group B Streptococcus (GBS), this is a leading cause of neonatal sepsis and meningitis but is not a common cause of liver abscesses in older children with skin infections. **Clinical Pearls for NEET-PG:** * **Most common cause of pyogenic liver abscess (overall):** *E. coli* (adults/portal route); *S. aureus* (children/hematogenous). * **Amoebic Liver Abscess:** Caused by *Entamoeba histolytica*; look for "anchovy sauce" pus and a history of bloody diarrhea. * **CGD Link:** If a child has recurrent *S. aureus* or *Serratia marcescens* abscesses, the screening test of choice is the **Dihydrorhodamine (DHR) 123 assay** or the Nitroblue Tetrazolium (NBT) test.

Question 1

A 70-year-old patient presents with high-grade fever, dry cough, and abdominal pain. Sputum sample collected from the patient shows Gram-negative organisms that are able to grow only on charcoal yeast extract medium. What is the most likely organism?

Accepted Answer

Legionella

Answer

Haemophilus influenzae

Answer

Listeria monocytogenes

Answer

Moraxella catarrhalis

Question 2

In a splenectomized patient, which of the following organisms is least likely to cause infection?

Accepted Answer

Staph. aureus

Answer

Pneumococci

Answer

Klebsiella

Answer

H. influenzae

Question 3

Which of the following microorganisms is a known cause of necrotizing enterocolitis?

Accepted Answer

Clostridium perfringens type C

Answer

Clostridium botulinum

Answer

Clostridium tetani

Answer

Clostridium perfringens type A

Question 4

Which of the following is false regarding H. pylori?

Accepted Answer

With chronic infection, the urease breath test can become negative.

Answer

H. pylori infection remains lifelong if untreated.

Answer

Endoscopy is diagnostic.

Answer

Toxigenic strains usually cause ulcers.

Question 5

What is the causative organism of hemolytic uremic syndrome?

Accepted Answer

Shigella

Answer

Enterotoxigenic Escherichia coli (ETEC)

Answer

Salmonella

Answer

Pseudomonas

Question 6

A 35-year-old patient complains of abdominal cramps along with profuse diarrhea. The treating physician wants to process the stool specimen for isolation of Campylobacter jejuni. Which of the following is the method of choice for the culture of stool?

Accepted Answer

Culture on Skirrow's medium incubated at 42°C under micro-aerophilic conditions.

Answer

Culture on TCBS medium incubated at 37°C under aerobic conditions.

Answer

Culture on MacConkey's medium incubated at 42°C under anaerobic conditions.

Answer

Culture on Wilson and Blair's medium incubated at 37° under microaerophilic conditions.

Question 7

A painful vaginal ulcer with inguinal lymphadenopathy and a "school of fish" appearance on microscopy are characteristic of which sexually transmitted infection?

Accepted Answer

Chancroid

Answer

Syphilis

Answer

Lymphogranuloma venereum (LGV)

Answer

Granuloma inguinale (Donovanosis)

Question 8

Which of the following is NOT a metastatic complication of gonococci infection?

Accepted Answer

Nephritis

Answer

Endocarditis

Answer

Meningitis

Answer

Arthritis

Question 9

Most of the clinical cases of Listeria monocytogenes infections are attributed to which serovar?

Accepted Answer

4b

Answer

1

Answer

4a

Answer

6

Question 10

A child with repeated skin infections presents with a liver abscess. What is the most likely causative organism?

Accepted Answer

Staphylococcus aureus

Answer

Streptococcus milleri

Answer

Bacteroides fragilis

Answer

Streptococcus agalactiae

Bacteriology — MCQs

Bacteriology — MCQs

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