Bacteriology Practice Questions

Q: A patient presented with pyrexia of unknown origin. Blood culture in special laboratory media was positive for gram-negative coccobacilli which were oxidase positive. Which one of the following is the likely organism grown in culture?

Brucella spp.. ### Explanation **Correct Option: D. Brucella spp.** The clinical presentation of Pyrexia of Unknown Origin (PUO) combined with the laboratory findings of **Gram-negative coccobacilli** that are **oxidase positive** strongly points toward *Brucella*. * **Microbiology:** *Brucella* are small, aerobic, non-motile, Gram-negative coccobacilli. They are catalase and oxidase positive. * **Culture:** They are fastidious and require "special laboratory media" (e.g., Castaneda’s biphasic medium) for growth. Blood culture is the gold standard for diagnosing Brucellosis, especially during the febrile phase. **Analysis of Incorrect Options:** * **A. Pasteurella spp.:** While these are Gram-negative coccobacilli and oxidase positive, they are typically associated with animal bites (cats/dogs) and cause cellulitis or abscesses rather than classic PUO. They show characteristic bipolar staining ("safety-pin appearance"). * **B. Francisella spp.:** *Francisella tularensis* causes Tularemia. Although it is a Gram-negative coccobacillus, it is **oxidase negative**. It also requires cysteine-enriched media (like BCYE) for growth. * **C. Bartonella spp.:** These are Gram-negative bacilli but are typically **oxidase negative**. They are associated with Cat Scratch Disease or Trench Fever, not the classic undulant fever pattern of PUO seen in Brucellosis. **NEET-PG High-Yield Pearls:** * **Undulant Fever:** The classic temperature pattern associated with Brucellosis. * **Castaneda’s Medium:** The specific biphasic medium used to reduce the risk of laboratory-acquired infections (a significant hazard with *Brucella*). * **Rose Bengal Plate Test (RBPT):** A rapid screening test for Brucellosis. * **Standard Agglutination Test (SAT):** Detects antibodies; a titer of 1:160 or more is significant. * **Drug of Choice:** Doxycycline + Rifampicin (or Streptomycin).

Q: Which one of the following factors released by heating a suspension of sheep erythrocytes is required for the growth of Haemophilus Influenzae in chocolate agar?

Nicotinamide adenine dinucleotide (NAD). ### Explanation *Haemophilus influenzae* is a fastidious organism that requires two specific growth factors found in blood: **Factor X (Hemin)** and **Factor V (NAD)**. **Why Option B is Correct:** While sheep blood agar contains both factors, Factor V (NAD) is trapped inside the erythrocytes. Furthermore, sheep blood contains **NADases** (enzymes) that can inactivate Factor V. Heating the blood to approximately 75–80°C to create **Chocolate Agar** serves two purposes: 1. It lyses the red blood cells, **releasing NAD (Factor V)** into the medium. 2. It inactivates the heat-labile NADases, ensuring the NAD remains available for the bacteria. Therefore, the specific factor released and made available by heating is NAD. **Analysis of Incorrect Options:** * **A & C (Hemin/Hemoglobin):** Factor X (Hemin) is derived from hemoglobin. While essential for *H. influenzae*, it is heat-stable and already available in basic blood agar; heating is not specifically required to "release" it for growth in the same way it is for NAD. * **D (Hemolysin):** This is an exotoxin produced by certain bacteria to lyse RBCs; it is not a growth factor required by *Haemophilus*. **High-Yield NEET-PG Pearls:** * **Satellitism:** *H. influenzae* can grow on sheep blood agar near colonies of *Staphylococcus aureus* because *S. aureus* produces NAD as a metabolic byproduct. * **Culture Media:** Chocolate agar is the medium of choice. For selective isolation, **Levinthal’s medium** or **Fildes’ agar** can be used. * **Species Differentiation:** *H. influenzae* requires both X and V; *H. parainfluenzae* requires only V; *H. ducreyi* (Chancroid) requires only X.

Q: All the following species of Haemophilus show satellitism, except?

H. ducreyi. **Explanation:** The phenomenon of **satellitism** occurs when certain *Haemophilus* species grow on Blood Agar only in the vicinity of colonies of *Staphylococcus aureus*. This happens because *S. aureus* synthesizes and releases **Factor V (NAD)** into the medium, while the Blood Agar itself provides **Factor X (Hemin)**. **Why H. ducreyi is the correct answer:** *Haemophilus ducreyi* is unique among the *Haemophilus* species because it **requires only Factor X** for growth and is **Factor V independent**. Since it does not require Factor V, it does not exhibit the satellitism phenomenon. **Analysis of Incorrect Options:** * **H. influenzae:** A classic "X and V factor dependent" organism. It shows robust satellitism around *S. aureus* on Blood Agar. * **H. aegypticus:** Closely related to *H. influenzae* (biotype III), it also requires both Factors X and V, thus showing satellitism. * **H. haemolyticus:** This species is also dependent on both Factors X and V, exhibiting satellitism and often causing confusion with *S. pyogenes* due to its hemolytic properties. **High-Yield NEET-PG Pearls:** * **Factor X:** Hemin (Heat stable). * **Factor V:** NAD - Nicotinamide Adenine Dinucleotide (Heat labile). * **H. ducreyi Clinical Link:** Causes **Chancroid** (Soft, painful chancre). On Gram stain, it shows a characteristic **"School of fish"** or "Railroad track" appearance. * **Culture Media:** While satellitism is seen on Blood Agar, the gold standard for *H. influenzae* is **Chocolate Agar** (where RBCs are lysed to release both X and V factors).

Q: Which of the following mycobacteria can cause disease in an HIV+ve patient with a CD4 count of 600/cu.mm?

M. tuberculosis. **Explanation:** The risk and type of opportunistic infections in HIV patients are directly correlated with the **CD4+ T-lymphocyte count**. A CD4 count of 600/cu.mm is relatively high (near the normal range of 500–1500/cu.mm), indicating only mild immunosuppression. **Why M. tuberculosis is correct:** *Mycobacterium tuberculosis* (MTB) is highly virulent. Unlike most opportunistic pathogens, it does not require profound immunosuppression to cause disease. It is the **most common opportunistic infection** in HIV patients worldwide and can occur at **any CD4 count**. When the CD4 count is >500/cu.mm, MTB typically presents with classic pulmonary features (upper lobe cavitations), similar to HIV-negative individuals. **Why the other options are incorrect:** * **B. MAC (Mycobacterium avium complex):** This is a "late-stage" opportunistic infection. It typically occurs only when the CD4 count drops significantly, usually **below 50/cu.mm**. It presents as disseminated disease with fever, night sweats, and lymphadenopathy. * **C & D. M. chelonei and M. fortuitum:** These are Rapidly Growing Mycobacteria (RGM). While they can cause skin and soft tissue infections, they rarely cause systemic disease in HIV patients unless there is severe immunosuppression or direct inoculation (e.g., post-surgical or trauma). **NEET-PG High-Yield Pearls:** 1. **MTB vs. CD4:** MTB is the earliest opportunistic infection in HIV. As CD4 counts decline (<200), the presentation shifts from "typical" (cavitary) to "atypical" (lower lobe infiltrates, no cavities, extrapulmonary spread). 2. **MAC Prophylaxis:** Historically initiated when CD4 <50/cu.mm (usually with Azithromycin), though guidelines now focus on rapid ART initiation. 3. **Chest X-ray:** In an HIV patient with a high CD4 count and TB, the CXR looks "normal/classic." In low CD4 counts, the CXR may show intrathoracic lymphadenopathy or a miliary pattern.

Q: A 3-year-old boy sustained a leg injury while playing. A few days later, crepitations are felt in the injured area. What is the probable causative organism?

Clostridium Welchii. ### Explanation The clinical presentation of an injury followed by the presence of **crepitations** (the sensation of air bubbles under the skin) is a classic hallmark of **Gas Gangrene** (Clostridial Myonecrosis). **1. Why Clostridium Welchii is correct:** *Clostridium welchii* (now commonly known as **Clostridium perfringens**) is the most common cause of gas gangrene. It is an anaerobic, spore-forming Gram-positive bacillus. When introduced into deep, traumatic wounds with low oxygen tension, it produces various toxins (notably **Alpha-toxin/Lecithinase**). These toxins cause tissue necrosis and ferment muscle carbohydrates, leading to the production of gas. This gas accumulates in the tissues, resulting in the characteristic clinical finding of **crepitus**. **2. Why the other options are incorrect:** * **Corynebacterium diphtheriae:** Causes Diphtheria, characterized by a "pseudomembrane" in the throat and exotoxin-mediated myocarditis or neuropathy, not wound gas production. * **Pseudomonas aeruginosa:** Often infects burns or chronic wounds, producing blue-green pigment (pyocyanin) and a fruity odor, but it does not cause gas gangrene or crepitations. * **Clostridium tetani:** While also found in soil and wounds, it causes **Tetanus**. It produces tetanospasmin, leading to muscle spasticity (lockjaw, opisthotonus) rather than tissue necrosis or gas formation. **3. NEET-PG Clinical Pearls:** * **Nagler’s Reaction:** A biochemical test used to identify *C. perfringens* based on its lecithinase activity on egg yolk agar. * **Morphology:** It is described as "box-car shaped" bacilli and is unique among Clostridia for being **non-motile** and **capsulated**. * **Radiology:** X-rays of the affected limb typically show "feathering" patterns due to gas between muscle fascicles. * **Management:** Requires urgent surgical debridement and high-dose Penicillin G.

Q: A box of ham sandwiches with mayonnaise, prepared by a person with a boil on his neck, was left unrefrigerated for on-call interns. Three doctors became violently ill approximately 2 hours after eating the sandwiches. What is the most likely causative agent?

Staphylococcus aureus enterotoxin. **Explanation:** The clinical presentation describes a classic case of **Staphylococcal Food Poisoning**. **Why Option D is Correct:** * **Source:** *Staphylococcus aureus* is a common inhabitant of the skin. The "boil on the neck" of the food handler is the source of contamination. * **Mechanism:** When protein-rich foods (like ham or mayonnaise) are left unrefrigerated, the bacteria multiply and produce **preformed enterotoxins**. These toxins are heat-stable and resistant to gastric enzymes. * **Incubation Period:** This is the highest-yield clue. *S. aureus* has a very short incubation period (**1–6 hours**) because the toxin is already present in the food (intoxication), not produced after ingestion (infection). * **Symptoms:** Rapid onset of projectile vomiting, nausea, and abdominal cramps. **Why Other Options are Incorrect:** * **Option A:** *Clostridium perfringens* typically has a longer incubation period (8–16 hours) and primarily causes watery diarrhea rather than violent vomiting. It is associated with reheated meat/gravy. * **Option B:** Coagulase is an enzyme used for laboratory identification of *S. aureus*, but it is not the virulence factor responsible for food poisoning symptoms; the enterotoxin is. * **Option C:** Penicillinase (beta-lactamase) is an enzyme that confers antibiotic resistance; it has no role in causing acute gastrointestinal illness. **NEET-PG High-Yield Pearls:** 1. **Shortest Incubation:** *S. aureus* (1–6 hrs) and *Bacillus cereus* (emetic type: 1–5 hrs) have the shortest incubation periods. 2. **Heat Stability:** The enterotoxin is stable at 100°C for 30 minutes; reheating food does not prevent the illness. 3. **Mechanism:** The toxin acts as a **Superantigen**, stimulating the vagus nerve and the vomiting center in the brain. 4. **Diagnosis:** Primarily clinical; culture of the food can confirm the presence of the same strain found in the carrier’s lesion.

Q: Which of the following bacteria is NOT typically encapsulated?

Escherichia coli. **Explanation:** The presence of a polysaccharide capsule is a major virulence factor for many bacteria, as it inhibits phagocytosis. While several pathogens are encapsulated, **Escherichia coli** is generally considered **non-encapsulated** in its typical commensal and most pathogenic forms. Although specific strains like the **K1 serotype** (associated with neonatal meningitis) do possess a capsule, E. coli is not classified as a "typically" encapsulated organism in the context of standard microbiology and NEET-PG patterns. **Analysis of Options:** * **Klebsiella pneumoniae:** Known for its prominent, thick polysaccharide capsule that gives its colonies a characteristic **mucoid appearance** on culture media. * **Haemophilus influenzae:** Type b (Hib) is the most virulent strain due to its polyribosylribitol phosphate (PRP) capsule. * **Bacillus anthracis:** A unique exception in bacteriology; it possesses a capsule made of **poly-D-glutamic acid (polypeptide)** rather than polysaccharide. This is a high-yield distinction. **NEET-PG High-Yield Pearls:** 1. **Mnemonic for Encapsulated Organisms:** *"**S**ome **K**illers **H**ave **N**ice **S**hiny **B**odies"* (**S**treptococcus pneumoniae, **K**lebsiella, **H**aemophilus influenzae, **N**eisseria meningitidis, **S**almonella Typhi, **B**acillus anthracis). 2. **Quellung Reaction:** This "capsular swelling" test is used to identify encapsulated bacteria. 3. **Asplenic Patients:** Individuals with functional or anatomical asplenia are at high risk for sepsis from encapsulated organisms (especially *S. pneumoniae*) because the spleen is the primary site for opsonization and clearance of these bacteria.

Q: What is the most common organism responsible for symptomatic native valve infective endocarditis in children?

Staphylococcus aureus. **Explanation:** **Staphylococcus aureus** is currently the most common cause of symptomatic native valve infective endocarditis (IE) in children. While historically Viridans group streptococci (Alpha-hemolytic) were more prevalent, the epidemiology has shifted due to increased use of indwelling central venous catheters and more frequent cardiac interventions. *S. aureus* is highly virulent, often leading to acute, fulminant presentations with rapid valvular destruction and systemic embolization, even in children without prior structural heart disease. **Analysis of Incorrect Options:** * **Alpha-hemolytic streptococci (Viridans group):** These were previously the leading cause of IE in children, typically associated with dental procedures and underlying congenital heart disease (CHD). While still a major cause of subacute IE, they have been surpassed by *S. aureus* in overall frequency. * **Beta-hemolytic streptococci (e.g., Group A):** These are more commonly associated with Acute Rheumatic Fever (ARF) rather than direct infective endocarditis. * **Group D streptococci (e.g., S. bovis/gallolyticus):** These are rare in the pediatric population and are more typically associated with colonic pathology in adults. **High-Yield Pearls for NEET-PG:** * **Most common underlying risk factor:** Congenital Heart Disease (CHD), specifically VSD, TOF, and Left-sided obstructive lesions. * **Most common site:** Mitral valve (overall), but Tricuspid valve in cases of IV drug use or central lines. * **Duke Criteria:** The gold standard for diagnosis (requires 2 Major, 1 Major + 3 Minor, or 5 Minor criteria). * **Prophylaxis:** Indicated only for high-risk cardiac conditions (e.g., prosthetic valves, cyanotic CHD) before dental procedures involving gingival manipulation.

Question 1

A patient presented with pyrexia of unknown origin. Blood culture in special laboratory media was positive for gram-negative coccobacilli which were oxidase positive. Which one of the following is the likely organism grown in culture?

Accepted Answer

Brucella spp.

Answer

Pasteurella spp.

Answer

Francisella spp.

Answer

Bartonella spp.

Question 2

A man presents to a sexually transmitted infection clinic with urethritis and urethral discharge. Gram stain shows numerous pus cells but no microorganisms. The culture is negative on routine laboratory media. What is the most likely causative agent?

Accepted Answer

Chlamydia trachomatis

Answer

Haemophilus ducreyi

Answer

Treponema pallidum

Answer

Neisseria gonorrhoeae

Question 3

Which one of the following factors released by heating a suspension of sheep erythrocytes is required for the growth of Haemophilus Influenzae in chocolate agar?

Accepted Answer

Nicotinamide adenine dinucleotide (NAD)

Answer

Hemin

Answer

Hemoglobin

Answer

Hemolysin

Question 4

All the following species of Haemophilus show satellitism, except?

Accepted Answer

H. ducreyi

Answer

H. influenzae

Answer

H. aegypticus

Answer

H. haemolyticus

Question 5

Which of the following mycobacteria can cause disease in an HIV+ve patient with a CD4 count of 600/cu.mm?

Accepted Answer

M. tuberculosis

Answer

MAC (Mycobacterium avium complex)

Answer

M. chelonei

Answer

M. fortuitum

Question 6

A 3-year-old boy sustained a leg injury while playing. A few days later, crepitations are felt in the injured area. What is the probable causative organism?

Accepted Answer

Clostridium Welchii

Answer

Corynebacterium Diphtheriae

Answer

Pseudomonas Aeruginosa

Answer

Clostridium Tetani

Question 7

A box of ham sandwiches with mayonnaise, prepared by a person with a boil on his neck, was left unrefrigerated for on-call interns. Three doctors became violently ill approximately 2 hours after eating the sandwiches. What is the most likely causative agent?

Accepted Answer

Staphylococcus aureus enterotoxin

Answer

Clostridium perfringens toxin

Answer

Coagulase from S. aureus in the ham

Answer

Penicillinase given to inactivate penicillin in the pork

Question 8

Which of the following bacteria is NOT typically encapsulated?

Accepted Answer

Escherichia coli

Answer

Klebsiella pneumoniae

Answer

Haemophilus influenzae

Answer

Bacillus anthracis

Question 9

What is the most common organism responsible for symptomatic native valve infective endocarditis in children?

Accepted Answer

Staphylococcus aureus

Answer

Beta-hemolytic streptococci

Answer

Alpha-hemolytic streptococci

Answer

Group D streptococci

Question 10

Which of the following infections does NOT involve lymphatic spread?

Accepted Answer

Erysipelas

Answer

Actinomycosis

Answer

Ludwig's Angina

Answer

Madura foot

Bacteriology — MCQs

Bacteriology — MCQs

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