Bacteriology Practice Questions

Q: A 53-year-old woman, three months after surgery and chemotherapy for breast cancer, developed a cough productive of purulent sputum four weeks ago. Two weeks ago, she noted progressive weakness of her left arm and leg. Chest examination revealed rales over the left upper back with deep breathing, and chest radiography showed a left upper lobe infiltrate. Neurologic examination confirmed left arm and leg weakness, and contrast-enhanced computed tomography revealed two lesions in the right hemisphere. Gram stain of a purulent sputum specimen showed branching Gram-positive rods that were partially acid-fast. Which of the following organisms is the cause of this patient's current illness?

Nocardia. ### Explanation The correct answer is **Nocardia**. **Why it is correct:** The clinical presentation describes a classic triad of **Nocardiosis**: pulmonary infection (cough, purulent sputum, upper lobe infiltrate), dissemination to the brain (hemiparesis, ring-enhancing lesions on CT), and an immunocompromised state (post-chemotherapy). The definitive diagnostic clue is the Gram stain finding: **branching, filamentous Gram-positive rods** that are **partially acid-fast** (due to mycolic acids in the cell wall). *Nocardia asteroides* is the most common species causing this systemic presentation. **Why incorrect options are wrong:** * **Actinomyces israelii:** While it also shows branching Gram-positive filaments, it is **not acid-fast**. It typically causes "lumpy jaw" (cervicofacial) with sulfur granules and is an anaerobe, unlike the aerobic Nocardia. * **Corynebacterium pseudodiphtheriticum:** These are Gram-positive rods but appear in "Chinese-letter" or palisading patterns, not branching filaments, and are not acid-fast. * **Aspergillus fumigatus:** This is a fungus showing septate hyphae with 45-degree branching. It does not appear as Gram-positive rods on a Gram stain. **NEET-PG High-Yield Pearls:** * **Staining:** Nocardia is **Modified Ziehl-Neelsen (kinyoun) positive** (uses 1% sulfuric acid instead of 20% used for *M. tuberculosis*). * **Habitat:** It is an exogenous infection found in **soil** (inhalation or traumatic inoculation). * **Target Organs:** It has a high tropism for the **Central Nervous System**, often presenting as multi-loculated brain abscesses. * **Treatment:** The drug of choice is **Sulfonamides (TMP-SMX)**. (Mnemonic: **SNAP** – **S**ulfonamides for **N**ocardia, **A**ctinomyces treated with **P**enicillin).

Q: A patient complained to his dentist about a draining lesion in his mouth. A Gram's stain of the pus showed a few Gram-positive cocci, leukocytes, and many branched Gram-positive rods. What is the most likely causative organism?

Actinomyces israelii. **Explanation:** The clinical presentation of a **draining oral lesion** (cervicofacial abscess) combined with the microscopic finding of **branched Gram-positive rods** is a classic description of **Actinomycosis**. **1. Why Actinomyces israelii is correct:** *Actinomyces israelii* is the most common cause of human actinomycosis. It is a commensal of the oral cavity that becomes pathogenic following mucosal trauma (e.g., dental procedures). Key diagnostic features include: * **Morphology:** Gram-positive, non-acid-fast, filamentous (branched) rods. * **Clinical Sign:** "Lumpy jaw" with multiple draining sinus tracts. * **Pus Examination:** Presence of **"Sulfur granules"** (yellowish colonies of bacteria), which appear as sunburst patterns on microscopy. **2. Why the other options are incorrect:** * **Actinomyces viscosus:** While part of the oral flora and associated with dental plaque/gingivitis, it is rarely the primary cause of invasive actinomycotic abscesses compared to *A. israelii*. * **C. diphtheriae:** These are Gram-positive rods but are **club-shaped** (coryneform) and arranged in "Chinese letter" patterns, not branched filaments. They cause pseudomembranous pharyngitis, not draining oral abscesses. * **Propionibacterium acnes:** These are Gram-positive pleomorphic rods (anaerobic diphtheroids) primarily associated with acne vulgaris and prosthetic joint infections, not cervicofacial actinomycosis. **NEET-PG High-Yield Pearls:** * **Anaerobic Nature:** *Actinomyces* are strict/facultative anaerobes (unlike *Nocardia*, which is aerobic). * **Acid-Fastness:** *Actinomyces* is **Non-Acid Fast**, distinguishing it from *Nocardia* (which is weakly acid-fast). * **Treatment:** The drug of choice is **Penicillin G** (long-term). "ACTino" = **A**naerobic, **C**ervicofacial, **T**reatment is Penicillin.

Q: Tellurite stimulates the growth of which of the following bacteria?

Corynebacterium diphtheriae. **Explanation:** The correct answer is **Corynebacterium diphtheriae**. **Why it is correct:** Potassium tellurite is a selective agent used in media such as **McLeod’s medium** and **Potassium Tellurite Blood Agar (PTBA)**. *Corynebacterium diphtheriae* has the unique ability to reduce potassium tellurite to metallic tellurium. This process serves two purposes: 1. **Selectivity:** Tellurite inhibits the growth of most other upper respiratory tract commensals (like Staphylococci and Streptococci). 2. **Differential Identification:** The reduction of tellurite results in the formation of characteristic **black or greyish-black colonies**, which is a hallmark diagnostic feature for *C. diphtheriae*. **Why the other options are incorrect:** * **Escherichia coli & Salmonella:** These are Gram-negative enteric bacilli. They are typically cultured on MacConkey agar or selective media like DCA/XLD. Tellurite is generally inhibitory to most Gram-negative bacteria unless specifically used in specialized formulations for certain rare strains (like EHEC O157). * **Clostridium tetani:** This is an obligate anaerobe. It requires anaerobic culture conditions (e.g., Robertson’s Cooked Meat medium) and is not identified using tellurite-based media. **High-Yield Clinical Pearls for NEET-PG:** * **Löffler's Serum Slope:** The best medium for rapid growth (6-8 hours) of *C. diphtheriae*; it enhances the development of **metachromatic granules** (Volutin/Babes-Ernst granules). * **Albert’s Stain:** Used to visualize these granules (granules appear bluish-black, while the body appears green). * **Elek’s Gel Precipitation Test:** The gold standard *in vitro* test for detecting the **toxigenicity** of the isolate. * **Toxin Mechanism:** Diphtheria toxin inhibits protein synthesis by ADP-ribosylation of **Elongation Factor-2 (EF-2)**.

Q: Which of the following bacteria is known as the Ebeh Gaaffky bacillus?

Salmonella. **Explanation:** The correct answer is **Salmonella (specifically *Salmonella Typhi*)**. **Why it is correct:** In the history of microbiology, *Salmonella Typhi* is referred to as the **Eberth-Gaffky bacillus**. It was first observed in the abdominal lymph nodes and spleen of typhoid patients by **Karl Joseph Eberth** in 1880. Later, in 1884, **Georg Gaffky** successfully isolated the organism in pure culture. This eponym is a frequent "one-liner" in medical entrance exams. **Analysis of Incorrect Options:** * **Klebsiella:** Also known as **Friedländer’s bacillus**, it is a common cause of community-acquired pneumonia characterized by "currant jelly" sputum. * **Haemophilus aegyptius:** Known as the **Koch-Weeks bacillus**, it is a major causative agent of acute contagious conjunctivitis (pink eye). * **Corynebacterium:** Specifically *C. diphtheriae*, it is known as the **Klebs-Löffler bacillus**. It causes diphtheria and is characterized by a pseudomembrane in the throat. **High-Yield Clinical Pearls for NEET-PG:** * **Salmonella Typhi:** Causes Enteric Fever. Key diagnostic tests include the **Widal test** (after the 1st week) and **Blood Culture** (most sensitive in the 1st week; mnemonic: **BASU** - Blood, Agglutination/Widal, Stool, Urine). * **Morphology:** Gram-negative, motile (peritrichous flagella), non-lactose fermenting bacilli. * **Other Eponyms to Remember:** * *Mycobacterium leprae*: Hansen’s bacillus. * *Mycobacterium tuberculosis*: Koch’s bacillus. * *Bordetella pertussis*: Bordet-Gengou bacillus. * *Haemophilus influenzae*: Pfeiffer’s bacillus.

Q: Which of the following are features of congenital syphilis?

Perforation in the cartilaginous part of the nose. **Explanation:** Congenital syphilis is divided into early (manifesting before 2 years of age) and late (manifesting after 2 years) stages. **Why Option B is Correct:** Syphilis is characterized by an affinity for the **cartilaginous structures**. In congenital syphilis, the inflammatory process (gummatous destruction) specifically targets the **cartilaginous part of the nasal septum**. This leads to perforation and the subsequent collapse of the nasal bridge. **Analysis of Incorrect Options:** * **A. Snuffles:** This is a highly infectious nasal discharge (syphilitic rhinitis) and is an **early** manifestation, typically appearing within the first few weeks of life. * **C. Mulberry Molars:** These are permanent first molars with multiple poorly developed cusps. While seen in congenital syphilis, they are a **late** manifestation (appearing after age 2), and the question specifically highlights the cartilaginous involvement as a primary pathological feature. * **D. Saddle Nose Deformity:** While this is a classic sign of congenital syphilis, it results from the destruction of the **bony** part of the nasal bridge (the bridge itself), whereas the primary pathological perforation occurs in the **cartilaginous** septum. **High-Yield Clinical Pearls for NEET-PG:** * **Hutchinson’s Triad:** Interstitial keratitis, Hutchinson’s teeth (notched incisors), and 8th nerve deafness. * **Early Signs:** Snuffles, Condyloma lata, Pemphigus syphiliticus (bullous lesions on palms/soles), and Osteochondritis (Wimberger’s sign). * **Late Signs:** Clutton’s joints (painless knee swelling), Sabre shin (anterior bowing of tibia), and Rhagades (linear scars around the mouth). * **Screening:** VDRL/RPR are used for screening; TPHA/FTA-ABS are confirmatory. Treatment of choice is **Penicillin G**.

Q: Meningococci differ from gonococci in that they

Possess a capsule. ### Explanation The primary taxonomic and structural difference between *Neisseria meningitidis* (Meningococci) and *Neisseria gonorrhoeae* (Gonococci) lies in the presence of a **polysaccharide capsule**. **1. Why the correct answer is right:** *Neisseria meningitidis* possesses a prominent polysaccharide capsule that is essential for its virulence. This capsule allows the bacteria to resist phagocytosis and survive in the bloodstream, leading to systemic dissemination (meningitis and septicemia). In contrast, *Neisseria gonorrhoeae* is **non-capsulated**, which is why it typically causes localized mucosal infections rather than systemic spread. **2. Why the incorrect options are wrong:** * **Option A (Intracellular):** Both organisms are characteristically seen as Gram-negative diplococci located within polymorphonuclear leucocytes (intracellular) during acute infections. * **Option C (Fermentation of glucose):** Both species ferment glucose. However, they are differentiated by **maltose fermentation**: Meningococci ferment **M**altose and glucose, while Gonococci ferment **only** glucose. * **Option D (Oxidase positive):** Both organisms belong to the genus *Neisseria*, and all members of this genus are oxidase-positive. **3. High-Yield Clinical Pearls for NEET-PG:** * **Capsule & Vaccines:** Because Meningococci have a capsule, vaccines are available against specific serogroups (A, C, W-135, Y). No vaccine exists for Gonococci due to high antigenic variation of its pili. * **Maltose Trick:** Remember **M**eningococci = **M**altose + Glucose; **G**onococci = **G**lucose only. * **Portal of Entry:** Meningococci enter via the nasopharynx (respiratory), whereas Gonococci enter via the genitourinary tract (sexual contact). * **Media:** Both grow on Thayer-Martin Medium (Selective media).

Q: What is responsible for Legionella pathogenicity?

Failure of oxidative burst. **Explanation:** *Legionella pneumophila* is a facultative intracellular pathogen that primarily infects alveolar macrophages. Its pathogenicity is fundamentally tied to its ability to survive and replicate within these immune cells. **Why Option D is Correct:** The hallmark of *Legionella* virulence is the **inhibition of phagosome-lysosome fusion** and the **failure of the oxidative burst**. Upon being phagocytosed, the bacteria utilize a **Type IV Secretion System (Dot/Icm)** to inject effector proteins into the macrophage. These proteins remodel the phagosome into a "Legionella-containing vacuole" (LCV) that mimics the endoplasmic reticulum. This prevents the activation of NADPH oxidase, thereby failing to trigger the oxidative burst (production of reactive oxygen species) that would normally kill the bacteria. **Why Other Options are Incorrect:** * **A. Capsule:** Unlike *Streptococcus pneumoniae* or *Haemophilus influenzae*, *Legionella* does not possess a polysaccharide capsule as its primary virulence factor. * **B. Toxin:** While *Legionella* produces enzymes (like phospholipase C), it does not produce a potent exotoxin or endotoxin responsible for its primary clinical manifestations. * **C. Bacteriophage:** Bacteriophages are responsible for virulence in organisms like *Corynebacterium diphtheriae* (Diphtheria toxin) or *Vibrio cholerae*, but not *Legionella*. **NEET-PG High-Yield Pearls:** * **Stain:** Poorly gram-staining (Gram-negative rod); visualized better with **Dieterle silver stain**. * **Culture:** Requires **BCYE (Buffered Charcoal Yeast Extract) agar** supplemented with **L-cysteine** and **Iron**. * **Diagnosis:** **Urinary Antigen Test** is the rapid test of choice (detects Serogroup 1). * **Clinical:** Causes **Pontiac fever** (mild) and **Legionnaires' disease** (severe pneumonia with hyponatremia and diarrhea).

Q: Which culture medium is used for Campylobacter jejuni?

Skirrow's medium. **Explanation:** **Campylobacter jejuni** is a fastidious, microaerophilic organism that requires selective media for isolation from stool samples, as it is often overgrown by normal intestinal flora. **1. Why Skirrow’s medium is correct:** Skirrow’s medium is a selective blood-based agar specifically designed for *Campylobacter*. It contains antibiotics (**Vancomycin, Polymyxin B, and Trimethoprim**) which inhibit the growth of Gram-positive bacteria, most Gram-negative bacteria, and fungi, respectively, allowing the thermophilic *C. jejuni* to grow optimally at 42°C. Other selective media for *Campylobacter* include **Butzler’s** and **Campy-BAP**. **2. Why other options are incorrect:** * **BCYE (Buffered Charcoal Yeast Extract) medium:** Used for the isolation of ***Legionella pneumophila***. It contains L-cysteine and iron, which are essential for its growth. * **Thayer-Martin medium:** A selective medium used for the isolation of ***Neisseria gonorrhoeae*** and ***Neisseria meningitidis***. It is essentially Chocolate agar supplemented with Vancomycin, Colistin, Nystatin, and Trimethoprim (VCNT). * **TCBS (Thiosulfate Citrate Bile salts Sucrose) agar:** The gold standard selective medium for ***Vibrio cholerae***, where it produces characteristic yellow colonies due to sucrose fermentation. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** *C. jejuni* shows a characteristic **"seagull-wing"** appearance on Gram stain and exhibits **darting motility**. * **Culture Conditions:** It is **thermophilic** (grows best at 42°C) and **microaerophilic** (requires 5% $O_2$ and 10% $CO_2$). * **Clinical Association:** It is the most common antecedent infection associated with **Guillain-Barré Syndrome (GBS)** due to molecular mimicry (anti-GM1 antibodies).

Question 1

A 53-year-old woman, three months after surgery and chemotherapy for breast cancer, developed a cough productive of purulent sputum four weeks ago. Two weeks ago, she noted progressive weakness of her left arm and leg. Chest examination revealed rales over the left upper back with deep breathing, and chest radiography showed a left upper lobe infiltrate. Neurologic examination confirmed left arm and leg weakness, and contrast-enhanced computed tomography revealed two lesions in the right hemisphere. Gram stain of a purulent sputum specimen showed branching Gram-positive rods that were partially acid-fast. Which of the following organisms is the cause of this patient's current illness?

Accepted Answer

Nocardia

Answer

Actinomyces israelii

Answer

Corynebacterium pseudodiphtheriticum

Answer

Aspergillus fumigatus

Question 2

A patient complained to his dentist about a draining lesion in his mouth. A Gram's stain of the pus showed a few Gram-positive cocci, leukocytes, and many branched Gram-positive rods. What is the most likely causative organism?

Accepted Answer

Actinomyces israelii

Answer

Actinomyces viscosus

Answer

C. diphtheriae

Answer

Propionibacterium acnes

Question 3

The Montoux test is:

Accepted Answer

A test where boosting is maximum after 4 weeks of the initial test

Answer

A diagnostic test for TB

Answer

A test where a false positive reaction is seen in the immunocompromised

Answer

A highly sensitive test with low specificity

Question 4

Tellurite stimulates the growth of which of the following bacteria?

Accepted Answer

Corynebacterium diphtheriae

Answer

Escherichia coli

Answer

Clostridium tetani

Answer

Salmonella

Question 5

Which of the following bacteria is known as the Ebeh Gaaffky bacillus?

Accepted Answer

Salmonella

Answer

Klebsiella

Answer

Haemophilus aegyptius

Answer

Corynebacterium

Question 6

Which of the following are features of congenital syphilis?

Accepted Answer

Perforation in the cartilaginous part of the nose

Answer

Snuffles is a late manifestation

Answer

Mulberry molars

Answer

Saddle nose deformity

Question 7

Meningococci differ from gonococci in that they

Accepted Answer

Possess a capsule

Answer

Are intracellular

Answer

Cause fermentation of glucose

Answer

Are oxidase positive

Question 8

Which of the following is NOT true about Borrelia recurrentis?

Accepted Answer

It is transmitted by the Pediculus humanus humanus (body louse)

Answer

Causes epidemic relapsing fever

Answer

No other known animal reservoir of B. recurrentis exists

Answer

It infects the person's mucous membranes

Question 9

What is responsible for Legionella pathogenicity?

Accepted Answer

Failure of oxidative burst

Answer

Capsule

Answer

Toxin

Answer

Bacteriophage

Question 10

Which culture medium is used for Campylobacter jejuni?

Accepted Answer

Skirrow's medium

Answer

BYCE medium

Answer

Thayer-Martin medium

Answer

TCBS medium

Bacteriology — MCQs

Bacteriology — MCQs

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