Bacteriology Practice Questions

Q: Which of the following is true regarding El Tor?

All the above. This question focuses on the differentiating characteristics between the two biotypes of *Vibrio cholerae* O1: **Classical** and **El Tor**. Understanding these differences is high-yield for NEET-PG, as El Tor is the biotype responsible for the current (7th) cholera pandemic. ### **Explanation of Options:** * **A. Resistant to Polymyxin B:** This is a classic laboratory marker. El Tor strains are resistant to Polymyxin B (50 units), whereas Classical strains are sensitive. * **B. Higher Chronic Carrier State:** El Tor is known for better environmental survival and a higher tendency to produce biliary tract carriers compared to the Classical biotype. * **C. Lower Rate of Secondary Attack:** While El Tor is more hardy and spreads faster in a community, it often results in a higher ratio of asymptomatic or mild infections. Because many cases are subclinical, the "secondary attack rate" (the probability that infection occurs among susceptible persons within a specific group) is statistically lower compared to the more virulent Classical biotype. ### **Why "All the Above" is Correct:** El Tor is characterized by its resilience. It survives longer in the environment, is resistant to Polymyxin B and Vibriophage IV, and produces more asymptomatic carriers, making it more successful at pandemic spread than the Classical biotype. ### **High-Yield Clinical Pearls for NEET-PG:** | Feature | Classical Biotype | El Tor Biotype | | :--- | :--- | :--- | | **Hemolysis** | Non-hemolytic | **Hemolytic** (Greig test +ve) | | **Voges-Proskauer (VP)** | Negative | **Positive** | | **Polymyxin B (50U)** | Sensitive | **Resistant** | | **Chick Erythrocyte Agglutination** | Negative | **Positive** | | **Phage Susceptibility** | Group IV sensitive | **Group V sensitive** | * **Current Pandemic:** The 7th pandemic (ongoing since 1961) is caused by **El Tor**. * **Haldane’s Rule:** El Tor vibrios survive longer in the environment than Classical vibrios.

Q: Which of the following statements regarding Mycobacterium is FALSE?

All of the above statements are true.. ### Explanation The genus *Mycobacterium* is characterized by a unique, complex cell wall structure that dictates its staining properties, virulence, and resistance to environmental stress. **1. Why the correct answer is "All of the above":** * **High Lipid Content (Option A):** Approximately 60% of the mycobacterial cell wall weight consists of lipids. This high lipid concentration makes the bacteria hydrophobic and resistant to common disinfectants and many antibiotics. * **Mycolic Acids and LAM (Option B):** The "waxy coat" is primarily composed of **Mycolic acids** (long-chain fatty acids) and **Lipoarabinomannan (LAM)**. LAM is functionally similar to the O-antigen of Gram-negative LPS and plays a crucial role in modulating the host immune response. * **Lack of Toxins (Option C):** Unlike many other pathogenic bacteria, *Mycobacterium tuberculosis* does not produce classic **exotoxins or endotoxins**. Its pathogenicity is derived from its ability to survive and replicate within macrophages, leading to a delayed-type hypersensitivity (Type IV) reaction and granuloma formation. **2. Analysis of Options:** Since statements A, B, and C are all scientifically accurate descriptions of the genus *Mycobacterium*, Option D is the only logical choice. **3. NEET-PG High-Yield Pearls:** * **Acid-Fastness:** Due to mycolic acids, these bacilli resist decolorization by 20% sulfuric acid (Ziehl-Neelsen stain). * **Cord Factor:** A surface glycolipid (Trehalose dimycolate) responsible for the parallel "serpentine cord" growth pattern; it is correlated with virulence. * **Generation Time:** *M. tuberculosis* has a very slow doubling time (12–20 hours), leading to the chronic nature of the disease and long culture periods (up to 6–8 weeks on LJ medium). * **LAM:** It is a key diagnostic marker that can be detected in the **urine** of HIV-positive patients with disseminated tuberculosis.

Q: Which factor of Staphylococcus aureus is responsible for the Coagglutination reaction?

Protein A. **Explanation:** The correct answer is **Protein A**. **Why Protein A is correct:** Protein A is a surface component of *Staphylococcus aureus* cell walls. It possesses a unique biological property: it binds to the **Fc portion of IgG molecules** (except IgG3), leaving the **Fab (antigen-binding) sites** facing outwards. In the **Coagglutination reaction**, specific antibodies are coated onto *S. aureus* (Cowan 1 strain). When these "armed" bacteria are mixed with a sample containing the corresponding antigen, the free Fab ends bind the antigen, causing visible clumping (agglutination). This technique is highly sensitive and used for detecting bacterial antigens in body fluids (e.g., *S. pneumoniae*, *H. influenzae*). **Why other options are incorrect:** * **Clumping factor (Bound Coagulase):** This is responsible for the **Slide Coagulase test**. It converts fibrinogen directly to fibrin, causing the bacteria to clump in plasma. It is not involved in the immunological Coagglutination reaction. * **Hemolysin:** These are exotoxins (Alpha, Beta, Gamma, Delta) that cause lysis of red blood cells and are involved in tissue damage, not antibody binding. * **Teichoic acid:** This is a major cell wall component that mediates mucosal attachment and induces septic shock by triggering the complement cascade; it does not bind the Fc region of IgG. **High-Yield NEET-PG Pearls:** * **Protein A** also acts as a virulence factor by preventing **opsonization** and phagocytosis (since the antibody is bound "backwards"). * **Cowan 1 Strain:** The specific strain of *S. aureus* used for Coagglutination because it is rich in Protein A. * **Free Coagulase:** Detected by the **Tube Coagulase test** (requires CRF - Coagulase Reacting Factor).

Q: Which of the following bacteria produces a red pigment?

Serratia marcescens. **Explanation:** The correct answer is **Serratia marcescens**. **1. Why Serratia marcescens is correct:** *Serratia marcescens* is a Gram-negative rod belonging to the Enterobacteriaceae family. Its most defining characteristic in laboratory diagnosis is the production of a non-diffusible, bright red/pink pigment called **prodigiosin**. This pigment is typically produced when the bacteria are grown at room temperature (20–25°C) rather than at 37°C. In clinical settings, this can sometimes be seen as a "pink film" in bathrooms or on medical equipment. **2. Analysis of Incorrect Options:** * **Escherichia coli:** A Gram-negative rod that is a lactose fermenter. On MacConkey agar, it produces pink colonies due to acid production, but it does not produce an endogenous red pigment. * **Bordetella parapertussis:** This organism is known for producing a **brown pigment** on Bordet-Gengou or Regan-Lowe medium, not red. * **Pseudomonas aeruginosa:** Famous for producing various pigments, most notably **Pyocyanin** (blue-green) and **Pyoverdin** (fluorescent yellow-green). It does not produce red pigment. **3. NEET-PG High-Yield Pearls:** * **Serratia marcescens:** Often associated with hospital-acquired infections (UTIs, pneumonia) and endocarditis in IV drug users. It is frequently multi-drug resistant. * **Other Pigment Producers:** * *Staphylococcus aureus:* Golden yellow (Staphyloxanthin). * *Pseudomonas aeruginosa:* Blue-green (Pyocyanin). * *Mycobacterium kansasii:* Yellow/Orange (Photochromogen). * *Corynebacterium erythrasma:* Coral red fluorescence under Wood’s lamp (due to porphyrins). * **Mnemonic:** Remember **"Red Serratia"** to recall the pigment association quickly during the exam.

Q: A 65-year-old man with a long history of diabetes mellitus was hospitalized for treatment of a left great toe ulcer. Below-knee amputation was performed due to non-resolution despite multiple courses of oral antibiotics. On the first postoperative day, he developed fever, which worsened by day 2. His amputation stump was mottled with purplish discoloration and distal necrosis. Crepitus was palpable up to his patella. What is the likely pathogen causing these postoperative complications?

Clostridium perfringens. **Explanation:** The clinical presentation of rapid-onset postoperative fever, purplish skin discoloration (mottling), distal necrosis, and **palpable crepitus** (gas in tissues) is classic for **Gas Gangrene (Clostridial Myonecrosis)**. **1. Why Clostridium perfringens is correct:** *Clostridium perfringens* is a Gram-positive, anaerobic, spore-forming bacillus. In patients with diabetes and peripheral vascular disease, the low-oxygen environment of an amputation stump is ideal for its growth. It produces the **Alpha-toxin (lecithinase)**, which destroys cell membranes, leading to rapid tissue necrosis, hemolysis, and systemic toxicity. The "crepitus" is caused by gas produced during fermentation by the bacteria, a hallmark of this infection. **2. Why the other options are incorrect:** * **Escherichia coli:** While it can cause necrotizing infections (often polymicrobial), it is not the primary cause of classical gas gangrene with rapid myonecrosis. * **Actinomyces israelii:** This causes chronic, slow-growing "lumpy jaw" abscesses with sulfur granules, not acute postoperative gas gangrene. * **Staphylococcus aureus:** This is the most common cause of surgical site infections, typically presenting with localized pus, erythema, and warmth, but it does not produce gas or cause rapid crepitant myonecrosis. **Clinical Pearls for NEET-PG:** * **Incubation Period:** Very short (usually <24 hours). * **Diagnosis:** Primarily clinical; X-ray may show "feathering" of gas in muscle planes. * **Management:** Emergency surgical debridement (or re-amputation), high-dose Penicillin G + Clindamycin, and Hyperbaric Oxygen Therapy. * **Key Sign:** Dishwater-colored discharge with a foul/sweet odor.

Q: Which is a venereal form of treponemal infection?

Syphilis. **Explanation:** The genus *Treponema* consists of several species and subspecies that are morphologically identical but differ in clinical presentation and mode of transmission. **1. Why Syphilis is correct:** Syphilis is caused by ***Treponema pallidum* subspecies *pallidum***. It is the only treponemal infection primarily transmitted through **sexual contact (venereal)** or vertically from mother to fetus (congenital). It is characterized by three clinical stages (primary, secondary, and tertiary) and can affect almost every organ system. **2. Why the other options are incorrect:** The other three options are collectively known as **Endemic (Non-venereal) Treponematoses**. They are typically transmitted via direct skin-to-skin contact or shared utensils, usually in childhood under poor hygienic conditions. * **Yaws (Option A):** Caused by *T. pallidum* subsp. *pertenue*. It primarily affects the skin, bones, and joints in humid tropical regions. * **Pinta (Option B):** Caused by ***T. carateum***. It is the mildest form, restricted solely to the skin (dyschromic lesions), and found in Central and South America. * **Bejel (Option C):** Also known as Endemic Syphilis, caused by *T. pallidum* subsp. *endemicum*. It is found in arid regions (Middle East/Africa) and often presents with oral mucosal lesions. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Treponemes are thin, spiral organisms that cannot be grown on culture media (non-cultivable). They are visualized using **Dark Ground Microscopy (DGM)**. * **Serology:** All treponemal infections (venereal and non-venereal) give **positive results** on both non-specific (VDRL/RPR) and specific (TPHA/FTA-ABS) tests. * **Drug of Choice:** **Penicillin G** remains the treatment of choice for all treponemal infections. * **Rabbit Inoculation:** Since they don't grow in vitro, *T. pallidum* is maintained in the testes of rabbits (**Nichol’s strain**).

Q: Which organism grows in alkaline pH?

Vibrio. **Explanation:** The correct answer is **Vibrio**. **1. Why Vibrio is correct:** *Vibrio cholerae* is a halophilic (salt-loving) Gram-negative rod that is uniquely **alkaliphilic**. It thrives in environments with a high pH, typically ranging from **8.5 to 9.5**. This physiological characteristic is exploited in laboratory diagnostics through the use of selective enrichment media like **Alkaline Peptone Water (APW)** and solid media like **TCBS (Thiosulfate Citrate Bile Salts Sucrose) agar**, which has a pH of 8.6. The alkaline environment inhibits the growth of most other intestinal commensals, allowing *Vibrio* to flourish. **2. Why the other options are incorrect:** * **Klebsiella, Pseudomonas, and E. coli:** These are common Gram-negative bacteria that are generally **neutrophilic**. They grow optimally at a physiological pH of approximately **7.2 to 7.4**. While they can tolerate slight variations, they do not require or prefer highly alkaline conditions for growth. In fact, many of these organisms (like *E. coli*) produce acids during fermentation that lower the pH of their environment. **3. High-Yield Clinical Pearls for NEET-PG:** * **Selective Media for Vibrio:** Remember **TCBS** (Large yellow colonies due to sucrose fermentation) and **Monsur’s Tellurite Taurocholate Gelatin Agar**. * **Enrichment Media:** **Alkaline Peptone Water** and **Monsur’s enrichment medium**. * **Transport Media:** **Venkataraman-Ramakrishnan (VR) medium** and **Cary-Blair medium**. * **The "String Test":** Used to identify *Vibrio* species; colonies emulsified in 0.5% sodium deoxycholate lose viability and release DNA, forming a viscous "string." * **Darting Motility:** A classic description of *Vibrio cholerae* movement under dark-ground microscopy.

Q: A diabetic person presents with multiple abscesses in the leg. Microscopic examination of pus shows Gram-negative bacilli. Staining with methylene blue shows bipolar staining. What is the most likely causative agent?

B. pseudomallei. **Explanation:** The clinical presentation of multiple abscesses in a diabetic patient, combined with the microscopic finding of Gram-negative bacilli showing **bipolar staining** (safety-pin appearance) with methylene blue, is classic for **Burkholderia pseudomallei**, the causative agent of **Melioidosis**. 1. **Why A is correct:** *B. pseudomallei* is a saprophyte found in soil and water. Diabetics are highly predisposed to this infection. It characteristically shows bipolar staining (Waysen stain or Methylene blue) and produces a "wrinkled" colony appearance on agar. It can present as acute localized infection (abscesses), pneumonia, or septicemia. 2. **Why B is incorrect:** *Yersinia pestis* also shows classic bipolar staining. However, it is associated with flea bites, rodents, and presents as bubonic (lymphadenopathy) or pneumonic plague, not typically as chronic leg abscesses in a diabetic. 3. **Why C is incorrect:** *Burkholderia mallei* causes Glanders, primarily a disease of horses/donkeys. Human infection is rare and usually occurs in laboratory workers or those with direct equine contact. 4. **Why D is incorrect:** Botromycosis is a chronic granulomatous bacterial infection (often *S. aureus*) that mimics fungal grains. While it causes skin abscesses, it is not characterized by Gram-negative bacilli with bipolar staining. **High-Yield Clinical Pearls for NEET-PG:** * **Melioidosis** is often called the "Vietnamese Time Bomb" because it can remain latent for years. * **Drug of Choice:** Ceftazidime (Initial intensive phase) followed by Co-trimoxazole (Maintenance phase). * **Ashdown’s Medium:** The selective culture medium used for *B. pseudomallei*. * **Bipolar Staining Mnemonic:** "You (Yersinia) Better (Burkholderia) Be (Brucella) Past (Pasteurella) Him (H. influenzae)."

Q: An infant with a past history of ear infection is suspected to have meningitis. What is the organism commonly responsible for such a presentation?

Streptococcus pneumoniae. ### Explanation **Correct Answer: D. Streptococcus pneumoniae** **Concept:** *Streptococcus pneumoniae* (Pneumococcus) is the most common cause of community-acquired meningitis in children and adults. There is a strong clinical association between **Otitis Media (ear infection)**, sinusitis, or pneumonia and the subsequent development of meningitis. This occurs via direct spread through the anatomical plates or hematogenous dissemination from the primary focus in the middle ear. **Analysis of Options:** * **Streptococcus pneumoniae (Correct):** It is the leading cause of both Otitis Media and Meningitis in infants and children. The history of a preceding ear infection is a classic "red flag" for Pneumococcal meningitis. * **Haemophilus influenzae (Incorrect):** While *H. influenzae* type b (Hib) was previously a leading cause, its incidence has drastically declined due to widespread vaccination. It remains a differential but is less common than *S. pneumoniae* in vaccinated populations. * **Moraxella catarrhalis (Incorrect):** This is a common cause of Otitis Media and respiratory infections, but it is an extremely rare cause of meningitis. * **Pseudomonas (Incorrect):** This is typically associated with chronic suppurative otitis media (CSOM) or healthcare-associated meningitis (post-neurosurgery), rather than acute community-acquired presentations in infants. **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause of meningitis overall:** *S. pneumoniae*. * **Meningitis + Petechial rash:** Think *Neisseria meningitidis*. * **Meningitis in Neonates (0-3 months):** Group B Streptococcus (S. agalactiae), E. coli, and Listeria monocytogenes. * **CSF Findings in Bacterial Meningitis:** Increased neutrophils, increased protein, and decreased glucose (<40 mg/dL). * **Drug of Choice:** Empiric treatment usually involves 3rd generation cephalosporins (Ceftriaxone/Cefotaxime) plus Vancomycin.

Question 1

Which of the following is true regarding El Tor?

Accepted Answer

All the above

Answer

Resistant to polymyxin B

Answer

Higher chronic carrier state

Answer

Lower rate of secondary attack

Question 2

Which of the following statements regarding Mycobacterium is FALSE?

Accepted Answer

All of the above statements are true.

Answer

The cell wall has a high lipid content.

Answer

Mycolic acids and lipoarabinomannan (LAM) contribute to the waxy coat.

Answer

Mycobacterium species generally lack exotoxins and endotoxins.

Question 3

Which factor of Staphylococcus aureus is responsible for the Coagglutination reaction?

Accepted Answer

Protein A

Answer

Clumping factor

Answer

Hemolysin

Answer

Teichoic acid

Question 4

Which of the following bacteria produces a red pigment?

Accepted Answer

Serratia marcescens

Answer

Escherichia coli

Answer

Bordetella parapertussis

Answer

Pseudomonas aeruginosa

Question 5

A 65-year-old man with a long history of diabetes mellitus was hospitalized for treatment of a left great toe ulcer. Below-knee amputation was performed due to non-resolution despite multiple courses of oral antibiotics. On the first postoperative day, he developed fever, which worsened by day 2. His amputation stump was mottled with purplish discoloration and distal necrosis. Crepitus was palpable up to his patella. What is the likely pathogen causing these postoperative complications?

Accepted Answer

Clostridium perfringens

Answer

Escherichia coli

Answer

Actinomyces israelii

Answer

Staphylococcus aureus

Question 6

Helicobacter pylori is carcinogenic due to all mechanisms EXCEPT:

Accepted Answer

Causes RNA splicing

Answer

Cag A gene

Answer

Vac A gene

Answer

Elaboration of interleukin 8

Question 7

Which is a venereal form of treponemal infection?

Accepted Answer

Syphilis

Answer

Yaws

Answer

Pinta

Answer

Bejel

Question 8

Which organism grows in alkaline pH?

Accepted Answer

Vibrio

Answer

Klebsiella

Answer

Pseudomonas

Answer

E.coli

Question 9

A diabetic person presents with multiple abscesses in the leg. Microscopic examination of pus shows Gram-negative bacilli. Staining with methylene blue shows bipolar staining. What is the most likely causative agent?

Accepted Answer

B. pseudomallei

Answer

Y. pestis

Answer

Pseudomonas mallei

Answer

Botromycosis

Question 10

An infant with a past history of ear infection is suspected to have meningitis. What is the organism commonly responsible for such a presentation?

Accepted Answer

Streptococcus pneumoniae

Answer

Haemophilus influenzae

Answer

Moraxella catarrhalis

Answer

Pseudomonas

Bacteriology — MCQs

Bacteriology — MCQs

On this page

Practice by Chapter

Want unlimited practice?