Bacteriology Practice Questions

Q: Which of the following is NOT true of Treponema pallidum?

Motile by peritrichous flagella. ### Explanation **Treponema pallidum**, the causative agent of Syphilis, is a thin, delicate spirochete. Understanding its unique morphology and laboratory characteristics is essential for NEET-PG. **Why Option B is the Correct Answer (The False Statement):** *Treponema pallidum* does **not** possess peritrichous flagella. Instead, it has **endoflagella** (also known as axial filaments or periplasmic flagella) located in the periplasmic space between the inner membrane and outer sheath. These filaments allow the organism to move in a characteristic **corkscrew motion**, which is vital for penetrating host tissues. Peritrichous flagella are typical of bacteria like *E. coli* or *Salmonella*. **Analysis of Other Options:** * **Option A:** *T. pallidum* cannot be grown on artificial culture media (it is an obligate parasite). It is maintained in vivo via serial passage in **rabbit testes** (Nichol’s strain). * **Option C:** Because they are too thin (0.1–0.2 µm) to be seen under a standard light microscope and do not take up Gram stain well, **Dark Field Microscopy (DFM)** is the gold standard for visualizing live motile spirochetes from primary chancre exudates. * **Option D:** The **TPI (Treponema pallidum Immobilization) test** is a highly specific treponemal test. While largely replaced by FTA-ABS and TPHA in modern labs, it remains the "gold standard" for specificity in confirming syphilis. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** 6–14 regular spirals with a pitch of 1 µm. * **Staining:** Use **Silver Impregnation stains** (Fontana for films, Levaditi for tissues). * **Treatment:** **Benzathine Penicillin G** remains the drug of choice. * **Jarisch-Herxheimer Reaction:** An acute febrile reaction following the start of antibiotic treatment due to the release of endotoxins from dying spirochetes.

Q: What is the recommended treatment for penicillin-resistant gonorrhea?

Ceftriaxone. **Explanation:** The correct answer is **Ceftriaxone**. **Why Ceftriaxone is correct:** *Neisseria gonorrhoeae* has developed widespread resistance to penicillin due to the production of **plasmid-mediated beta-lactamase (PPNG)** and chromosomal mutations. Consequently, third-generation cephalosporins, specifically **Ceftriaxone**, have become the first-line treatment [1]. Ceftriaxone is highly bactericidal, has a long half-life, and provides high cure rates for mucosal gonorrhea (urethral, cervical, and pharyngeal) [1]. **Why the other options are incorrect:** * **Ciprofloxacin:** Once a standard treatment, fluoroquinolone resistance (QRNG) is now globally prevalent, making it unreliable for empirical therapy [1]. * **Streptomycin:** This aminoglycoside is not effective against *N. gonorrhoeae* and is primarily used in the treatment of tuberculosis or plague. * **Erythromycin:** While used for neonatal prophylaxis (ophthalmia neonatorum), it is not the drug of choice for adult gonorrhea due to poor efficacy and gastrointestinal side effects. **High-Yield Clinical Pearls for NEET-PG:** 1. **Dual Therapy Concept:** To cover potential co-infection with *Chlamydia trachomatis*, the current CDC/WHO guidelines often recommend combining Ceftriaxone with **Azithromycin** (though some recent guidelines suggest Ceftriaxone monotherapy at higher doses, e.g., 500mg IM, if Chlamydia is ruled out) [1]. 2. **Disseminated Gonococcal Infection (DGI):** Ceftriaxone remains the drug of choice for systemic manifestations like arthritis-dermatitis syndrome. 3. **Spectinomycin:** This is an alternative for patients who are allergic to cephalosporins. 4. **Culture Media:** Remember **Thayer-Martin medium** (VPN: Vancomycin, Polymyxin, Nystatin) is the selective medium for *N. gonorrhoeae*.

Q: Chlamydiae are classified as which of the following?

Bacteria. **Explanation:** **Chlamydiae** are classified as **obligate intracellular bacteria**. Although they were once thought to be viruses due to their small size and inability to grow on artificial media, they possess all the structural and metabolic hallmarks of bacteria. **Why Bacteria is the Correct Answer:** 1. **Cell Wall:** They possess a rigid cell wall containing peptidoglycan (though modified) and an outer membrane similar to Gram-negative bacteria. 2. **Nucleic Acids:** They contain both **DNA and RNA** (viruses contain only one). 3. **Reproduction:** They divide by **binary fission** rather than assembly. 4. **Metabolism:** They possess their own ribosomes and synthesize their own proteins and lipids. 5. **Antibiotic Sensitivity:** They are susceptible to antibacterial antibiotics (e.g., Azithromycin, Doxycycline) but not antivirals. **Why Other Options are Incorrect:** * **Viruses:** Viruses lack a cellular structure, do not have both DNA and RNA simultaneously, and cannot replicate via binary fission. * **Fungi & Protozoa:** These are **eukaryotic** organisms. Chlamydiae are **prokaryotic**, lacking a nuclear membrane and membrane-bound organelles. **High-Yield Clinical Pearls for NEET-PG:** * **Energy Parasitism:** Chlamydiae are "energy parasites" because they cannot synthesize their own ATP and must derive it from the host cell. * **Unique Life Cycle:** They exist in two forms: the **Elementary Body (EB)**, which is infectious and extracellular, and the **Reticulate Body (RB)**, which is the metabolically active, replicative intracellular form. * **Staining:** They are best visualized using **Giemsa or Castaneda stains**; they are poorly visualized on Gram stain. * **Serotypes:** *C. trachomatis* serotypes A, B, Ba, and C cause **Trachoma**, while D-K cause **NGU (Non-Gonococcal Urethritis)** and Inclusion Conjunctivitis. L1, L2, and L3 cause **Lymphogranuloma Venereum (LGV)**.

Q: Which carbohydrate does gonococcus ferment?

Glucose. **Explanation:** The differentiation of pathogenic *Neisseria* species is primarily based on their carbohydrate fermentation patterns (acid production from sugars). *Neisseria gonorrhoeae* (Gonococcus) is biochemically characterized by its ability to ferment **only Glucose**. **1. Why Glucose is correct:** *Neisseria gonorrhoeae* utilizes glucose oxidatively to produce acid, but it lacks the enzymes necessary to ferment other sugars. A helpful mnemonic for NEET-PG is: **G**onococcus ferments **G**lucose only. **2. Analysis of Incorrect Options:** * **Maltose:** This is the key differentiating factor. While *N. meningitidis* (Meningococcus) ferments both **G**lucose and **M**altose (**M**eningococcus = **M**altose), *N. gonorrhoeae* cannot ferment maltose. * **Sucrose & Fructose:** These sugars are typically fermented by non-pathogenic, commensal *Neisseria* species (e.g., *N. sicca*). Pathogenic species like Gonococcus and Meningococcus are sucrose-negative. **Clinical Pearls for NEET-PG:** * **Culture Media:** The gold standard for isolation is **Thayer-Martin medium** (a selective Chocolate agar containing Vancomycin, Colistin, and Nystatin). * **Gram Stain:** Appears as Gram-negative, kidney-shaped diplococci found within polymorphonuclear leukocytes (intracellular). * **Oxidase/Catalase:** All *Neisseria* species are **Oxidase positive** and **Catalase positive**. * **Key Distinction:** Unlike Meningococcus, Gonococcus does **not** have a polysaccharide capsule and does **not** ferment maltose.

Q: Crepitus in a wound is produced by which of the following microorganisms?

Clostridium welchii. **Explanation:** **Correct Answer: C. Clostridium welchii (Clostridium perfringens)** The presence of **crepitus** (a crackling sensation felt on palpation) indicates the presence of gas within the soft tissues. This is the hallmark clinical sign of **Gas Gangrene** (Clostridial Myonecrosis). * **Mechanism:** *Clostridium welchii* is an anaerobic, Gram-positive spore-forming bacillus. In an anaerobic environment (like deep crush injuries), it ferments carbohydrates in the muscle tissue, producing significant amounts of insoluble gas (CO₂ and H₂). This gas dissects through tissue planes, leading to crepitus, severe pain, and rapid tissue necrosis. The primary virulence factor responsible for tissue destruction is the **Alpha toxin** (Lecithinase). **Why other options are incorrect:** * **A. Staphylococcus aureus:** Typically causes pyogenic (pus-forming) infections like abscesses or cellulitis. While it can cause necrotizing fasciitis in rare cases, it is not a primary gas-producer. * **B. Clostridium tetani:** While also an anaerobe, it does not cause tissue necrosis or gas production. It produces **Tetanospasmin**, a neurotoxin that leads to spastic paralysis (Lockjaw). * **C. Pseudomonas:** Known for causing opportunistic infections in burns and producing a characteristic blue-green pigment (Pyocyanin). It does not typically produce gas in tissues. **NEET-PG High-Yield Pearls:** * **Nagler’s Reaction:** Used for rapid identification of *C. perfringens* (detects lecithinase activity on egg yolk agar). * **Morphology:** Described as "Box-car shaped" bacilli; notably **non-motile** (unlike most Clostridia). * **Stormy Fermentation:** Characteristic reaction in litmus milk. * **Target Hemolysis:** Double zone of hemolysis on blood agar (inner complete zone by theta toxin, outer partial zone by alpha toxin).

Q: What is the most common opportunistic infection in AIDS patients?

Tuberculosis. **Explanation:** In the context of the HIV/AIDS pandemic, **Tuberculosis (TB)**, caused by *Mycobacterium tuberculosis*, is the most common opportunistic infection (OI) and the leading cause of mortality worldwide. While HIV-infected individuals are susceptible to a wide range of pathogens, TB is unique because it can occur at any CD4 T-cell count, although the risk increases significantly as the count drops. In India and other developing nations, TB remains the most frequent co-infection encountered in clinical practice. **Analysis of Options:** * **Tuberculosis (A):** Correct. It is the most common OI globally and in India. It often presents atypically in advanced AIDS (extrapulmonary or disseminated forms). * **Pneumocystis jirovecii pneumonia (C):** This was historically the most common opportunistic infection in the Western world before the widespread use of prophylactic Co-trimoxazole. It typically occurs when the CD4 count falls below 200 cells/µL. * **Cryptococcosis (B):** This is the most common fungal meningitis in AIDS patients, usually occurring at CD4 counts <100 cells/µL, but it is less frequent than TB. * **Histoplasmosis (D):** This is a regional fungal infection (endemic in specific river valleys) and is not as globally prevalent as TB or PCP. **High-Yield Clinical Pearls for NEET-PG:** * **Most common OI overall:** Tuberculosis. * **Most common fungal OI:** Candidiasis (Oral Thrush). * **Most common CNS mass lesion:** Toxoplasmosis. * **Most common cause of blindness:** CMV Retinitis (CD4 <50 cells/µL). * **Screening:** All HIV-positive patients should be screened for TB using the four-symptom complex (cough, fever, night sweats, and weight loss).

Q: The phage-encoded exotoxin of Vibrio cholerae resembles which toxin of E. coli?

Heat labile toxin. **Explanation:** The correct answer is **Heat labile toxin (LT)**. **1. Why Heat Labile Toxin (LT) is correct:** The *Vibrio cholerae* enterotoxin (Cholera Toxin/CT) and the Heat Labile toxin (LT) of Enterotoxigenic *E. coli* (ETEC) are structurally, functionally, and immunologically nearly identical. * **Mechanism:** Both toxins are A-B subunit toxins. The 'B' subunit binds to the **GM1 ganglioside receptor** on enterocytes, while the 'A' subunit enters the cell to ADP-ribosylate the **Gs protein**. This leads to the permanent activation of **adenylate cyclase**, increasing intracellular **cAMP**. High cAMP levels result in the hypersecretion of water and electrolytes (Cl⁻, Na⁺, K⁺, and HCO₃⁻) into the intestinal lumen, causing profuse watery diarrhea. **2. Why other options are incorrect:** * **Heat stable toxin (ST):** Produced by ETEC, this toxin increases **cGMP** (not cAMP) by activating guanylate cyclase. It is a small peptide, not an A-B toxin. * **Shiga-like toxin (Verocytotoxin):** Produced by EHEC (e.g., O157:H7), this toxin inhibits protein synthesis by removing adenine from the 28S rRNA of the 60S ribosomal subunit. It is associated with Hemolytic Uremic Syndrome (HUS). **3. High-Yield Clinical Pearls for NEET-PG:** * **Genetic Origin:** Cholera toxin is encoded by the **CTXφ (CTX phage)**, a lysogenic bacteriophage. * **Mnemonic for cAMP vs. cGMP:** * **L**abile **A**ctivates **A**denylate cyclase (**cAMP**). * **S**table **A**ctivates **G**uanylate cyclase (**cGMP**). * *(Mnemonic: "cAMP is Labile, cGMP is Stable")* * **Stool Appearance:** *V. cholerae* causes "Rice water stools" with a fishy odor, lacking blood or inflammatory cells.

Q: Which of the following is a late lactose fermenter?

Shigella sonnei. **Explanation:** The classification of Enterobacteriaceae is primarily based on their ability to ferment lactose, which is detected on **MacConkey Agar**. Lactose fermentation requires two essential enzymes: 1. **Lactose Permease:** Transports lactose into the bacterial cell. 2. **$\beta$-galactosidase:** Breaks down lactose into glucose and galactose. **Shigella sonnei** is a classic example of a **Late Lactose Fermenter (LLF)**. These organisms possess the gene for $\beta$-galactosidase but lack or have a sluggish Lactose Permease. Consequently, they take 48–72 hours (instead of the usual 18–24 hours) to ferment lactose and produce pink colonies on MacConkey agar. **Analysis of Incorrect Options:** * **E. coli & Klebsiella:** These are **Rapid Lactose Fermenters (RLF)**. They possess both enzymes and produce bright pink colonies within 18–24 hours. Klebsiella is further distinguished by its mucoid colonies due to a prominent capsule. * **Salmonella:** This is a **Non-Lactose Fermenter (NLF)**. It lacks both enzymes and produces pale, colorless colonies on MacConkey agar. **NEET-PG High-Yield Pearls:** * **Other Late Lactose Fermenters:** *Vibrio cholerae* (specifically the O1 biotype) and *Serratia marcescens*. * **Shigella Classification:** *S. sonnei* belongs to Serogroup D. It is the only Shigella species that is a late lactose fermenter and is also **indole negative** and **ornithine decarboxylase positive**. * **Clinical Context:** *S. sonnei* is the most common cause of shigellosis (bacillary dysentery) in developed countries, whereas *S. flexneri* is more common in developing nations like India.

Q: Which of the following mycobacteria does not produce pigment?

M. Avium. ### Explanation The classification of Non-Tuberculous Mycobacteria (NTM) is based on the **Runyon Classification**, which categorizes species according to their growth rate and ability to produce pigment (carotenoids) in the presence or absence of light. **Why M. avium is correct:** *M. avium-intracellulare complex* (MAC) belongs to **Runyon Group III (Non-photochromogens)**. These bacteria are characterized by their inability to produce significant pigment regardless of light exposure. Their colonies typically appear buff, tan, or off-white. **Analysis of Incorrect Options:** * **A. M. kansasii:** This is a **Runyon Group I (Photochromogen)**. It produces a yellow-orange pigment only after exposure to light. * **B. M. simiae:** Also a **Runyon Group I (Photochromogen)**. It is unique among photochromogens for being niacin-positive, often mimicking *M. tuberculosis*. * **D. M. scrofulaceum:** This is a **Runyon Group II (Scotochromogen)**. It produces pigment (usually yellow-orange) in both the light and the dark. **NEET-PG High-Yield Pearls:** 1. **Runyon Classification Summary:** * **Group I (Photochromogens):** Pigment in light only (*M. kansasii, M. marinum, M. simiae*). * **Group II (Scotochromogens):** Pigment in light and dark (*M. scrofulaceum, M. szulgai, M. gordonae*). * **Group III (Non-photochromogens):** No pigment (*MAC, M. ulcerans*). * **Group IV (Rapid Growers):** Growth in <7 days (*M. fortuitum, M. chelonae, M. abscessus*). 2. **Clinical Context:** *M. avium* is the most common NTM causing systemic infection in HIV/AIDS patients (CD4 count <50 cells/mm³). 3. **M. marinum:** Known as "Swimming pool granuloma" or "Fish tank granuloma."

Question 1

Which of the following is NOT true of Treponema pallidum?

Accepted Answer

Motile by peritrichous flagella

Answer

Can be maintained in rabbit testis

Answer

Dark field microscopy is used for visualization

Answer

The TPI test is very useful

Question 2

What is the recommended treatment for penicillin-resistant gonorrhea?

Accepted Answer

Ceftriaxone

Answer

Ciprofloxacin

Answer

Streptomycin

Answer

Erythromycin

Question 3

All are true regarding Staphylococcus aureus, EXCEPT:

Accepted Answer

Methicillin-resistant S. aureus (MRSA) will not develop in individuals not previously exposed to antibiotics.

Answer

25-50% of healthy individuals are colonized by S. aureus.

Answer

Individuals using injectable insulin have an increased risk of S. aureus colonization.

Answer

S. aureus is the most common cause of surgical wound infections.

Question 4

Chlamydiae are classified as which of the following?

Accepted Answer

Bacteria

Answer

Viruses

Answer

Fungi

Answer

Protozoa

Question 5

Which carbohydrate does gonococcus ferment?

Accepted Answer

Glucose

Answer

Maltose

Answer

Sucrose

Answer

Fructose

Question 6

Crepitus in a wound is produced by which of the following microorganisms?

Accepted Answer

Clostridium welchii

Answer

Staphylococcus aureus

Answer

Clostridium tetani

Answer

Pseudomonas

Question 7

What is the most common opportunistic infection in AIDS patients?

Accepted Answer

Tuberculosis

Answer

Cryptococcosis

Answer

Pneumocystis jirovecii pneumonia

Answer

Histoplasmosis

Question 8

The phage-encoded exotoxin of Vibrio cholerae resembles which toxin of E. coli?

Accepted Answer

Heat labile toxin

Answer

Heat stable toxin

Answer

Shiga-like toxin

Answer

Verocytotoxin

Question 9

Which of the following is a late lactose fermenter?

Accepted Answer

Shigella sonnei

Answer

E. coli

Answer

Klebsiella

Answer

Salmonella

Question 10

Which of the following mycobacteria does not produce pigment?

Accepted Answer

M. Avium

Answer

M. kansasii

Answer

M. simiae

Answer

M. scrofulaceum

Bacteriology — MCQs

Bacteriology — MCQs

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