Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 901: All the following species of Haemophilus show satellitism, except?

A. H. influenzae
B. H. aegypticus
C. H. haemolyticus
D. H. ducreyi (Correct Answer)

Explanation: **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).

Question 902: What is the primary energy currency of the cell?

A. Adenosine monophosphate
B. Adenosine diphosphate
C. Adenosine triphosphate (Correct Answer)
D. Adenosine quadriphosphate

Explanation: **Explanation:** **Adenosine Triphosphate (ATP)** is the universal energy currency of all living cells, including bacteria. It consists of an adenine ring, a ribose sugar, and three phosphate groups. The energy is stored in the **high-energy phosphoanhydride bonds** between the phosphate groups. When the terminal phosphate bond is hydrolyzed (converting ATP to ADP), a significant amount of free energy (~7.3 kcal/mol) is released to drive endergonic reactions such as active transport, protein synthesis, and motility. **Analysis of Options:** * **Option A (AMP):** Adenosine monophosphate contains only one phosphate group and lacks high-energy bonds. It often acts as a signaling molecule (e.g., cyclic AMP) rather than an energy donor. * **Option B (ADP):** Adenosine diphosphate is the precursor to ATP. While it contains one high-energy bond, it represents a "lower charge" state of the cell's energy battery. * **Option D (AQP):** Adenosine quadriphosphate is not a standard biological molecule used for energy transfer in cellular metabolism. **High-Yield Facts for NEET-PG:** 1. **Generation:** In bacteria, ATP is generated via **Oxidative Phosphorylation** (in the cytoplasmic membrane) or **Substrate-Level Phosphorylation** (during glycolysis/TCA cycle). 2. **Proton Motive Force (PMF):** Bacteria use the electron transport chain to create a proton gradient across the cell membrane, which drives the **ATP synthase** enzyme to produce ATP. 3. **Clinical Relevance:** Certain antibiotics, like **Bedaquiline** (used in MDR-TB), work by specifically inhibiting the mycobacterial ATP synthase, effectively starving the bacteria of energy.

Question 903: Which one of the following bacteria is cell wall deficient?

A. Escherichia coli
B. Salmonella typhi
C. Mycoplasma (Correct Answer)
D. Treponema pallidum

Explanation: ### Explanation **Correct Answer: C. Mycoplasma** **1. Why Mycoplasma is correct:** *Mycoplasma* species are unique among bacteria because they naturally **lack a peptidoglycan cell wall**. Instead, their cell membrane contains **sterols** (acquired from the host or culture media), which provide structural integrity and osmotic stability. Because they lack a cell wall, they are pleomorphic (can change shape) and are inherently **resistant to Beta-lactam antibiotics** (like Penicillins and Cephalosporins) which act by inhibiting cell wall synthesis. **2. Why the other options are incorrect:** * **A. Escherichia coli:** A Gram-negative rod that possesses a thin peptidoglycan layer and an outer membrane. * **B. Salmonella typhi:** A Gram-negative bacterium with a standard cell wall structure containing lipopolysaccharides (LPS). * **D. Treponema pallidum:** A spirochete that, while very thin and difficult to stain with traditional Gram staining, still possesses a cytoplasmic membrane and a periplasmic peptidoglycan layer. **3. NEET-PG High-Yield Clinical Pearls:** * **Smallest Free-living Organisms:** Mycoplasma are the smallest organisms capable of self-replication. * **Fried Egg Appearance:** On specialized media (like PPLO agar), *Mycoplasma* colonies exhibit a characteristic "fried egg" appearance. * **Staining:** They do not take up Gram stain; they are best visualized using Giemsa or Dienes stain. * **L-forms vs. Mycoplasma:** Do not confuse them with **L-forms**, which are bacteria that *normally* have cell walls but have lost them due to adverse conditions (like antibiotic pressure). Mycoplasma *never* have a cell wall. * **Drug of Choice:** Since they lack a cell wall, infections (like Atypical Pneumonia) are treated with protein synthesis inhibitors like **Macrolides** (Azithromycin) or Tetracyclines.

Question 904: Toxic shock syndrome is caused by which of the following organisms?

A. Staphylococci (Correct Answer)
B. E. coli
C. Pneumococci
D. Meningococci

Explanation: **Explanation:** **Toxic Shock Syndrome (TSS)** is a life-threatening multisystem illness primarily caused by **Staphylococcus aureus**. The pathophysiology is driven by the **Toxic Shock Syndrome Toxin-1 (TSST-1)**, which acts as a **superantigen**. Unlike regular antigens, superantigens bypass normal antigen processing and bind directly to the MHC Class II molecules and the Vβ region of T-cell receptors. This leads to a massive, non-specific activation of T-cells (up to 20%), resulting in a "cytokine storm" (IL-1, IL-2, TNF-α, and IFN-γ), which causes high fever, hypotension, and multi-organ failure. **Analysis of Options:** * **A. Staphylococci (Correct):** *S. aureus* is the classic cause, historically associated with highly absorbent tampon use, though non-menstrual cases (post-surgical wounds, burns) are now common. Note: *Streptococcus pyogenes* can also cause a similar "Toxic Shock-like Syndrome" (TSLS). * **B. E. coli:** A gram-negative rod that typically causes sepsis via **endotoxins** (Lipid A), leading to septic shock, but not the specific clinical entity of TSS. * **C. Pneumococci:** Causes lobar pneumonia and meningitis; it does not produce superantigens associated with TSS. * **D. Meningococci:** Causes Waterhouse-Friderichsen syndrome and meningococcemia, characterized by petechial rashes and adrenal hemorrhage, but not TSS. **High-Yield Clinical Pearls for NEET-PG:** 1. **Clinical Triad:** Fever, hypotension, and a diffuse macular erythroderma (sunburn-like rash) that desquamates (peels) on the palms and soles 1–2 weeks after onset. 2. **TSST-1 vs. SpeA:** TSST-1 is associated with *S. aureus*, while Streptococcal TSLS is often mediated by **Pyrogenic Exotoxin A (SpeA)**. 3. **Vaginal Colonization:** While *S. aureus* is part of the normal flora in ~5% of women, TSS occurs when specific toxin-producing strains multiply in aerobic conditions (e.g., tampons).

Question 905: What is the mechanism of action of diphtheria toxin?

A. ADP ribosylation (Correct Answer)
B. Activation of adenylyl cyclase
C. Activation of guanylyl cyclase
D. Inhibition

Explanation: **Explanation:** **Mechanism of Action (Option A):** Diphtheria toxin, produced by *Corynebacterium diphtheriae* (strains lysogenized by the **beta-phage**), is a classic A-B exotoxin. The 'B' subunit binds to the host cell receptor (HB-EGF), allowing the 'A' (Active) subunit to enter the cytosol. The A subunit catalyzes the **ADP-ribosylation of Elongation Factor-2 (EF-2)**. This modification inactivates EF-2, halting protein synthesis and leading to cell death. **Analysis of Incorrect Options:** * **Option B (Adenylyl Cyclase):** This is the mechanism for toxins like **Cholera toxin** (*Vibrio cholerae*) and Heat-Labile (LT) toxin of *E. coli*. They increase cAMP, leading to electrolyte and water loss. * **Option C (Guanylyl Cyclase):** This is the mechanism for the Heat-Stable (ST) toxin of *E. coli*, which increases cGMP levels. * **Option D (Inhibition):** This is too vague. While the toxin "inhibits" protein synthesis, the specific biochemical mechanism required for the exam is ADP-ribosylation. **High-Yield Clinical Pearls for NEET-PG:** * **Target Organ:** The toxin has a predilection for the **myocardium** (causing myocarditis) and **peripheral nerves** (causing demyelination/paralysis). * **Schick Test:** Used to demonstrate immunity against diphtheria. * **Elek’s Gel Precipitation Test:** The gold standard for demonstrating the **toxigenicity** of a *C. diphtheriae* strain. * **Culture:** Use **Loeffler’s Serum Slope** (rapid growth) or **Potassium Tellurite Agar** (black colonies). * **Similar Mechanism:** *Pseudomonas aeruginosa* (Exotoxin A) also acts by ADP-ribosylating EF-2.

Question 906: What is the best selective medium for Salmonella?

A. Thiosulphate citrate bile salt agar (TCBS)
B. Wilson and Blair's brilliant-green bismuth sulphite agar medium (Correct Answer)
C. Sorbitol MacConkey agar
D. All of the above

Explanation: **Explanation:** The correct answer is **Wilson and Blair's brilliant-green bismuth sulphite agar medium**. This is considered the most sensitive and specific selective medium for the isolation of *Salmonella Typhi*. **Why it is correct:** Wilson and Blair’s medium contains **brilliant green** and **bismuth sulphite**, which inhibit the growth of most commensal Enterobacteriaceae (like *E. coli*) and Gram-positive bacteria. *Salmonella Typhi* reduces bismuth sulphite to metallic bismuth in the presence of glucose, resulting in characteristic **jet-black colonies with a metallic sheen**. This distinct morphology allows for easy identification even in heavily contaminated fecal samples. **Why the other options are incorrect:** * **A. Thiosulphate citrate bile salt agar (TCBS):** This is the highly selective medium for ***Vibrio cholerae***. The high pH (8.6) and bile salts inhibit most other intestinal flora. * **C. Sorbitol MacConkey agar (SMAC):** This is the selective and differential medium used to identify **Enterohemorrhagic *E. coli* (EHEC/O157:H7)**. Unlike most *E. coli*, O157:H7 does not ferment sorbitol and appears as colorless colonies. **High-Yield Clinical Pearls for NEET-PG:** * **Enrichment Media for Salmonella:** Selenite F broth and Tetrathionate broth are used to increase the yield of *Salmonella* before plating on selective media. * **Other Selective Media:** XLD (Xylose Lysine Deoxycholate) and DCA (Deoxycholate Citrate Agar) are also used; *Salmonella* typically produces red colonies with black centers on XLD due to $H_2S$ production. * **Specimen Timing:** For Enteric Fever, remember the **BASU** mnemonic: **B**lood culture (1st week), **A**ntibody/Widal (2nd week), **S**tool culture (3rd week), and **U**rine culture (4th week).

Question 907: Which of the following mycobacteria can cause disease in an HIV+ve patient with a CD4 count of 600/cu.mm?

A. M. tuberculosis (Correct Answer)
B. MAC (Mycobacterium avium complex)
C. M. chelonei
D. M. fortuitum

Explanation: **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.

Question 908: The CAMP test is used to identify which of the following bacteria?

A. Group A Streptococcus
B. Group B Streptococcus (Correct Answer)
C. Group C Streptococcus
D. Group D Streptococcus

Explanation: **Explanation:** The **CAMP test** (Christie, Atkins, and Munch-Petersen) is a diagnostic laboratory test used specifically to identify **Group B Streptococcus (GBS)**, also known as *Streptococcus agalactiae*. **Why Group B Streptococcus is correct:** *Streptococcus agalactiae* produces a diffusible, heat-stable extracellular protein known as the **CAMP factor**. When GBS is cultured on blood agar alongside *Staphylococcus aureus* (which produces β-hemolysin), the CAMP factor acts synergistically with the staphylococcal hemolysin. This synergy results in an enhanced zone of hemolysis, classically appearing as an **"arrowhead" shape** at the junction of the two bacterial growths. **Why the other options are incorrect:** * **Group A Streptococcus (*S. pyogenes*):** It is CAMP negative. It is primarily identified by its sensitivity to **Bacitracin** and a positive PYR test. * **Group C and D Streptococcus:** These groups do not produce the CAMP factor. Group D (e.g., *Enterococcus*) is typically identified by its ability to grow in 6.5% NaCl and hydrolyze bile esculin. **High-Yield Clinical Pearls for NEET-PG:** * **Reverse CAMP Test:** Used to identify *Clostridium perfringens*. In this version, *S. agalactiae* is used to detect the alpha-toxin of *C. perfringens*. * **Clinical Significance:** GBS is the leading cause of **neonatal sepsis, meningitis, and pneumonia**. Pregnant women are screened at 35–37 weeks of gestation via vaginal/rectal swabs. * **Morphology:** GBS shows a narrow zone of β-hemolysis on blood agar and is catalase-negative.

Question 909: Inclusion bodies present in psittacosis are known as:

A. HP body
B. Miyagawa corpuscles
C. Henderson-Peterson inclusion bodies (Correct Answer)
D. Negri bodies

Explanation: **Explanation:** The correct answer is **Henderson-Peterson inclusion bodies**. Psittacosis is caused by *Chlamydia psittaci*, an obligate intracellular bacterium. Like other Chlamydiae, it undergoes a unique life cycle involving elementary bodies (infectious) and reticulate bodies (replicative). During this process, large, basophilic intracytoplasmic inclusion bodies are formed within the host cell. In the context of *C. psittaci*, these are specifically referred to as **Henderson-Peterson bodies**. **Analysis of Options:** * **HP body (Halberstaedter-Prowazek bodies):** These are the characteristic intracytoplasmic inclusion bodies seen in **Trachoma** (caused by *Chlamydia trachomatis* serotypes A, B, Ba, and C). * **Miyagawa corpuscles:** These are the inclusion bodies associated with **Lymphogranuloma Venereum (LGV)**, caused by *Chlamydia trachomatis* serotypes L1, L2, and L3. * **Negri bodies:** These are pathognomonic eosinophilic intracytoplasmic inclusions found in pyramidal cells of the hippocampus and Purkinje cells of the cerebellum in **Rabies**. **High-Yield Clinical Pearls for NEET-PG:** * **Psittacosis (Ornithosis):** Primarily a zoonotic disease transmitted to humans from infected birds (parrots, pigeons). It typically presents as atypical pneumonia with splenomegaly. * **Staining:** Chlamydial inclusions are best visualized using **Giemsa, Castaneda, or Gimenez stains**. They do not stain well with Gram stain. * **Levinthal-Cole-Lillie (LCL) bodies:** Another term sometimes used synonymously with the inclusions of *C. psittaci*. * **Drug of Choice:** Tetracyclines (Doxycycline) are the preferred treatment for all Chlamydial infections.

Question 910: 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?

A. Corynebacterium Diphtheriae
B. Pseudomonas Aeruginosa
C. Clostridium Tetani
D. Clostridium Welchii (Correct Answer)

Explanation: ### 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.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

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