Bacteriology Practice Questions

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: What is the primary energy currency of the cell?

Adenosine triphosphate. **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.

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

Mycoplasma. ### 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.

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

Staphylococci. **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).

Q: What is the mechanism of action of diphtheria toxin?

ADP ribosylation. **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.

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

Group B Streptococcus. **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.

Q: Inclusion bodies present in psittacosis are known as:

Henderson-Peterson inclusion bodies. **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.

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.

Question 1

All the following species of Haemophilus show satellitism, except?

Accepted Answer

H. ducreyi

Answer

H. influenzae

Answer

H. aegypticus

Answer

H. haemolyticus

Question 2

What is the primary energy currency of the cell?

Accepted Answer

Adenosine triphosphate

Answer

Adenosine monophosphate

Answer

Adenosine diphosphate

Answer

Adenosine quadriphosphate

Question 3

Which one of the following bacteria is cell wall deficient?

Accepted Answer

Mycoplasma

Answer

Escherichia coli

Answer

Salmonella typhi

Answer

Treponema pallidum

Question 4

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

Accepted Answer

Staphylococci

Answer

E. coli

Answer

Pneumococci

Answer

Meningococci

Question 5

What is the mechanism of action of diphtheria toxin?

Accepted Answer

ADP ribosylation

Answer

Activation of adenylyl cyclase

Answer

Activation of guanylyl cyclase

Answer

Inhibition

Question 6

What is the best selective medium for Salmonella?

Accepted Answer

Wilson and Blair's brilliant-green bismuth sulphite agar medium

Answer

Thiosulphate citrate bile salt agar (TCBS)

Answer

Sorbitol MacConkey agar

Answer

All of the above

Question 7

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 8

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

Accepted Answer

Group B Streptococcus

Answer

Group A Streptococcus

Answer

Group C Streptococcus

Answer

Group D Streptococcus

Question 9

Inclusion bodies present in psittacosis are known as:

Accepted Answer

Henderson-Peterson inclusion bodies

Answer

HP body

Answer

Miyagawa corpuscles

Answer

Negri bodies

Question 10

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

Bacteriology — MCQs

Bacteriology — MCQs

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