Bacteriology — MCQs

A urine sample from a patient suspected of a urinary tract infection (UTI) was sent to the microbiology lab. On culture, the colony shows hemolysis on blood agar, a positive indole test, and a positive MUG test. The organism also shows characteristic growth on Eosin Methylene Blue (EMB) agar. What is the most likely etiologic organism?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 691: Pertussis toxin acts by all of the following mechanisms, EXCEPT:

A. ADP ribosylation of proteins associated with receptors
B. Increase cyclic AMP
C. Increased calcium release from sarcoplasmic reticulum (Correct Answer)
D. Acts through G alpha subunit

Explanation: **Explanation:** **Pertussis toxin (PTX)**, produced by *Bordetella pertussis*, is a classic A-B subunit exotoxin. Its primary mechanism involves the **ADP-ribosylation** of the **Gi (inhibitory) protein** alpha subunit. 1. **Why Option C is Correct:** Increased calcium release from the sarcoplasmic reticulum is a mechanism associated with toxins like **Ryanodine** or certain physiological muscle contractions, but it is **not** a feature of Pertussis toxin. PTX primarily targets signaling pathways involving cyclic nucleotides, not direct calcium sequestration in the sarcoplasmic reticulum. 2. **Why the Other Options are Incorrect:** * **Option A & D:** PTX acts by catalyzing the **ADP-ribosylation** of the **Gαi subunit** of heterotrimeric G proteins. This modification locks the Gi protein in an inactive state, preventing it from inhibiting adenylate cyclase. * **Option B:** Because the "inhibitory" G protein (Gi) is inactivated, the enzyme **adenylate cyclase** remains constitutively active. This leads to a massive **increase in intracellular cyclic AMP (cAMP)** levels, which triggers the systemic effects of the disease (e.g., lymphocytosis, insulin hypoglycemia). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism Mnemonic:** **P**ertussis toxin **P**revents Gi (Inhibitory) action, leading to increased cAMP. (Contrast this with **C**holera toxin, which **C**onstitutively activates Gs). * **Biological Effects:** PTX is also known as **Lymphocytosis-promoting factor** because it prevents lymphocytes from entering lymphoid tissue, causing a characteristic high peripheral white cell count. * **Other Toxins:** *B. pertussis* also produces an **Invasive Adenylate Cyclase toxin** (which acts like *B. anthracis* Edema Factor) and **Tracheal Cytotoxin** (which damages ciliated epithelium).

Question 692: Which of the following is NOT true regarding scrub typhus?

A. It is caused by Rickettsia tsutsugamushi.
B. It is spread by mites.
C. It is caused by the bite of an adult mite on a vertebrate host. (Correct Answer)
D. Tetracycline is an effective treatment.

Explanation: **Explanation:** Scrub typhus is a zoonotic rickettsial infection caused by **Orientia tsutsugamushi** (formerly *Rickettsia tsutsugamushi*). The disease is transmitted to humans through the bite of infected **larval mites (chiggers)** of the family Trombiculidae. **Why Option C is the correct answer (False statement):** The transmission occurs exclusively via the **larval stage (chigger)**. Adult and nymphal mites do not feed on vertebrate hosts; they live in the soil and feed on other arthropods. Therefore, the statement that it is caused by the bite of an **adult** mite is incorrect. **Analysis of other options:** * **Option A:** It is indeed caused by *Orientia tsutsugamushi*. While the genus name has changed, many exams still use the older nomenclature *Rickettsia tsutsugamushi*. * **Option B:** The vector is the trombiculid mite (*Leptotrombidium deliense*). * **Option D:** Tetracyclines (specifically **Doxycycline**) are the gold standard and highly effective treatment for scrub typhus. **High-Yield Facts for NEET-PG:** * **The Eschar:** A necrotic, black, crusty lesion at the site of the chigger bite is a pathognomonic clinical sign. * **Weil-Felix Test:** Scrub typhus shows a positive reaction with **OX-K** antigens (negative for OX-2 and OX-19). * **Tsutsugamushi Triangle:** Refers to the geographical area (including India, SE Asia, and Northern Australia) where the disease is endemic. * **Reservoir:** The mite acts as both a vector and a reservoir through **transovarial transmission**.

Question 693: All of the following strains of Salmonella are motile by peritrichous flagella except?

A. S. bongor
B. S. Gallinarum pullorum (Correct Answer)
C. S. choleraesuis
D. Salmonella enterica

Explanation: **Explanation:** The genus *Salmonella* is generally characterized as Gram-negative, facultatively anaerobic bacilli that are **motile** via **peritrichous flagella**. However, there are two notable exceptions to this rule: **Salmonella Gallinarum** and **Salmonella Pullorum**. **1. Why Option B is Correct:** *Salmonella enterica* serovar **Gallinarum-Pullorum** is uniquely **non-motile**. These strains lack functional flagella, which is a key biochemical feature used to differentiate them from other *Salmonella* species in the laboratory. In the context of veterinary and clinical microbiology, these are the primary pathogens causing fowl typhoid and pullorum disease in poultry. **2. Why the other options are Incorrect:** * **Option A (S. bongori):** This is one of the two main species of the genus *Salmonella*. Like most members of the genus, it is motile by peritrichous flagella. * **Option C (S. choleraesuis):** This is a highly invasive serotype of *S. enterica* (subspecies *enterica*) that typically causes septicaemia. It follows the general rule of being motile. * **Option D (Salmonella enterica):** This species contains over 2,500 serotypes (including *S. Typhi* and *S. Paratyphi*). With the specific exception of the Gallinarum-Pullorum biotypes, the members of this species are motile. **Clinical Pearls for NEET-PG:** * **Flagellar (H) Antigen:** Motility in *Salmonella* is linked to the H antigen, which is used for serotyping via the **Kauffman-White scheme**. * **Culture:** Non-motile strains will not show "swarming" or spreading on semi-solid agar (e.g., Craigie’s tube method). * **High-Yield Exception:** Always remember: **"Salmonella are motile EXCEPT Gallinarum-Pullorum."** This is a classic "except" type question frequently tested in PG entrance exams.

Question 694: Haemophilus influenzae requires which of the following for its growth?

A. Factor V
B. Factor X
C. Factor V and X (Correct Answer)
D. Factor VII

Explanation: **Explanation:** *Haemophilus influenzae* is a fastidious gram-negative coccobacillus that requires specific growth factors found in blood for its cultivation. These are known as **Factor X** and **Factor V**. 1. **Factor X (Hemin):** This is a heat-stable iron-protoporphyrin compound required for the synthesis of essential iron-containing enzymes like cytochromes, catalase, and peroxidase. 2. **Factor V (NAD/NADP):** This is a heat-labile nicotinamide adenine dinucleotide (NAD) or its phosphate (NADP). It acts as a coenzyme in oxidation-reduction reactions. **Why Option C is correct:** *H. influenzae* lacks the enzymes to synthesize these factors independently. While it can grow on **Chocolate Agar** (where heat lyses RBCs to release both X and V factors and inactivates V-factor inhibitors), it cannot grow on standard Blood Agar because Factor V remains sequestered inside intact RBCs. **Analysis of Incorrect Options:** * **Options A & B:** While both factors are necessary, neither alone is sufficient for the growth of *H. influenzae*. *H. parainfluenzae*, by contrast, requires only Factor V. * **Option D:** Factor VII is a clotting factor and has no role in the metabolic requirements of *Haemophilus* species. **High-Yield Clinical Pearls for NEET-PG:** * **Satellitism:** *H. influenzae* grows as small colonies around *Staphylococcus aureus* on blood agar. *S. aureus* provides Factor V (via hemolysis/secretion), while the blood agar provides Factor X. * **Culture Media:** Chocolate agar is the medium of choice. Levinthal’s medium is used for serotyping. * **Capsule:** The Type b (Hib) capsule is made of **Polyribosyl Ribitol Phosphate (PRP)**, which is the primary virulence factor and the target for the Hib vaccine.

Question 695: The diagnosis of gonorrhea is established by which of the following tests?

A. Complement fixation tests (Correct Answer)
B. Pili agglutination tests
C. Hemagglutination tests
D. All of the above tests

Explanation: The diagnosis of *Neisseria gonorrhoeae* is primarily clinical and microbiological (Gram stain and culture); however, serological tests play a specific role in diagnosing complications. ### **Explanation of the Correct Answer** **A. Complement Fixation Tests (CFT):** While not used for acute urethritis, the **Gonococcal Complement Fixation Test (GCFT)** is a classic serological method used to diagnose chronic or systemic gonococcal infections, such as **Gonococcal Arthritis** or **Pelvic Inflammatory Disease (PID)**. It detects antibodies against gonococcal antigens. Although its sensitivity is low in acute cases, it remains a recognized diagnostic tool in medical literature for disseminated gonococcal infection (DGI). ### **Explanation of Incorrect Options** * **B. Pili Agglutination Tests:** While pili are major virulence factors used for attachment, specific "pili agglutination" is not a standardized or routine diagnostic test used in clinical practice for establishing a diagnosis of gonorrhea. * **C. Hemagglutination Tests:** Passive hemagglutination tests have been researched for detecting gonococcal antibodies, but they lack the specificity and clinical validation required to be the standard diagnostic answer compared to the established GCFT. * **D. All of the above:** Since options B and C are not standard diagnostic modalities, this option is incorrect. ### **NEET-PG High-Yield Pearls** * **Gold Standard:** Culture on **Thayer-Martin Medium** (a selective VCN medium: Vancomycin, Colistin, Nystatin). * **Test of Choice (Modern):** Nucleic Acid Amplification Test (**NAAT**) is now the preferred diagnostic method due to high sensitivity. * **Microscopy:** Gram-negative intracellular diplococci (kidney-bean shaped) within polymorphonuclear leukocytes. * **Biochemical Fact:** *N. gonorrhoeae* is **Oxidase positive** and ferments **only Glucose** (not Maltose, unlike *N. meningitidis*).

Question 696: What is a cold abscess related to?

A. Periapical pathology
B. Mycobacterium tuberculosis (Correct Answer)
C. Streptococcus viridans
D. None of the above

Explanation: ### Explanation **Correct Answer: B. Mycobacterium tuberculosis** A **cold abscess** is a hallmark clinical presentation of tuberculosis, most commonly associated with **tuberculous lymphadenitis** (scrofula) or **Pott’s spine** (tuberculosis of the vertebrae). The term "cold" is used because the abscess lacks the classic signs of acute inflammation—**calor** (heat), **rubor** (redness), and **dolor** (pain). In a pyogenic infection, the body mounts a rapid neutrophilic response leading to heat and inflammation. In contrast, *M. tuberculosis* triggers a **Type IV hypersensitivity reaction** and granulomatous inflammation. The "pus" in a cold abscess is actually **caseous necrosis** (liquefied dead tissue) rather than true purulent exudate, resulting in a fluctuant swelling that is neither warm nor tender. **Why other options are incorrect:** * **Option A (Periapical pathology):** This typically leads to acute dental abscesses or chronic granulomas. These are usually associated with pain and inflammatory markers, unlike the indolent nature of a cold abscess. * **Option C (Streptococcus viridans):** This is a pyogenic bacterium. Infections caused by *S. viridans* (like subacute bacterial endocarditis or dental abscesses) trigger an acute inflammatory response characterized by warmth and pain. **NEET-PG High-Yield Pearls:** * **Most common site:** The cervical lymph nodes are the most frequent site for a tuberculous cold abscess. * **Pott’s Spine:** A cold abscess in the lumbar spine can track down the **psoas sheath** to manifest as a swelling in the inguinal region (Psoas abscess). * **Histology:** Look for **Langhans giant cells**, epithelioid cells, and central caseous necrosis. * **Differential Diagnosis:** Actinomycosis can also present with "cold" features but is typically characterized by "sulfur granules" and multiple draining sinuses.

Question 697: A 30-year-old male presented with an ulcerative lesion on the glans penis. Wright-Giemsa stain showed 1-2 rounded structures within macrophage vacuoles. What is the etiology?

A. Chlamydia trachomatis
B. N. gonorrhoea
C. H. ducreyi
D. Calymatobacterium granulomatis (Correct Answer)

Explanation: ### Explanation The clinical presentation and microscopic findings are diagnostic of **Granuloma Inguinale (Donovanosis)**, caused by *Klebsiella granulomatis* (formerly known as *Calymatobacterium granulomatis*). **Why the correct answer is right:** The hallmark of Granuloma Inguinale is the presence of **Donovan bodies**. On Wright-Giemsa or Leishman stain, these appear as 1–2 µm, rounded or safety-pin shaped, bipolar-staining bodies located within the vacuoles of large mononuclear cells (macrophages). Clinically, it presents as a chronic, painless, "beefy-red" ulcerative lesion that bleeds easily on touch (friable), often without associated lymphadenopathy (though "pseudobuboes" may occur due to subcutaneous granulation). **Why the incorrect options are wrong:** * **Chlamydia trachomatis (Serotypes L1-L3):** Causes Lymphogranuloma Venereum (LGV). It typically presents with a transient, painless papule followed by painful, suppurative regional lymphadenopathy (the "Groove sign"). It does not show Donovan bodies. * **N. gonorrhoeae:** Primarily causes urethritis (purulent discharge) rather than ulcerative lesions. Gram stain would show Gram-negative intracellular diplococci. * **H. ducreyi:** Causes Chancroid. These ulcers are characteristically **painful** and "soft." Microscopic examination shows Gram-negative coccobacilli in a "school of fish" or "railroad track" appearance. **NEET-PG High-Yield Pearls:** * **Donovan Bodies:** Pathognomonic for Granuloma Inguinale; described as "safety-pin" appearance. * **Clinical Feature:** Painless, beefy-red ulcer; "Pseudobubo" (swelling in the groin that is actually granulomatous tissue, not a lymph node). * **Treatment of Choice:** Azithromycin (1g orally once a week or 500mg daily) for at least 3 weeks. * **Staining:** Best visualized with Wright, Giemsa, or Leishman stain; the organism is difficult to culture.

Question 698: Chlamydia trachomatis infection causes the following except?

A. Pneumonitis
B. Rhinitis (Correct Answer)
C. Conjunctivitis
D. Urethritis

Explanation: **Explanation:** *Chlamydia trachomatis* is an obligate intracellular bacterium with several serovars (A-K and L1-L3) that show a specific tropism for **columnar and transitional epithelium**. It primarily affects the ocular and genitourinary tracts, as well as the respiratory system in neonates. **Why Rhinitis is the Correct Answer:** *Chlamydia trachomatis* does not typically involve the nasal mucosa or cause **Rhinitis**. Rhinitis is more commonly associated with viral infections (Rhinovirus, Adenovirus) or allergic reactions. While *C. trachomatis* can cause respiratory symptoms in infants, it manifests as a distinct "staccato cough" pneumonitis rather than nasal inflammation. **Analysis of Incorrect Options:** * **Pneumonitis:** Serovars D-K cause **Infantile Pneumonitis**, typically acquired during birth via an infected birth canal. It presents between 2–19 weeks of age with a characteristic staccato cough and hyperinflation, usually without fever. * **Conjunctivitis:** This is a hallmark of Chlamydial infection. Serovars A-C cause **Trachoma** (leading cause of preventable blindness), while serovars D-K cause **Inclusion Conjunctivitis** in both neonates (Ophthalmia neonatorum) and adults. * **Urethritis:** *C. trachomatis* (D-K) is the most common cause of **Non-Gonococcal Urethritis (NGU)**. In males, it leads to urethral discharge; in females, it can cause cervicitis and Pelvic Inflammatory Disease (PID). **High-Yield Clinical Pearls for NEET-PG:** * **Staccato Cough:** Pathognomonic for Chlamydial pneumonia in infants. * **Eosinophilia:** Often seen in peripheral blood in Chlamydial pneumonitis. * **Drug of Choice:** Azithromycin (single dose) or Doxycycline (for 7 days). For neonatal infections, Erythromycin is preferred. * **Diagnosis:** **NAAT (Nucleic Acid Amplification Test)** is the gold standard. Cytology shows **Halberstaedter-Prowazek (HP) inclusion bodies**.

Question 699: Carrom coin appearance of pneumococcus is due to which of the following?

A. Capsular polysaccharide
B. C- Carbohydrate antigen
C. Pneumolysin
D. Autolysin (Correct Answer)

Explanation: **Explanation:** The **"Carrom coin"** or **"draughtsman"** appearance is a classic morphological feature of *Streptococcus pneumoniae* (Pneumococcus) when grown on blood agar. **Why Autolysin is correct:** As *S. pneumoniae* colonies age (usually after 24 hours of incubation), they undergo a process of **self-digestion** mediated by an endogenous enzyme called **Autolysin** (specifically *N-acetylmuramoyl-L-alanine amidase*). This enzyme breaks down the peptidoglycan in the center of the colony where the bacteria are oldest. This results in the collapse or "sinking" of the central portion of the colony, while the periphery remains raised, creating a flat, indented appearance resembling a carrom coin or a draughtsman piece. **Analysis of Incorrect Options:** * **Capsular polysaccharide:** This is the primary virulence factor and is responsible for the **mucoid** appearance of Type 3 pneumococci and the positive Quellung reaction, but it does not cause the central indentation. * **C-Carbohydrate antigen:** This is a cell wall component that reacts with C-Reactive Protein (CRP). It is used for grouping but does not influence colony morphology. * **Pneumolysin:** This is a membrane-damaging toxin (cytotoxin) released during autolysis that inhibits ciliary action and immune cells; it is not responsible for the physical shape of the colony. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, lancet-shaped diplococci. * **Culture:** Alpha-hemolytic (greenish discoloration) on blood agar. * **Bile Solubility Test:** Pneumococci are **bile soluble** because bile salts (like sodium deoxycholate) activate the same **autolytic enzymes** mentioned above, leading to rapid lysis. * **Optochin Sensitivity:** Pneumococci are sensitive to Optochin (ethylhydrocupreine hydrochloride), distinguishing them from *S. viridans*.

Question 700: A urine sample from a patient suspected of a urinary tract infection (UTI) was sent to the microbiology lab. On culture, the colony shows hemolysis on blood agar, a positive indole test, and a positive MUG test. The organism also shows characteristic growth on Eosin Methylene Blue (EMB) agar. What is the most likely etiologic organism?

A. Salmonella typhi
B. Proteus vulgaris
C. Klebsiella pneumoniae
D. E. coli (Correct Answer)

Explanation: **Explanation:** The clinical presentation and laboratory findings are classic for **Escherichia coli**, the most common cause of community-acquired urinary tract infections. **Why E. coli is the correct answer:** * **Culture Characteristics:** Most uropathogenic strains of *E. coli* exhibit **beta-hemolysis** on blood agar. On **EMB agar**, it produces a characteristic **metallic green sheen** due to vigorous lactose fermentation. * **Biochemical Profile:** *E. coli* is part of the IMViC group "++--" (Indole positive, Methyl Red positive, Voges-Proskauer negative, Citrate negative). * **MUG Test:** This is a highly specific rapid test for *E. coli*. The enzyme **beta-glucuronidase** (produced by 97% of *E. coli* strains) cleaves the MUG substrate to release a fluorescent compound visible under UV light. **Why other options are incorrect:** * **Salmonella typhi:** It is a non-lactose fermenter (NLF), indole negative, and typically causes enteric fever rather than primary UTIs. * **Proteus vulgaris:** While indole positive, it is famous for "swarming motility" on agar and being **urease positive** (leading to alkaline urine and struvite stones), not a metallic sheen on EMB. * **Klebsiella pneumoniae:** It produces large, mucoid colonies (lactose fermenter) but is **indole negative** and **citrate positive**. **High-Yield NEET-PG Pearls:** * **EMB Agar:** Selective and differential; the metallic green sheen is a "must-know" for *E. coli*. * **Catalase/Oxidase:** *E. coli* is Catalase positive and **Oxidase negative** (like all Enterobacteriaceae). * **P-fimbriae:** The most important virulence factor for *E. coli* causing pyelonephritis. * **MUG Test:** Remember that *E. coli* O157:H7 (EHEC) is a notable exception and is typically MUG negative.

Practice by Chapter

Staphylococci

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Streptococci and Enterococci

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Neisseria and Moraxella

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Corynebacterium and Listeria

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Bacillus and Clostridium

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Enterobacteriaceae

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Vibrio, Aeromonas, and Plesiomonas

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Pseudomonas and Related Bacteria

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Haemophilus and HACEK Group

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Bordetella and Brucella

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Mycobacteria

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Spirochetes

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