Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1221: The Weil-Felix reaction is a test for:

A. Precipitation
B. Agglutination (Correct Answer)
C. Complement fixation test (CFT)
D. Immunoassay

Explanation: **Explanation:** The **Weil-Felix reaction** is a classic example of a **heterophile agglutination test** used for the presumptive diagnosis of Rickettsial infections. **1. Why Agglutination is correct:** The test is based on a shared antigenicity (cross-reactivity) between certain species of **Rickettsia** and specific strains of **Proteus vulgaris** (OX-19, OX-2) and **Proteus mirabilis** (OX-K). When a patient’s serum containing anti-rickettsial antibodies is mixed with these Proteus antigens, visible clumping or **agglutination** occurs. This is a qualitative and semi-quantitative method to detect the presence of specific antibodies. **2. Why other options are incorrect:** * **Precipitation:** This involves the interaction of soluble antigens with antibodies to form an insoluble precipitate. In Weil-Felix, the antigen (Proteus bacilli) is particulate, not soluble. * **Complement Fixation Test (CFT):** This is a complex serological test that measures the consumption of complement. While used for some rickettsial diseases historically, it is not the principle behind the Weil-Felix test. * **Immunoassay:** This is a broad category (e.g., ELISA, RIA). While modern rickettsial diagnosis uses Indirect Immunofluorescence Assays (IFA), the specific Weil-Felix reaction is a simple tube or slide agglutination test. **High-Yield Clinical Pearls for NEET-PG:** * **Epidemic & Endemic Typhus:** Positive for OX-19. * **Scrub Typhus:** Positive for OX-K (Proteus mirabilis). * **Rocky Mountain Spotted Fever:** Positive for OX-19 and OX-2. * **Q Fever:** The Weil-Felix test is **negative** (High-yield exception). * **Limitations:** The test lacks high sensitivity and specificity; definitive diagnosis is now preferred via IFA or PCR.

Question 1222: Dark ground microscopy is used for the diagnosis of which type of microorganism?

A. Spirochaetes (Correct Answer)
B. Mycobacterium tuberculosis
C. Mycobacterium leprae
D. Mycoplasma

Explanation: **Explanation:** **Dark Ground Microscopy (DGM)** is a specialized technique where the condenser prevents direct light from entering the objective lens. Only light reflected or scattered by the specimen enters the lens, making the organism appear bright against a dark background. **1. Why Spirochaetes is correct:** Spirochaetes (such as *Treponema pallidum*, *Leptospira*, and *Borrelia*) are extremely thin (approx. 0.2 µm) and have a refractive index similar to surrounding medium/glass. They are below the resolution limit of standard light microscopy and do not stain well with aniline dyes (Gram stain). DGM allows for the visualization of these organisms in their **live, motile state**, which is crucial for the primary diagnosis of syphilis from chancre fluid. **2. Why other options are incorrect:** * **Mycobacterium tuberculosis & M. leprae:** These are Acid-Fast Bacilli (AFB). They are thick enough to be seen under light microscopy using the **Ziehl-Neelsen (ZN) stain** or fluorescent microscopy (Auramine-Rhodamine). * **Mycoplasma:** These are the smallest free-living organisms and lack a cell wall. While they are thin, they are typically identified via specialized culture media (PPLO agar) or PCR, rather than DGM. **High-Yield Clinical Pearls for NEET-PG:** * **Treponema pallidum:** Shows characteristic **corkscrew motility** on DGM. * **Leptospira:** Shows **spinning/hooked-end motility** (often described as "shepherd's crook"). * **Alternative for Spirochaetes:** If DGM is unavailable, **Silver Impregnation Stains** (e.g., Fontana or Levaditi) are used to increase the apparent thickness of the organism for visualization. * **Limitation:** DGM cannot be used for oral samples to diagnose syphilis because non-pathogenic commensal spirochaetes are part of the normal oral flora.

Question 1223: All clostridia species cause myonecrosis except?

A. Clostridium septicum
B. Clostridium difficile (Correct Answer)
C. Clostridium novyi
D. Clostridium welchii

Explanation: **Explanation:** The correct answer is **Clostridium difficile**. The underlying medical concept here is the distinction between **histotoxic clostridia** (which cause tissue destruction and gas gangrene) and **enterotoxic clostridia**. **Why Clostridium difficile is the correct answer:** *C. difficile* is primarily an enteric pathogen. It produces two main toxins: **Toxin A (Enterotoxin)** and **Toxin B (Cytotoxin)**. These toxins cause mucosal inflammation and damage in the colon, leading to **Antibiotic-Associated Diarrhea** and **Pseudomembranous Colitis**. It does not possess the alpha-toxin (lecithinase) or other collagenases required to cause myonecrosis (muscle death). **Why the other options are incorrect:** * **Clostridium welchii (C. perfringens):** This is the most common cause of gas gangrene (myonecrosis). It produces the lethal **Alpha-toxin**, which degrades cell membranes. * **Clostridium novyi:** A potent histotoxic organism often involved in gas gangrene, particularly in soil-contaminated wounds. * **Clostridium septicum:** A major cause of non-traumatic gas gangrene. Its presence in the bloodstream is highly associated with underlying **colon cancer** or hematologic malignancies. **High-Yield Clinical Pearls for NEET-PG:** * **Gas Gangrene Trio:** The three most common species causing myonecrosis are *C. perfringens* (Type A), *C. novyi*, and *C. septicum*. * **C. difficile Diagnosis:** Look for "volcano-like" lesions on endoscopy and detect toxins in stool via ELISA or PCR. * **Drug of Choice:** Oral Vancomycin or Fidaxomicin is now preferred over Metronidazole for *C. difficile* infections. * **Nagler’s Reaction:** Used to identify *C. perfringens* by demonstrating lecithinase activity on egg yolk agar.

Question 1224: Which species of Shigella produces exotoxin?

A. Shigella sonnei
B. Shigella flexneri
C. Shigella boydii
D. Shigella dysenteriae (Correct Answer)

Explanation: **Explanation:** The genus *Shigella* is primarily characterized by its ability to invade the colonic mucosa. However, **Shigella dysenteriae type 1** (also known as Shiga bacillus) is unique among the species because it produces a potent exotoxin known as the **Shiga toxin (Stx)**. **Why the correct answer is right:** * **Shigella dysenteriae:** This species produces the Shiga toxin, which has three primary activities: enterotoxic (causing fluid secretion), cytotoxic (inhibiting protein synthesis by inactivating the 60S ribosomal subunit), and neurotoxic. This toxin is responsible for the increased severity of the disease, leading to complications like Hemolytic Uremic Syndrome (HUS). **Why the other options are wrong:** * **Shigella sonnei, S. flexneri, and S. boydii:** While these species are pathogenic and cause bacillary dysentery through mucosal invasion (mediated by large virulence plasmids), they **do not produce the Shiga exotoxin**. *S. sonnei* is the most common cause of shigellosis in developed countries and typically causes the mildest form of the disease. *S. flexneri* is the most common species in developing countries. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Shiga toxin acts as an N-glycosidase that cleaves a specific adenine residue from the 28S rRNA of the **60S ribosomal subunit**, halting protein synthesis. * **HUS Association:** Shiga toxin damages glomerular endothelial cells, leading to the triad of HUS: Microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. * **Infective Dose:** *Shigella* has a very low infective dose (as few as 10–100 organisms) because it is highly resistant to gastric acid. * **Culture:** On MacConkey agar, *Shigella* species appear as **late-lactose fermenters** (*S. sonnei*) or non-lactose fermenters (others).

Question 1225: The maculopapular, copper-coloured rash is characteristic of which condition?

A. Measles
B. Dengue
C. Rubella
D. Secondary syphilis (Correct Answer)

Explanation: **Explanation:** The correct answer is **Secondary syphilis**. This stage of syphilis, caused by the spirochete *Treponema pallidum*, occurs 2–10 weeks after the primary chancre heals. It is characterized by a generalized, non-pruritic, **maculopapular rash** that classically has a **copper-colored** or "raw ham" appearance. A key diagnostic feature for NEET-PG is that this rash characteristically involves the **palms and soles**, a rare finding in other eruptive fevers. **Why other options are incorrect:** * **Measles:** Presents with a bright red (morbilliform) maculopapular rash that begins behind the ears and spreads cephalocaudally. It is associated with the "3 Cs" (Cough, Coryza, Conjunctivitis) and pathognomonic **Koplik spots**. * **Dengue:** Typically features a "white islands in a sea of red" rash during the recovery phase. The initial rash is a transient facial flushing or a maculopapular eruption, but it lacks the copper hue and palmar involvement of syphilis. * **Rubella:** Also known as German Measles, it presents with a pinkish-red maculopapular rash that spreads rapidly (within 24 hours) and disappears by the third day. It is often associated with post-auricular lymphadenopathy. **High-Yield Clinical Pearls for NEET-PG:** * **Condyloma Lata:** Flat, moist, wart-like lesions in intertriginous areas, also characteristic of secondary syphilis (highly infectious). * **Snail-track ulcers:** Found on the oral mucosa in secondary syphilis. * **Diagnosis:** Screening is done via non-specific tests (**VDRL/RPR**), while confirmation requires specific treponemal tests (**FTA-ABS/TPHA**). * **Treatment:** The drug of choice for all stages of syphilis is **Benzathine Penicillin G**.

Question 1226: A labourer involved with repair-work of sewers was admitted with fever, jaundice, and renal failure. What is the most appropriate test to diagnose the infection in this patient?

A. Dark field microscopy
B. Paul Bunnell test
C. Microscopic agglutination (Correct Answer)
D. Micro immunofluorescence

Explanation: **Explanation:** The clinical presentation of fever, jaundice, and renal failure in a patient with occupational exposure to sewage (rodent urine) is classic for **Weil’s Disease**, the severe form of **Leptospirosis**. **1. Why Microscopic Agglutination Test (MAT) is correct:** MAT is the **gold standard** serological test for diagnosing Leptospirosis. It involves mixing the patient's serum with live antigens from various *Leptospira* serovars and observing for agglutination under a dark-field microscope. It is highly specific and can identify the infecting serovar. While PCR and blood cultures are useful in the first week (leptospiremic phase), MAT becomes the investigation of choice from the second week onwards (immune phase). **2. Why other options are incorrect:** * **Dark field microscopy (DFM):** While *Leptospira* can be visualized via DFM of blood or urine, it has very low sensitivity and a high rate of false positives due to artifacts (like fibrin threads), making it unreliable for definitive diagnosis. * **Paul Bunnell test:** This is a heterophile antibody test used to diagnose Infectious Mononucleosis caused by the Epstein-Barr virus (EBV). * **Micro immunofluorescence (MIF):** This is the gold standard for diagnosing *Chlamydia* infections, not Leptospirosis. **Clinical Pearls for NEET-PG:** * **Transmission:** Contact with water/soil contaminated by urine of infected rodents (rats). * **Biphasic illness:** Initial febrile/septicemic phase followed by an immune phase. * **Weil’s Syndrome Triad:** Jaundice, Renal failure (often non-oliguric), and Hemorrhage (e.g., epistaxis or pulmonary hemorrhage). * **Culture:** Uses **EMJH medium** or **Fletcher’s medium** (growth takes weeks). * **Drug of Choice:** Doxycycline (prophylaxis/mild cases) or IV Penicillin G (severe cases).

Question 1227: Which selective medium is used for Corynebacterium diphtheriae?

A. Loffeler's serum slop
B. Tellurite blood agar (Correct Answer)
C. Chocolate agar
D. Tinsdale medium

Explanation: **Explanation:** *Corynebacterium diphtheriae* requires specific media for isolation and identification. The correct answer is **Tellurite Blood Agar (TBA)**. **1. Why Tellurite Blood Agar is correct:** Tellurite blood agar (e.g., McLeod’s medium) is a **selective medium**. Potassium tellurite inhibits the growth of most normal upper respiratory flora (like Streptococci and Staphylococci) while allowing *C. diphtheriae* to grow. Furthermore, *C. diphtheriae* reduces tellurite to metallic tellurium, which results in the characteristic **grey-to-black colored colonies**, aiding in presumptive identification. **2. Analysis of other options:** * **Loffler’s Serum Slope (LSS):** This is an **enriched medium**, not selective. It is used for the rapid growth of *C. diphtheriae* (6-8 hours) and is excellent for demonstrating the characteristic **metachromatic granules** (Volutin/Babes-Ernst granules) via Albert’s stain. * **Chocolate Agar:** This is a non-selective enriched medium used primarily for fastidious organisms like *Haemophilus influenzae* and *Neisseria*. It does not inhibit commensal flora. * **Tinsdale Medium:** While Tinsdale is also a selective medium for *C. diphtheriae* (producing black colonies with a brown halo), **Tellurite Blood Agar** is the classic, more frequently tested primary selective medium in the context of this standard question. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, non-motile bacilli showing "Chinese letter" or cuneiform arrangement. * **Staining:** Albert’s stain is used to visualize metachromatic granules (granules appear bluish-black, body appears green). * **Toxin Detection:** The **Elek’s Gel Precipitation Test** is the gold standard for detecting toxigenicity (distinguishing *C. diphtheriae* from diphtheroids). * **Culture sequence:** Clinical samples are usually inoculated onto LSS (for speed) and TBA (for selectivity) simultaneously.

Question 1228: Which of the following is true about Clostridium tetani?

A. It is gram-positive.
B. It has a drumstick appearance. (Correct Answer)
C. It grows in an aerobic environment.
D. It is gram-negative.

Explanation: **Explanation:** *Clostridium tetani* is a clinically significant, spore-forming bacterium responsible for tetanus. The correct answer is **Option B** because of its unique morphology. 1. **Why Option B is correct:** *C. tetani* produces **terminal, spherical spores** that are wider than the vegetative body of the bacillus. This specific arrangement gives the organism a characteristic **"drumstick" or "tennis racket" appearance** under the microscope, which is a classic morphological hallmark used for identification. 2. **Why other options are incorrect:** * **Options A & D:** *Clostridium* species are **Gram-positive** bacilli. While they may occasionally appear Gram-variable in older cultures due to cell wall degradation, they are taxonomically and fundamentally classified as Gram-positive. Therefore, Option D is factually incorrect. (Note: While Option A is technically true, in the context of medical exams, the "drumstick appearance" is the pathognomonic feature most frequently tested). * **Option C:** *C. tetani* is an **obligate anaerobe**. It cannot grow in an aerobic environment; it requires a low oxidation-reduction potential to germinate and produce its potent exotoxin. **High-Yield Clinical Pearls for NEET-PG:** * **Motility:** It exhibits **swarming growth** on blood agar (similar to *Proteus*), mediated by peritrichous flagella. * **Toxin:** It produces **Tetanospasmin**, an AB-type exotoxin that blocks the release of inhibitory neurotransmitters (**GABA and Glycine**) from Renshaw cells in the spinal cord, leading to spastic paralysis. * **Clinical Signs:** Look for keywords like **Risus sardonicus** (grimace), **Trismus** (lockjaw), and **Opisthotonus** (archback posturing). * **Treatment:** Management involves wound debridement, Metronidazole (preferred over Penicillin), and Tetanus Immunoglobulin (TIG).

Question 1229: Apart from dental caries, what other condition can be caused by Streptococcus mutans?

A. Endocarditis (Correct Answer)
B. Food poisoning
C. Urinary tract infection
D. Pharyngitis

Explanation: **Explanation:** *Streptococcus mutans* is a member of the **Viridans group streptococci (VGS)**, which are normal commensals of the oral cavity. While it is primarily known for causing **dental caries** (by producing dextran from sucrose, allowing it to adhere to tooth enamel), it is also a significant cause of **Subacute Bacterial Endocarditis (SABE)**. **1. Why Endocarditis is correct:** When the oral mucosa is breached (e.g., during dental procedures, vigorous brushing, or scaling), *S. mutans* enters the bloodstream (transient bacteremia). It utilizes its extracellular polysaccharides (**dextrans/glucans**) to adhere to fibrin-platelet aggregates on damaged heart valves (e.g., mitral valve prolapse or rheumatic heart disease), leading to the formation of vegetations and endocarditis. **2. Why other options are incorrect:** * **Food poisoning:** Typically caused by *Staphylococcus aureus*, *Bacillus cereus*, or *Clostridium perfringens*. *S. mutans* does not produce enterotoxins. * **Urinary tract infection:** Most commonly caused by *E. coli*, *Staphylococcus saprophyticus*, or *Enterococcus*. * **Pharyngitis:** Primarily caused by **Group A Streptococcus** (*S. pyogenes*). Viridans group streptococci are normal flora in the throat and do not cause primary pharyngitis. **Clinical Pearls for NEET-PG:** * **Dextran production:** The key virulence factor for both dental plaque formation and heart valve adherence. * **Optochin Resistance:** Unlike *S. pneumoniae*, Viridans streptococci are Optochin resistant and Bile insoluble. * **Prophylaxis:** Patients with prosthetic valves or specific congenital heart defects require antibiotic prophylaxis (usually Amoxicillin) before invasive dental procedures to prevent VGS endocarditis.

Question 1230: What is the most common infecting organism responsible for boils on the face?

A. Streptococcus
B. Staphylococcus (Correct Answer)
C. Pseudomonas
D. None of the above

Explanation: ### Explanation **Correct Answer: B. Staphylococcus** **Medical Concept:** Boils (furuncles) are deep-seated infections of the hair follicles, characterized by painful, inflammatory nodules that eventually discharge pus. The most common causative agent is ***Staphylococcus aureus***. This organism is a normal commensal of the skin and anterior nares. It produces **coagulase** and various toxins that lead to tissue necrosis and localized abscess formation. Because the face has a high density of sebaceous glands and hair follicles, it is a frequent site for these infections. **Analysis of Incorrect Options:** * **A. Streptococcus:** While *Streptococcus pyogenes* (Group A Strep) is a major cause of skin infections like **erysipelas** and **impetigo** (non-bullous), it typically causes spreading infections rather than localized follicular abscesses (boils). * **C. Pseudomonas:** *Pseudomonas aeruginosa* is commonly associated with **"Hot tub folliculitis"** or infections in burn wounds and immunocompromised patients. It is not the primary cause of common facial boils in the general population. **High-Yield Clinical Pearls for NEET-PG:** * **Danger Triangle of the Face:** Boils in the area bounded by the bridge of the nose and the corners of the mouth are clinically significant. Infection here can spread via the facial and ophthalmic veins to the **cavernous sinus**, leading to life-threatening **Cavernous Sinus Thrombosis**. * **Carbuncle:** A cluster of interconnected boils with multiple inflammatory foci and drainage points, most commonly seen on the nape of the neck and back, often associated with **Diabetes Mellitus**. * **Treatment:** Small boils may resolve with warm compresses; however, larger or systemic lesions require incision and drainage (I&D) and anti-staphylococcal antibiotics (e.g., Cloxacillin or Clindamycin).

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

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free