Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1251: Which of the following statements is FALSE regarding Enterobacteriaceae?

A. Non-fastidious growth
B. They are oxidase positive (Correct Answer)
C. They are catalase positive
D. They reduce nitrates

Explanation: The **Enterobacteriaceae** family consists of a large group of Gram-negative bacilli that share specific biochemical characteristics. Understanding these "family rules" is essential for identifying pathogens like *E. coli*, *Salmonella*, and *Klebsiella*. ### Why Option B is the Correct Answer (The False Statement) All members of the Enterobacteriaceae family are **Oxidase negative** (except for the genus *Plesiomonas*). The oxidase test detects the presence of cytochrome c oxidase; Enterobacteriaceae lack this enzyme because they utilize different pigments in their electron transport chain. This is the primary biochemical test used to differentiate Enterobacteriaceae from other Gram-negative rods like *Pseudomonas* and *Vibrio*, which are oxidase positive. ### Analysis of Other Options * **A. Non-fastidious growth:** Most members grow readily on ordinary media like Nutrient Agar and MacConkey Agar. They do not require specialized growth factors. * **C. Catalase positive:** All members of this family possess the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen. * **D. Reduce nitrates:** They possess the enzyme nitrate reductase, which reduces nitrates to nitrites. This property is the basis for the **Nitrite test** on urine dipsticks used to diagnose Urinary Tract Infections (UTIs). ### High-Yield Clinical Pearls for NEET-PG * **The "Big Three" Rules:** To be classified as Enterobacteriaceae, an organism must be: 1. Oxidase negative 2. Glucose fermenters (ferment glucose with the production of acid or acid and gas) 3. Nitrate reducers * **MacConkey Agar:** Used to differentiate the family into **Lactose Fermenters** (Pink colonies: *E. coli, Klebsiella*) and **Non-Lactose Fermenters** (Pale colonies: *Salmonella, Shigella, Proteus*). * **Exceptions:** *Plesiomonas shigelloides* is the only member that is oxidase positive.

Question 1252: A child is suffering from watery diarrhea. The isolated organism is showing darting motility. What is the most probable diagnosis?

A. Campylobacter
B. Vibrio cholerae (Correct Answer)
C. Enterotoxigenic E. coli
D. Shigella

Explanation: ### Explanation The correct answer is **Vibrio cholerae**. **1. Why Vibrio cholerae is correct:** The clinical presentation of **watery diarrhea** (often described as "rice-water stools") combined with the classic laboratory finding of **darting motility** is pathognomonic for *Vibrio cholerae*. This rapid, jerky movement is due to a single polar flagellum. In a clinical setting, this motility can be visualized via hanging drop preparation of the stool sample and is inhibited by adding specific antisera (Pfeiffer’s phenomenon). **2. Why the other options are incorrect:** * **Campylobacter:** While *Campylobacter jejuni* also exhibits darting motility (often described as "corkscrew"), it typically causes **bloody diarrhea** (dysentery) and is a thermophilic organism (grows at 42°C). * **Enterotoxigenic E. coli (ETEC):** This is a common cause of traveler’s diarrhea (watery). However, it does not exhibit the characteristic "darting" motility; its motility is generally described as peritrichous/sluggish. * **Shigella:** This organism is **non-motile**. It typically presents with inflammatory diarrhea containing blood and mucus (bacillary dysentery). **3. High-Yield Clinical Pearls for NEET-PG:** * **Culture Media:** TCBS (Thiosulfate Citrate Bile Salts Sucrose) agar is the selective medium where *V. cholerae* produces **yellow colonies** (sucrose fermenter). * **Enrichment Media:** Alkaline Peptone Water (APW) and Monsur’s Taurocholate Tellurite Peptone Water. * **Transport Media:** Venkatraman-Ramakrishnan (VR) medium. * **Biochemical Test:** Oxidase positive (differentiates it from Enterobacteriaceae). * **String Test:** Positive (colonies lose turbidity and form a "string" when mixed with 0.5% sodium deoxycholate).

Question 1253: Which of the following bacteria is catalase positive and novobiocin susceptibility negative?

A. Staph epidermidis
B. Streptococcus pyogenes
C. Staph saprophyticus (Correct Answer)
D. Streptococcus viridans

Explanation: ### Explanation The question tests the ability to differentiate between Gram-positive cocci using biochemical markers and antibiotic susceptibility. **1. Why the Correct Answer is Right:** * **Catalase Test:** All members of the genus *Staphylococcus* are **catalase positive**, which distinguishes them from *Streptococcus* (catalase negative). * **Novobiocin Susceptibility:** Among the coagulase-negative staphylococci (CoNS), **_Staphylococcus saprophyticus_** is uniquely **resistant (negative susceptibility)** to Novobiocin. This is a key laboratory diagnostic feature used to identify it as a common cause of urinary tract infections (UTIs) in young, sexually active females. **2. Why the Incorrect Options are Wrong:** * **Option A: *Staph epidermidis*:** While it is catalase positive, it is **Novobiocin sensitive**. It is the most common CoNS and is typically associated with prosthetic valve endocarditis and catheter infections. * **Option B: *Streptococcus pyogenes*:** This is a Group A Streptococcus (GAS). It is **catalase negative** and shows a zone of inhibition with Bacitracin (Bacitracin sensitive). * **Option C: *Streptococcus viridans*:** These are **catalase negative** alpha-hemolytic streptococci. They are differentiated from *S. pneumoniae* by being Optochin resistant. **3. Clinical Pearls for NEET-PG:** * **Mnemonic for Novobiocin:** *"On the **Staph** retreat, there's **NO** **S**aprophyticus **E**ntrance"* (**NO**vobiocin: **S**aprophyticus is **R**esistant; **E**pidermidis is **S**ensitive). * **Clinical Association:** *S. saprophyticus* is the second most common cause of community-acquired UTI in young women (after *E. coli*), often referred to as "Honeymoon Cystitis." * **Coagulase Test:** Both *S. saprophyticus* and *S. epidermidis* are Coagulase-negative (CoNS), whereas *S. aureus* is Coagulase-positive.

Question 1254: Which of the following organisms' culture demonstrates the phenomenon of satellitism?

A. Haemophilus (Correct Answer)
B. Streptococcus
C. Klebsiella
D. Proteus

Explanation: ### Explanation **1. Why Haemophilus is Correct:** The phenomenon of **satellitism** is a classic laboratory characteristic of *Haemophilus influenzae*. This organism is fastidious and requires two essential growth factors: **Factor X (Hemin)** and **Factor V (NAD)**. While blood agar contains Factor X, the Factor V is sequestered inside red blood cells (RBCs). When *Staphylococcus aureus* is streaked across a blood agar plate inoculated with *H. influenzae*, the Staphylococci lyse the RBCs (releasing Factor V) and also synthesize additional NAD as a metabolic byproduct. Consequently, *H. influenzae* colonies grow only in the immediate vicinity of the *S. aureus* streak, appearing as "satellites." **2. Why Other Options are Incorrect:** * **Streptococcus:** Most species are non-fastidious or require enriched media (like blood agar) for hemolysis patterns, but they do not rely on other bacteria for specific growth factors via satellitism. * **Klebsiella:** This is a robust member of the Enterobacteriaceae family that grows easily on basic media like MacConkey agar without needing external factor supplementation. * **Proteus:** Known for its characteristic **"swarming motility"** on agar due to its peritrichous flagella, not satellitism. **3. NEET-PG High-Yield Pearls:** * **Culture Media:** *H. influenzae* grows best on **Chocolate Agar** because the heating process inactivates V-factor inhibitors and releases factors from RBCs. * **Factor Requirements:** *H. influenzae* requires both X and V; *H. parainfluenzae* requires only V; *H. ducreyi* (causes chancroid) requires only X. * **The "Satellite Test":** Used specifically to identify *Haemophilus* species in labs where chocolate agar is unavailable.

Question 1255: Which of the following is associated with Pseudomonas aeruginosa?

A. Produces heat-stable enterotoxin
B. Causes Shanghai fever (Correct Answer)
C. Cannot be destroyed at 55 degrees Celsius for 1 hour
D. Does not produce any pigment

Explanation: **Explanation:** **Pseudomonas aeruginosa** is a non-fermentative, Gram-negative bacillus known for its versatility and multi-drug resistance. 1. **Why Option B is Correct:** **Shanghai Fever** is a rare clinical syndrome caused by *Pseudomonas aeruginosa* septicemia. It typically presents in children with symptoms mimicking typhoid fever (fever, diarrhea, and prostration) but is often associated with characteristic skin lesions like **ecthyma gangrenosum**. 2. **Why Other Options are Incorrect:** * **Option A:** Heat-stable enterotoxins are characteristic of *Escherichia coli* (ST) or *Staphylococcus aureus*. Pseudomonas primarily produces **Exotoxin A**, which inhibits protein synthesis via ADP-ribosylation of EF-2 (similar to Diphtheria toxin). * **Option C:** *Pseudomonas aeruginosa* is not particularly heat-resistant. It is easily killed by moist heat at **55°C for 1 hour**. This distinguishes it from spore-forming bacteria or thermophilic organisms. * **Option D:** Pseudomonas is famous for producing various pigments, including **Pyocyanin** (blue-green, unique to *P. aeruginosa*), **Pyoverdin** (fluorescent yellow-green), and **Pyorubrin** (red). **High-Yield Clinical Pearls for NEET-PG:** * **Culture Characteristics:** Exhibits a characteristic **fruity/grape-like odor** due to aminoacetophenone production. * **Key Virulence Factor:** Alginate production leads to a mucoid phenotype, commonly seen in **Cystic Fibrosis** patients. * **Common Infections:** Ventilator-associated pneumonia (VAP), Otitis externa (Swimmer’s ear), and puncture wound osteomyelitis (especially through sneakers). * **Treatment:** Requires anti-pseudonal drugs like Piperacillin-Tazobactam, Ceftazidime, or Carbapenems.

Question 1256: A child presents with a white patch over the tonsils. Diagnosis is made by culture in which medium?

A. Loeffler medium (Correct Answer)
B. LJ medium
C. Blood agar
D. Tellurite medium

Explanation: The clinical presentation of a **white patch (pseudomembrane)** over the tonsils in a child is a classic hallmark of **Diphtheria**, caused by *Corynebacterium diphtheriae*. ### **Why Loeffler’s Serum Slope (LSS) is the correct answer:** Loeffler’s medium is the **enrichment medium of choice** for *C. diphtheriae*. It is preferred for initial diagnosis because: 1. It promotes **rapid growth** (6–8 hours), which is crucial for early diagnosis. 2. It enhances the development of characteristic **metachromatic granules** (Babes-Ernst granules), which are best visualized using Albert’s stain. 3. It inhibits the growth of common oral commensals to some extent, allowing the pathogen to outpace them. ### **Analysis of Incorrect Options:** * **LJ (Lowenstein-Jensen) Medium:** This is the gold standard solid medium for *Mycobacterium tuberculosis*. It contains egg, malachite green, and glycerol. * **Blood Agar:** While *C. diphtheriae* can grow on blood agar, it is non-specific. It is primarily used to rule out Streptococcal pharyngitis (which shows beta-hemolysis). * **Tellurite Medium (Potassium Tellurite/McLeod’s):** This is a **selective medium**, not the primary enrichment medium. While it helps in biotyping (mitis, intermedius, gravis) by producing black/grey colonies, it inhibits growth and takes longer (24–48 hours) compared to Loeffler’s. ### **NEET-PG High-Yield Pearls:** * **Morphology:** Gram-positive, non-motile, club-shaped bacilli arranged in "Chinese letter" or cuneiform patterns. * **Staining:** Albert’s stain shows bluish-black metachromatic granules against a green bacillary body. * **Toxin Detection:** The **Elek’s Gel Precipitation Test** is the gold standard for detecting toxin production (virulence). * **Selective Media:** Potassium Tellurite and Tinsdale Medium.

Question 1257: Lactobacilli are numerous in carious lesions because they:

A. a) can produce insoluble extracellular polysaccharides (Correct Answer)
B. b) can attach to smooth enamel surfaces
C. c) are more violently cariogenic for animals than Streptococcus mutans
D. d) are secondary invaders

Explanation: ### Explanation **Correct Option: A (Can produce insoluble extracellular polysaccharides)** The primary mechanism behind the proliferation of *Lactobacilli* in dental caries is their ability to synthesize **insoluble extracellular polysaccharides (glucans)** from dietary sucrose. These polysaccharides act as a sticky matrix, facilitating the formation of dental plaque (biofilm). Furthermore, *Lactobacilli* are highly **acidogenic** (produce lactic acid) and **aciduric** (tolerate low pH), allowing them to thrive in the acidic environment of a carious lesion where other bacteria perish. This acid production leads to the demineralization of tooth enamel and dentin. **Analysis of Incorrect Options:** * **Option B:** While attachment is crucial, *Streptococcus mutans* is the primary colonizer that attaches to smooth enamel. *Lactobacilli* are generally poor at initial attachment and usually colonize pre-existing lesions. * **Option C:** *Streptococcus mutans* is considered the most important and potent initiator of dental caries in both humans and animals. *Lactobacilli* are significant but typically considered secondary to *S. mutans* in terms of cariogenic potential. * **Option D:** While *Lactobacilli* are often considered "secondary invaders" because they proliferate *after* a lesion has started, this option describes their **timing** rather than the **reason** (mechanism) for their high numbers. The question asks *why* they are numerous, which is due to their metabolic capability (polysaccharide production). **High-Yield Clinical Pearls for NEET-PG:** * **Primary Initiator:** *Streptococcus mutans* (most common cause of dental caries). * **Progression of Caries:** *Lactobacillus* species are more involved in the **progression** of the lesion into the dentin rather than the initiation. * **Döderlein's Bacilli:** These are *Lactobacilli* found in the normal vaginal flora that maintain an acidic pH (via glycogen fermentation) to prevent bacterial vaginosis. * **Culture Media:** *Lactobacilli* grow well on **Rogosa SL agar** (selective medium).

Question 1258: Which of the following organisms has been implicated in the pathogenesis of atherosclerosis?

A. Pseudomonas
B. Streptococcus
C. Chlamydia (Correct Answer)
D. Yersinia

Explanation: ### Explanation The correct answer is **C. Chlamydia**. **Pathogenesis:** The specific organism implicated in atherosclerosis is ***Chlamydia pneumoniae***. It is an obligate intracellular bacterium primarily known for causing atypical pneumonia. Its role in cardiovascular disease is supported by the "Inflammatory Theory" of atherosclerosis. *C. pneumoniae* can infect vascular endothelial cells, smooth muscle cells, and macrophages (foam cells) within arterial plaques. It induces chronic inflammation, promotes lipid peroxidation, and triggers the release of pro-inflammatory cytokines, which destabilizes the plaque and leads to atheroma progression. **Analysis of Incorrect Options:** * **A. Pseudomonas:** Primarily an opportunistic pathogen causing nosocomial infections (ventilator-associated pneumonia, UTIs, and burn wound infections). It is not linked to chronic vascular inflammation. * **B. Streptococcus:** While *Streptococcus viridans* is a major cause of Infective Endocarditis and *S. pyogenes* leads to Rheumatic Heart Disease, neither is implicated in the long-term pathogenesis of atherosclerotic plaques. * **D. Yersinia:** *Yersinia pestis* causes plague, and *Y. enterocolitica* causes gastroenteritis/mesenteric lymphadenitis. There is no established clinical evidence linking them to atherosclerosis. **High-Yield Facts for NEET-PG:** * **Evidence:** *C. pneumoniae* has been identified in atherosclerotic lesions via PCR and electron microscopy. * **Other Organisms:** Besides *C. pneumoniae*, **Cytomegalovirus (CMV)** and **Helicobacter pylori** have also been studied for potential roles in atherosclerosis, though the evidence for *Chlamydia* remains the most frequently tested. * **Diagnosis:** *C. pneumoniae* is best diagnosed via PCR or MIF (Microimmunofluorescence) tests. * **Treatment Note:** Despite the association, large-scale clinical trials (like the ACES and WIZARD trials) showed that prolonged antibiotic treatment does not significantly reduce the risk of secondary cardiovascular events.

Question 1259: Which of the following is a rapid growth medium for Klebs-Loeffler Bacillus?

A. Cysteine-tellurite blood agar
B. Loeffler’s serum slope (Correct Answer)
C. Nutrient broth
D. None of these

Explanation: **Explanation:** **Klebs-Loeffler Bacillus (KLB)** is the eponym for *Corynebacterium diphtheriae*. The diagnosis of diphtheria relies heavily on specific culture media due to the fastidious nature of the organism. **1. Why Loeffler’s Serum Slope (LSS) is correct:** LSS is an **enriched medium** containing horse, ox, or sheep serum. It is specifically designed for the **rapid growth** of *C. diphtheriae*. The organism grows faster on LSS (within 6–8 hours) than on most other media. Furthermore, LSS enhances the development of the characteristic **metachromatic granules** (Volutin/Babes-Ernst granules) and preserves the typical morphology of the bacilli, which may be lost on more selective media. **2. Why other options are incorrect:** * **A. Cysteine-tellurite blood agar (CTBA):** This is a **selective medium** (e.g., McLeod’s or Hoyle’s medium). While it is essential for isolating *C. diphtheriae* from mixed flora (by inhibiting commensals), it is **slow-growing**, requiring 24–48 hours. Potassium tellurite is reduced to metallic tellurium, giving the colonies a characteristic grey-black color. * **C. Nutrient broth:** This is a basal medium. *C. diphtheriae* is fastidious and requires enrichment (serum or blood) for optimal growth; it will not show rapid or characteristic growth in simple nutrient broth. **NEET-PG High-Yield Pearls:** * **Staining:** Use **Albert’s stain** to visualize metachromatic granules (granules appear bluish-black, while the body appears green). * **Morphology:** Described as "Chinese letter" or "Cuneiform" arrangement due to incomplete separation during binary fission (snapping division). * **Toxigenicity Testing:** The **Elek’s gel precipitation test** is the gold standard for detecting toxin production. * **Culture sequence:** In clinical practice, a swab is usually inoculated onto both LSS (for rapid presumptive diagnosis) and a Tellurite medium (for selective isolation).

Question 1260: Which statement about Streptococcus is FALSE?

A. M protein is responsible for the production of mucoid colonies. (Correct Answer)
B. M protein is the major surface protein of group A streptococci.
C. Mucoid colonies are virulent.
D. Endotoxin causes the rash of scarlet fever.

Explanation: **Explanation** **1. Why Option A is False (The Correct Answer):** The mucoid appearance of *Streptococcus pyogenes* (Group A Strep) colonies is due to the **Hyaluronic acid capsule**, not the M protein. The capsule inhibits phagocytosis and gives the colonies a glistening, "dew-drop" appearance. While the M protein is a major virulence factor, it is a hair-like projection (fimbriae) on the cell wall and does not contribute to the mucoid morphology. **2. Analysis of Other Options:** * **Option B:** M protein is indeed the **chief virulence factor** and the major surface protein of Group A Streptococci. It is highly antigenic and is used for Serotyping (Griffith typing). * **Option C:** Mucoid colonies are highly virulent because the hyaluronic acid capsule is chemically similar to human connective tissue, allowing the bacteria to evade the host's immune system (molecular mimicry). * **Option D:** This is a common point of confusion. The rash of scarlet fever is caused by **Streptococcal Pyrogenic Exotoxins (SPE)**, specifically types A, B, and C (also known as Erythrogenic toxins). *Note: While the question implies "Endotoxin" is true, in strict microbiological terms, Gram-positive bacteria do not possess classical LPS endotoxin; however, in many PG-level MCQ contexts, the focus is on identifying the M-protein/Capsule distinction as the "most false" statement.* **High-Yield Clinical Pearls for NEET-PG:** * **M Protein:** Essential for virulence; it acts by inhibiting C3b opsonization. It is the basis for the development of acute rheumatic fever (Type II hypersensitivity). * **ASO Titre:** Useful for diagnosing post-streptococcal complications (Glomerulonephritis/Rheumatic fever), but notably **low or absent** in skin infections (Impetigo). * **Dick Test:** Historically used to identify susceptibility to scarlet fever. * **Schultz-Charlton Reaction:** A blanching test used to identify the scarlet fever rash.

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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