Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 601: Which of the following bacteria is the initiator of smooth surface caries?

A. Streptococcus mutans (Correct Answer)
B. Streptococcus sanguis
C. Lactobacillus
D. Actinomyces

Explanation: **Explanation:** Dental caries is a multifactorial disease involving the interplay of host factors, fermentable carbohydrates, and specific cariogenic bacteria. **Why Streptococcus mutans is the correct answer:** *Streptococcus mutans* is the primary **initiator** of dental caries, particularly on **smooth surfaces**. Its pathogenicity stems from its ability to produce **glucans** (extracellular polysaccharides) from sucrose using the enzyme **glucosyltransferase**. These glucans act as a "biological glue," allowing the bacteria to adhere tenaciously to the tooth enamel and form dental plaque. Furthermore, *S. mutans* is highly **acidogenic** (produces lactic acid) and **aciduric** (tolerates low pH), which leads to the demineralization of the tooth structure. **Analysis of Incorrect Options:** * **Streptococcus sanguis:** This is one of the earliest colonizers of the dental pellicle (the "pioneer" bacteria). While it is found in early plaque, it is not the primary initiator of the decay process itself. * **Lactobacillus:** These organisms are secondary invaders. They are poor at adhering to smooth surfaces but thrive in the acidic environment created by *S. mutans*. They are responsible for the **progression** of deep dentinal caries rather than initiation. * **Actinomyces:** These species (specifically *A. viscosus*) are primarily associated with **root surface caries** and gingival recession in older adults, rather than smooth surface enamel caries. **High-Yield NEET-PG Pearls:** * **Sucrose** is the most cariogenic sugar because it is the only substrate *S. mutans* can use to synthesize glucans. * **Stephan Curve:** Describes the rapid drop in plaque pH followed by a slow recovery after sugar consumption. * **Critical pH:** Enamel demineralization begins when the pH drops below **5.5**. * **Order of colonization:** *S. sanguis* (Initial) → *S. mutans* (Initiation of decay) → *Lactobacillus* (Progression).

Question 602: Which of the following is a feature of gram-positive bacteria rather than gram-negative bacteria?

A. Lipid A-containing lipopolysaccharide
B. Lipoprotein in periplasmic space
C. Outer membrane
D. Thick peptidoglycan cell wall (Correct Answer)

Explanation: ### Explanation The fundamental difference between Gram-positive and Gram-negative bacteria lies in the composition and architecture of their cell walls, which determines their staining characteristics and antibiotic susceptibility. **Why Option D is Correct:** Gram-positive bacteria possess a **thick, multi-layered peptidoglycan (murein) layer** (20–80 nm) that constitutes up to 90% of the cell wall weight. This dense meshwork traps the Crystal Violet-Iodine complex during the Gram staining procedure, preventing it from being washed away by alcohol (decolorizer), thus causing the bacteria to appear **purple**. **Analysis of Incorrect Options:** * **Option A (Lipid A/LPS):** Lipopolysaccharide (LPS), specifically the **Lipid A** component (endotoxin), is a hallmark of the outer membrane of **Gram-negative** bacteria. It triggers systemic inflammatory responses (sepsis). * **Option B (Periplasmic Space):** This is the space between the inner cytoplasmic membrane and the outer membrane. While a very small periplasmic space may exist in some Gram-positives, it is a distinct, functional compartment containing enzymes (like beta-lactamases) and transport proteins primarily in **Gram-negative** bacteria. * **Option C (Outer Membrane):** This is a lipid bilayer found **only in Gram-negative** bacteria. It acts as a protective barrier against certain antibiotics and detergents. **NEET-PG High-Yield Pearls:** * **Teichoic Acids:** Found only in Gram-positive cell walls; they act as surface antigens and help in attachment. * **Lysozyme Sensitivity:** Gram-positive bacteria are generally more susceptible to lysozyme (found in tears/saliva) because it directly digests the exposed peptidoglycan layer. * **Porins:** These transmembrane proteins are exclusive to the **outer membrane** of Gram-negative bacteria, regulating the entry of hydrophilic molecules.

Question 603: What is the primary cellular infiltrate observed in late Pseudomonas infection?

A. Neutrophils (Correct Answer)
B. Lymphocytes
C. Monocytes
D. Plasma cells

Explanation: **Explanation:** The primary cellular infiltrate in *Pseudomonas aeruginosa* infections, regardless of the stage (early or late), is **Neutrophils**. *Pseudomonas* is a classic **pyogenic (pus-forming) bacterium**. The pathogenesis involves the release of potent exotoxins and enzymes (like Elastase and Protease) that cause extensive tissue necrosis. This tissue damage, combined with the presence of bacterial Pathogen-Associated Molecular Patterns (PAMPs), triggers a robust acute inflammatory response. Even in chronic or "late" stages—such as in the lungs of Cystic Fibrosis patients—the hallmark of the infection is a persistent, massive recruitment of neutrophils. These neutrophils often fail to clear the bacteria due to biofilm formation, leading to "frustrated phagocytosis" and further tissue damage. **Analysis of Incorrect Options:** * **B. Lymphocytes:** These are characteristic of chronic granulomatous inflammation or viral infections, not acute pyogenic bacterial infections like *Pseudomonas*. * **C. Monocytes:** While present in later stages of many inflammations to clear debris, they are not the *primary* or dominant infiltrate in *Pseudomonas* lesions, which remain neutrophilic. * **D. Plasma Cells:** These are seen in specific chronic conditions (e.g., syphilis, chronic periodontitis) and represent a late-stage humoral immune response, not the primary cellular defense against *Pseudomonas*. **High-Yield Clinical Pearls for NEET-PG:** * **Ecthyma Gangrenosum:** A pathognomonic skin lesion in *Pseudomonas* septicemia characterized by a necrotic center with an erythematous halo. Histology shows bacterial invasion of blood vessels (perivascular cuffing) with a **lack of prominent inflammatory cells** in neutropenic patients, but typically, neutrophils are the mainstay. * **Pigments:** *Pseudomonas* produces **Pyocyanin** (blue-green) and **Pyoverdin** (fluorescent yellow-green). * **Odor:** Cultures typically have a characteristic **fruity/grape-like odor**. * **Virulence:** Its **Exotoxin A** acts similarly to Diphtheria toxin by inhibiting EF-2 (protein synthesis).

Question 604: Which species of Shigella is aerogenic?

A. Shigella flexneri Type 6 (Correct Answer)
B. Shigella dysenteriae Type 1
C. Shigella sonnei
D. Shigella boydii Type 4

Explanation: ### Explanation **Correct Option: A (Shigella flexneri Type 6)** The genus *Shigella* is characteristically defined as **non-motile, non-capsulated, and non-aerogenic** (does not produce gas from glucose fermentation). However, there are rare exceptions to this rule that are frequently tested in competitive exams like NEET-PG. **Shigella flexneri Type 6** (specifically the Newcastle and Manchester biotypes) is the only member of the genus that can produce gas during carbohydrate fermentation, making it **aerogenic**. **Analysis of Incorrect Options:** * **B. Shigella dysenteriae Type 1:** Also known as *S. shigae*, it is the most virulent species and produces the Shiga toxin. It is strictly anaerogenic and is also catalase-negative (unlike other Shigella species). * **C. Shigella sonnei:** This is a Group D Shigella and is a late lactose fermenter. It is anaerogenic and typically causes milder "sonnei dysentery." * **D. Shigella boydii Type 4:** While *S. boydii* encompasses many serotypes, they are all characteristically anaerogenic. **High-Yield Clinical Pearls for NEET-PG:** * **The "Exceptions" Rule:** * Most *Shigella* are catalase positive; **Exception:** *S. dysenteriae* Type 1 (catalase negative). * Most *Shigella* are anaerogenic; **Exception:** *S. flexneri* Type 6 (aerogenic). * Most *Shigella* are non-lactose fermenters; **Exception:** *S. sonnei* (late lactose fermenter). * **Infective Dose:** *Shigella* has a very low infective dose (as few as 10–100 organisms), making it highly communicable via the fecal-oral route. * **Culture Media:** Deoxycholate Citrate Agar (DCA) and XLD agar are used, where *Shigella* appears as pale, colorless (NLF) colonies.

Question 605: What are the most commonly encountered bacterial pathogens in the lungs of patients with cystic fibrosis?

A. Escherichia coli and alpha Streptococcus
B. Escherichia coli and Pseudomonas
C. Staphylococcus and Proteus
D. Staphylococcus and Pseudomonas (Correct Answer)

Explanation: **Explanation:** Cystic Fibrosis (CF) is characterized by defective chloride transport (CFTR gene mutation), leading to thick, viscid endobronchial secretions. This impaired mucociliary clearance creates a niche for specific bacterial pathogens that colonize the lungs in a predictable age-related sequence. **Why Option D is Correct:** * **Staphylococcus aureus:** This is typically the **most common** pathogen isolated from the respiratory tract of infants and young children with CF. * **Pseudomonas aeruginosa:** This is the **most common** pathogen in adults and the leading cause of chronic pulmonary infection and progressive lung damage in CF patients. By early adulthood, up to 80% of CF patients are colonized with *Pseudomonas*. **Why Other Options are Incorrect:** * **Options A & B (Escherichia coli):** While *E. coli* can cause neonatal pneumonia, it is not a characteristic or primary pathogen associated with the chronic pulmonary pathology of CF. * **Option A (Alpha Streptococcus):** These are generally considered normal oral flora and are not primary drivers of CF lung disease. * **Option C (Proteus):** *Proteus* species are more commonly associated with urinary tract infections and are rarely implicated in CF respiratory exacerbations. **High-Yield NEET-PG Pearls:** 1. **Age Sequence:** *S. aureus* and *H. influenzae* dominate in childhood; *P. aeruginosa* dominates in adulthood. 2. **Burkholderia cepacia complex:** A highly feared pathogen in CF; it is associated with "Cepacia syndrome" (rapid clinical decline) and is often a contraindication for lung transplantation. 3. **Pseudomonas Phenotype:** In CF, *Pseudomonas* often converts to a **mucoid phenotype** (alginate production), which forms biofilms, making it highly resistant to antibiotics and host immune responses. 4. **Other Pathogens:** *Stenotrophomonas maltophilia* and *Aspergillus fumigatus* (ABPA) are also frequently encountered.

Question 606: DCA (deoxycholate citrate agar) media is used for the differentiation of which infection?

A. Salmonella (Correct Answer)
B. Staphylococcus aureus
C. Haemophilus influenzae
D. Bordetella

Explanation: **Explanation:** **Deoxycholate Citrate Agar (DCA)** is a **selective and differential medium** primarily used for the isolation and differentiation of enteric pathogens, specifically **Salmonella** and **Shigella**, from clinical specimens like stool. 1. **Why Salmonella is correct:** DCA contains sodium deoxycholate and citrate salts, which inhibit the growth of Gram-positive bacteria and many coliforms. It differentiates bacteria based on **Lactose Fermentation**. Salmonella (and Shigella) are **Non-Lactose Fermenters (NLF)**; they appear as pale, colorless, or translucent colonies. Furthermore, Salmonella species that produce $H_2S$ (like *S. Typhi*) develop a **black center** on DCA, aiding in rapid identification. 2. **Why other options are incorrect:** * **Staphylococcus aureus:** This is a Gram-positive coccus. The high concentration of bile salts (deoxycholate) in DCA is inhibitory to almost all Gram-positive organisms. * **Haemophilus influenzae:** This is a fastidious organism that requires growth factors X (Hemin) and V (NAD), typically provided in **Chocolate Agar**. It will not grow on DCA. * **Bordetella:** *B. pertussis* requires specialized enriched media like **Regan-Lowe** or **Bordet-Gengou** medium (potato-blood-glycerol agar) for isolation. **High-Yield Clinical Pearls for NEET-PG:** * **DCA Composition:** It is a modification of MacConkey agar with increased selectivity. * **Salmonella vs. Shigella on DCA:** Both are NLF (pale colonies), but Salmonella often shows a black center ($H_2S$ production), whereas Shigella does not. * **Other Enteric Media:** Remember **Wilson and Blair’s Bismuth Sulfite Agar** is the highly specific "gold standard" for *Salmonella Typhi* (producing jet-black colonies with a metallic sheen). * **Selective Agent:** Sodium deoxycholate is the specific agent that makes this medium selective against Gram-positives.

Question 607: Fildes' technique is used for the cultivation of which bacterium?

A. Bacillus anthracis
B. Clostridium tetani (Correct Answer)
C. Mycoplasma pneumoniae
D. Yersinia pestis

Explanation: **Explanation:** **Clostridium tetani** is the correct answer. **Fildes' technique** is a specialized method used to isolate *C. tetani* from mixed cultures (like soil or wound swabs). It exploits the bacterium's characteristic **swarming growth**. In this technique, the inoculum is placed at the base of a slope of enriched agar (like Fildes' agar, which contains peptic digest of blood). *C. tetani* swarms rapidly upward, reaching the top of the slope before other non-motile or less motile organisms, allowing for pure sub-culturing from the top. **Analysis of Incorrect Options:** * **A. Bacillus anthracis:** It is a non-motile organism. It is typically cultured on Blood Agar (showing non-hemolytic "Medusa head" colonies) or PLET medium (selective). * **C. Mycoplasma pneumoniae:** These are the smallest free-living organisms and lack a cell wall. They require complex media containing sterols (PPLO agar) and show a characteristic "fried egg" appearance. * **D. Yersinia pestis:** This is the causative agent of plague. It grows on blood agar but shows a characteristic "stalactite growth" in ghee broth and "cracked glass" appearance on agar. **High-Yield Clinical Pearls for NEET-PG:** * **C. tetani Morphology:** Gram-positive bacilli with terminal, spherical spores giving a **"drumstick" appearance**. * **Swarming:** Apart from *C. tetani*, *Proteus* species also show swarming, but *Proteus* swarming is inhibited by increasing agar concentration (6%) or adding boric acid. * **Toxin:** Tetanospasmin (a neurotoxin) blocks the release of inhibitory neurotransmitters (GABA and Glycine), leading to spastic paralysis.

Question 608: Lemierre's disease, an oropharyngeal infection, is most commonly caused by which of the following organisms?

A. Streptococcus mutans
B. S. milleri
C. P. melanogenica
D. F. necrophorum (Correct Answer)

Explanation: **Explanation:** **Lemierre’s Disease** (also known as post-anginal septicemia) is a life-threatening condition characterized by a primary oropharyngeal infection (like tonsillitis or pharyngitis) that leads to **septic thrombophlebitis of the internal jugular vein (IJV)** and subsequent metastatic septic emboli, most commonly to the lungs. 1. **Why D is correct:** **_Fusobacterium necrophorum_** is the most common causative agent (found in >80% of cases). It is a Gram-negative, non-spore-forming, obligate anaerobic rod. It is highly virulent due to its ability to produce toxins (like leukocidin and hemolysin) and a potent endotoxin that promotes platelet aggregation and clot formation, leading to the characteristic thrombophlebitis. 2. **Why incorrect options are wrong:** * **A. *Streptococcus mutans*:** Primarily associated with dental caries and subacute bacterial endocarditis, not invasive oropharyngeal thrombophlebitis. * **B. *S. milleri* (Anginosus group):** While these can cause abscesses (brain, liver), they are less common primary drivers of Lemierre’s compared to *Fusobacterium*. * **C. *P. melanogenica* (Prevotella):** Though part of the oral flora and sometimes found in mixed anaerobic infections, it is not the classic primary pathogen for Lemierre’s syndrome. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Triad:** Recent oropharyngeal infection + Internal Jugular Vein thrombosis + Metastatic septic emboli (Lungs are the #1 site). * **Microscopy:** *F. necrophorum* appears as pleomorphic, long, thin Gram-negative rods with rounded ends. * **Drug of Choice:** Metronidazole or Carbapenems (must cover anaerobes). * **Key Imaging:** Contrast-enhanced CT of the neck is the gold standard to visualize the IJV thrombus.

Question 609: A 5-year-old child presents with acute onset of high fever, abdominal cramps, and malaise. The child passes small amounts of bloody mucopurulent stools with increasing tenesmus. What is the most likely cause?

A. Rotavirus
B. Vibrio cholerae
C. Shigella dysenteriae (Correct Answer)
D. Entamoeba histolytica

Explanation: ### **Explanation** The clinical presentation of **high fever, abdominal cramps, and bloody mucopurulent stools** (dysentery) associated with **tenesmus** (a distressing feeling of incomplete defecation) in a child is a classic description of **Shigellosis**, most commonly caused by *Shigella dysenteriae*. **Why Shigella dysenteriae is correct:** * **Pathogenesis:** *Shigella* invades the colonic mucosa via M cells, leading to intense inflammation, mucosal ulceration, and micro-abscess formation. * **Clinical Features:** It is characterized by low-volume, frequent stools containing blood and mucus. The **Shiga toxin** (produced by Serotype 1) contributes to severe disease and potential complications like Hemolytic Uremic Syndrome (HUS). * **Tenesmus:** This is a hallmark of distal colon/rectal involvement, typical of bacillary dysentery. **Why other options are incorrect:** * **Rotavirus:** The most common cause of pediatric diarrhea, but it typically causes **profuse watery diarrhea** (osmotic) without blood or mucus, usually accompanied by vomiting. * **Vibrio cholerae:** Causes "rice-water stools." It is a non-invasive secretory diarrhea leading to massive dehydration without fever or blood in stools. * **Entamoeba histolytica:** Causes amoebic dysentery. While it presents with bloody stools, it typically has a **gradual onset**, lower-grade fever, and stools are often described as "anchovy sauce" or mixed with blood/mucus rather than being purely mucopurulent. **NEET-PG High-Yield Pearls:** * **Infective Dose:** *Shigella* has a very low ID₅₀ (only 10–100 organisms), making it highly communicable. * **Gold Standard Diagnosis:** Stool culture on selective media like **DCA (Deoxycholate Citrate Agar)** or **XLD (Xylose Lysine Deoxycholate)** agar. * **Microscopy:** Characterized by numerous pus cells (neutrophils) and RBCs. * **Drug of Choice:** Ceftriaxone or Azithromycin (due to increasing Ciprofloxacin resistance).

Question 610: Which of the following organisms requires tryptophan for growth?

A. H. influenza
B. Vibrio
C. Gonococci
D. Salmonella typhi (Correct Answer)

Explanation: **Explanation:** The correct answer is **Salmonella typhi**. **1. Why Salmonella typhi is correct:** *Salmonella typhi* is a nutritionally fastidious organism compared to other members of the Enterobacteriaceae family. While most *Salmonella* species can grow on simple media, *S. typhi* specifically lacks the metabolic pathway to synthesize the essential amino acid **tryptophan**. Therefore, it requires an exogenous supply of tryptophan for growth. This biochemical characteristic is historically significant and is used in specific laboratory media formulations to differentiate it from other non-typhoidal Salmonellae. **2. Why the other options are incorrect:** * **H. influenzae:** This organism is highly fastidious but requires **Factor X (Hemin)** and **Factor V (NAD)** for growth, typically provided by Chocolate Agar. It does not have a specific diagnostic requirement for tryptophan. * **Vibrio:** *Vibrio cholerae* is non-fastidious and can grow on simple media. It is halophilic (salt-loving) and prefers alkaline conditions (pH 8.2–9.0), but it synthesizes its own amino acids. * **Gonococci (*N. gonorrhoeae*):** These are extremely fastidious and require enriched media (like Thayer-Martin or Chocolate agar) containing various amino acids, vitamins, and minerals, but tryptophan is not their defining growth requirement. **3. Clinical Pearls for NEET-PG:** * **Indole Test Connection:** The Indole test detects the ability of an organism to break down tryptophan into indole. Since *S. typhi* uses tryptophan for growth rather than just metabolizing it via tryptophanase, it is **Indole negative**. * **Culture Media:** For *S. typhi*, the gold standard for diagnosis in the first week of enteric fever is **Blood Culture** (using Castaneda’s biphasic medium). * **Selective Media:** Wilson and Blair’s Bismuth Sulfite Agar is highly specific for *S. typhi*, producing characteristic jet-black colonies with a metallic sheen due to $H_2S$ production.

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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