Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1471: Which bacterium is primarily implicated in dental caries?

A. Streptococcus mutans (Correct Answer)
B. Pneumococci
C. Streptococcus pyogenes
D. Staphylococcus aureus

Explanation: **Explanation:** **Streptococcus mutans** is the primary etiologic agent of dental caries due to its unique metabolic properties. It is an **acidogenic** (acid-producing) and **aciduric** (acid-tolerant) bacterium. It utilizes the enzyme **glucosyltransferase** to convert dietary sucrose into insoluble glucose polymers called **glucans (extracellular polysaccharides)**. These glucans act as a biological "glue," allowing the bacteria to adhere to the tooth enamel and form a biofilm known as **dental plaque**. Within this plaque, the fermentation of sugars produces lactic acid, which lowers the local pH below 5.5, leading to the demineralization of the tooth enamel. **Analysis of Incorrect Options:** * **B. Pneumococci (S. pneumoniae):** These are alpha-hemolytic, bile-soluble, encapsulated cocci primarily responsible for community-acquired pneumonia, meningitis, and otitis media, not oral pathologies. * **C. Streptococcus pyogenes (Group A Strep):** A beta-hemolytic bacterium known for causing pharyngitis, impetigo, and non-suppurative complications like Rheumatic Fever. It does not possess the mechanisms to colonize tooth enamel. * **D. Staphylococcus aureus:** A catalase and coagulase-positive organism typically associated with skin/soft tissue infections, abscesses, and osteomyelitis. **High-Yield Clinical Pearls for NEET-PG:** * **Sucrose** is the most cariogenic sugar because it is the only substrate *S. mutans* can use to synthesize glucans. * **Streptococcus sanguinis** is another viridans group organism often found in the mouth, but it is more commonly associated with **Subacute Bacterial Endocarditis (SBE)** following dental procedures. * **Lactobacilli** species are often involved in the *progression* of deep dentinal caries, whereas *S. mutans* is responsible for the *initiation*.

Question 1472: All of the following laboratory tests can be used to distinguish Streptococcus mutans from other oral streptococci, except?

A. Gram staining (Correct Answer)
B. Fermentation of mannitol and sorbitol
C. Production of intracellular and extracellular adherent polysaccharide
D. Colony morphology on saliva agar

Explanation: **Explanation:** The correct answer is **A. Gram staining**. **1. Why Gram staining is the correct answer:** Gram staining is a non-specific primary identification tool. All members of the genus *Streptococcus*, including *S. mutans*, *S. sanguinis*, and *S. salivarius*, are **Gram-positive cocci** arranged in chains. Therefore, Gram staining cannot be used to differentiate between species within the same genus, as they all share the same staining characteristics and morphology. **2. Analysis of incorrect options (Methods that DO distinguish S. mutans):** * **Fermentation of mannitol and sorbitol:** *S. mutans* is unique among most oral streptococci because it can ferment both mannitol and sorbitol, producing acid. This biochemical property is a key diagnostic feature used in selective media like Mitis Salivarius-Bacitracin (MSB) agar. * **Production of polysaccharides:** *S. mutans* produces extracellular **glucans (dextrans)** from sucrose using the enzyme glucosyltransferase. These sticky polysaccharides allow it to adhere to tooth enamel and form dental plaque, a feature not shared by all oral streptococci. * **Colony morphology on saliva agar:** On specialized media like Mitis Salivarius agar, *S. mutans* exhibits distinct morphology (typically small, blue, "frosted glass" or granular colonies), which helps differentiate it from the large, mucoid colonies of *S. salivarius*. **Clinical Pearls for NEET-PG:** * **Dental Caries:** *S. mutans* is the primary etiological agent of dental caries due to its acidogenic (acid-producing) and aciduric (acid-tolerant) nature. * **Viridans Group:** *S. mutans* belongs to the Viridans group of Streptococci (VGS), which are typically $\alpha$-hemolytic. * **Subacute Bacterial Endocarditis (SABE):** While *S. sanguinis* is more commonly associated with SABE following dental procedures, *S. mutans* is also a recognized cause.

Question 1473: Staphylococcus aureus causes vomiting in 2-6 hours. What is the mechanism of action?

A. Stimulation of cyclic AMP
B. Vagal stimulation (Correct Answer)
C. Stimulation of cyclic GMP
D. Acts through ganglioside GM receptor

Explanation: **Explanation:** *Staphylococcus aureus* food poisoning is caused by the ingestion of preformed **Enterotoxins** (most commonly Enterotoxin A) in contaminated food. These toxins are heat-stable and resistant to gut enzymes. **Mechanism of Action (Correct Option):** The enterotoxin acts as a **superantigen**, stimulating T-cells and leading to the release of inflammatory mediators. These toxins act locally in the gut to stimulate the **vagus nerve** and the sympathetic nervous system. This sends signals to the vomiting center in the medulla oblongata, resulting in the characteristic rapid onset (2–6 hours) of projectile vomiting and abdominal cramps. **Analysis of Incorrect Options:** * **Options A & C (cAMP and cGMP):** These mechanisms are associated with secretory diarrheas. *Vibrio cholerae* and Heat-Labile (LT) *E. coli* toxins increase **cAMP**, while Heat-Stable (ST) *E. coli* toxin increases **cGMP**. These lead to massive fluid loss rather than rapid-onset emesis. * **Option D (GM1 Ganglioside Receptor):** This is the specific binding site for the **Cholera toxin** (B subunit). *S. aureus* enterotoxins do not utilize this receptor. **NEET-PG High-Yield Pearls:** * **Incubation Period:** Shortest incubation (1–6 hours) among food-borne illnesses. * **Source:** Often associated with protein-rich foods (custards, milk products, processed meats) handled by carriers with skin lesions or nasal colonization. * **Key Feature:** Fever is usually absent. * **Superantigen:** Enterotoxins are superantigens, but unlike TSST-1, they are specifically associated with emesis.

Question 1474: All of the following statements about non-typhoidal Salmonella are true, except?

A. Humans are the only reservoirs. (Correct Answer)
B. Transmission is most commonly associated with eggs, poultry, and undercooked meat.
C. Common in immunocompromised individuals.
D. Resistance to fluoroquinolones has emerged.

Explanation: ### Explanation **1. Why Option A is the correct answer (The "Except" statement):** Unlike *Salmonella Typhi* and *Paratyphi* (the agents of enteric fever), which are strictly human pathogens, **Non-Typhoidal Salmonella (NTS)** are **zoonotic**. They have a wide host range including poultry, cattle, rodents, reptiles (turtles/iguanas), and pets. Therefore, humans are **not** the only reservoirs; animals serve as the primary source of infection. **2. Analysis of other options:** * **Option B:** Transmission is primarily fecal-oral, often through the consumption of contaminated animal products. **Poultry and eggs** are the most common vehicles because the bacteria can infect the ovaries of hens, contaminating the egg before the shell is formed. * **Option C:** NTS typically causes self-limiting gastroenteritis in healthy adults. However, it is a major cause of invasive disease (bacteremia) in **immunocompromised individuals**, particularly those with HIV (where it is an AIDS-defining illness), sickle cell anemia, or chronic granulomatous disease. * **Option D:** While third-generation cephalosporins and fluoroquinolones are used for invasive NTS, there is a rising global trend of **multidrug resistance (MDR)**, specifically against ciprofloxacin, necessitating the use of carbapenems in severe cases. ### NEET-PG High-Yield Pearls: * **Most common serotypes:** *Salmonella Typhimurium* and *Salmonella Enteritidis*. * **Sickle Cell Association:** NTS is the most common cause of **osteomyelitis** in patients with Sickle Cell Disease (due to intestinal infarcts allowing bacterial seeding). * **Clinical Presentation:** Primarily causes gastroenteritis (incubation period 6–72 hours); blood cultures are usually negative, but stool cultures are positive. * **Treatment:** Antibiotics are generally **not recommended** for uncomplicated NTS diarrhea as they may prolong fecal shedding; they are reserved for invasive disease or high-risk patients.

Question 1475: Which Streptococcus species is known to cause rheumatic heart disease?

A. Streptococcus milleri
B. Streptococcus mutans
C. Streptococcus pyogenes (Correct Answer)
D. Streptococcus equisimilis

Explanation: **Explanation:** **Streptococcus pyogenes (Group A Streptococcus - GAS)** is the correct answer because it is the only species listed that triggers the autoimmune sequelae known as **Acute Rheumatic Fever (ARF)** and subsequent **Rheumatic Heart Disease (RHD)**. The underlying mechanism is **Molecular Mimicry** (Type II Hypersensitivity): antibodies produced against the **M protein** of *S. pyogenes* cross-react with host tissues, specifically cardiac myosin, laminin, and valvular endothelium. This typically occurs 2–4 weeks after an untreated streptococcal pharyngitis (never after skin infections like impetigo). **Analysis of Incorrect Options:** * **Streptococcus milleri:** Part of the *S. anginosus* group; these are known for causing pyogenic abscesses (brain, liver, lung) rather than autoimmune complications. * **Streptococcus mutans:** A member of the Viridans group, it is the primary agent of dental caries and a common cause of Subacute Bacterial Endocarditis (SBE), but it does not trigger the rheumatic process. * **Streptococcus equisimilis:** This belongs to Group C or G Streptococcus. While it can cause pharyngitis, it is traditionally not associated with the development of ARF. **High-Yield Clinical Pearls for NEET-PG:** * **Jones Criteria:** Used for the diagnosis of ARF (Major: Joint/Polyarthritis, Carditis, Nodules, Erythema marginatum, Sydenham’s chorea). * **ASO Titer:** An elevated Anti-Streptolysin O titer is evidence of a preceding GAS infection. * **Aschoff Bodies:** Pathognomonic histological feature of RHD (granulomatous lesions with Anitschkow "caterpillar" cells). * **Prophylaxis:** Long-term Benzathine Penicillin is the gold standard to prevent recurrent ARF and worsening RHD.

Question 1476: Babes-Ernst bodies are characteristically seen in?

A. Diphtheria bacilli (Correct Answer)
B. Anthrax bacilli
C. Borrelia
D. Gonococci

Explanation: **Explanation:** **Babes-Ernst bodies** (also known as metachromatic granules or volutin granules) are characteristic intracellular inclusions found in **Corynebacterium diphtheriae** (Diphtheria bacilli). ### Why Diphtheria bacilli is correct: These granules represent stored reserves of **polymetaphosphate**. They appear as dark-staining spots at the poles of the bacilli, giving them a "beaded" appearance. They are called "metachromatic" because they stain a different color (reddish-purple) than the dye used (blue, such as Loeffler’s Methylene Blue or Albert’s stain). This is a vital diagnostic feature for identifying *C. diphtheriae*. ### Why other options are incorrect: * **Anthrax bacilli:** *Bacillus anthracis* is characterized by its large size, square ends (bamboo-stick appearance), and a polypeptide capsule. It does not possess metachromatic granules. * **Borrelia:** These are spirochetes identified via dark-ground microscopy or Giemsa/Wright stains. They do not show Babes-Ernst bodies. * **Gonococci:** *Neisseria gonorrhoeae* are Gram-negative diplococci (kidney-bean shaped) found typically within polymorphonuclear leukocytes. ### NEET-PG High-Yield Pearls: * **Stains for Babes-Ernst bodies:** Albert’s stain (most common), Neisser’s stain, and Ponder’s stain. * **Arrangement:** *C. diphtheriae* typically shows a "Chinese letter" or cuneiform arrangement due to incomplete separation during binary fission (snapping division). * **Culture Media:** Loeffler’s Serum Slope (rapid growth) and Potassium Tellurite Agar (selective media where colonies appear black). * **Other organisms with metachromatic granules:** *Gardnerella vaginalis* and *Agrobacterium tumefaciens*.

Question 1477: Cholera toxin has which of the following effects?

A. Increases the levels of intracellular cyclic GMP
B. Acts through the receptor for opiates
C. Causes continued activation of adenylate cyclase (Correct Answer)
D. Inhibits the enzyme phosphodiesterase

Explanation: **Explanation:** The pathogenesis of *Vibrio cholerae* is primarily mediated by the **Cholera Toxin (Choleragen)**, which is a classic A-B subunit exotoxin. **Mechanism of Action (Why C is correct):** The 'B' subunit binds to the **GM1 ganglioside receptor** on the intestinal epithelium, allowing the 'A' subunit to enter the cell. The A1 fragment catalyzes the **ADP-ribosylation of the Gs (stimulatory) protein**. This locks the Gs protein in its "active" state, preventing its GTPase activity. This leads to the **continued activation of Adenylate Cyclase**, resulting in a massive increase in intracellular **cyclic AMP (cAMP)**. High cAMP levels trigger the hypersecretion of water and electrolytes (sodium, potassium, and chloride) into the intestinal lumen, causing "rice-water" diarrhea. **Analysis of Incorrect Options:** * **Option A:** **Cyclic GMP** is the second messenger for the **Heat-Stable (ST) toxin** of *Enterotoxigenic E. coli (ETEC)* and *Guanylin*, not Cholera toxin. * **Option B:** Opiate receptors are targets for drugs like Loperamide (used to treat diarrhea), but they are not involved in the mechanism of Cholera toxin. * **Option D:** **Phosphodiesterase** is the enzyme that breaks down cAMP. While inhibiting it would increase cAMP levels (e.g., Caffeine, Theophylline), Cholera toxin works upstream by stimulating production rather than inhibiting breakdown. **High-Yield Clinical Pearls for NEET-PG:** * **Toxin Type:** ADP-ribosylating A-B toxin. * **Stool Characteristics:** "Rice-water" appearance (non-inflammatory, no blood/pus), fishy odor. * **Culture Media:** TCBS (Thiosulfate Citrate Bile salts Sucrose) agar – produces **yellow colonies** (sucrose fermentation). * **Transport Media:** Venkatraman-Ramakrishnan (VR) medium or Cary-Blair medium. * **Treatment Priority:** Aggressive fluid and electrolyte replacement (ORS/IV fluids). Doxycycline is the drug of choice to reduce shedding.

Question 1478: Outbreak of abscess following vaccine injection can be caused by which mycobacteria?

A. M. fortuitum (Correct Answer)
B. M. scrofulaceus
C. M. hordinae
D. M. avium

Explanation: **Explanation:** The correct answer is **M. fortuitum**. This organism belongs to the group of **Rapidly Growing Mycobacteria (RGM)**, which are classified under Runyon Group IV. **Why M. fortuitum is correct:** * **Mechanism:** *M. fortuitum* and *M. chelonae* are environmental saprophytes commonly found in soil and water. They are notorious for causing **post-injection abscesses** and surgical site infections (especially following cosmetic surgery, liposuction, or vaccinations) due to the use of contaminated needles, syringes, or skin antiseptics. * **Characteristics:** They grow rapidly on routine laboratory media (like Lowenstein-Jensen or blood agar) within **3 to 7 days**, unlike slow-growing mycobacteria which take weeks. **Why other options are incorrect:** * **M. scrofulaceus (Group II):** A scotochromatogen primarily associated with **cervical lymphadenitis** (scrofula) in children. * **M. avium (Group III):** Part of the *M. avium-intracellulare* complex (MAC), it typically causes disseminated disease in **HIV/AIDS patients** or chronic pulmonary disease, not localized post-vaccination abscesses. * **M. hordinae:** This is a non-pathogenic environmental mycobacterium and is not associated with human clinical outbreaks. **High-Yield Clinical Pearls for NEET-PG:** * **Runyon Classification:** Remember that Group IV (Rapid Growers) includes *M. fortuitum, M. chelonae,* and *M. abscessus*. * **Clinical Association:** If a question mentions "abscesses following tattooing," "pedicure-associated furunculosis," or "post-surgical wound infection" failing standard antibiotic therapy, always suspect Rapidly Growing Mycobacteria. * **Diagnosis:** They are unique because they can grow on **MacConkey agar** (without crystal violet), appearing as small, pin-point colonies.

Question 1479: Which of the following does NOT cause meningitis?

A. Listeria
B. Pneumococcus
C. Beta-Streptococci
D. Streptococcus type A (Correct Answer)

Explanation: The correct answer is **Streptococcus type A** (Group A Streptococcus or *S. pyogenes*). ### **Explanation** **Streptococcus type A (*S. pyogenes*)** is primarily a pathogen of the upper respiratory tract and skin. It is the leading cause of bacterial pharyngitis, impetigo, cellulitis, and non-suppurative sequelae like Rheumatic Fever and Glomerulonephritis. While it can cause invasive diseases like necrotizing fasciitis or toxic shock syndrome, it is **not** a recognized or common cause of meningitis. ### **Analysis of Other Options** * **Listeria monocytogenes:** A significant cause of meningitis in specific populations, including neonates, the elderly, and immunocompromised individuals. It is unique as a gram-positive motile rod. * **Pneumococcus (*S. pneumoniae*):** Currently the **most common cause** of bacterial meningitis in adults and children (following the decline of *H. influenzae* due to vaccination). It is characterized by lancet-shaped diplococci. * **Beta-Streptococci (specifically Group B):** *Streptococcus agalactiae* (Group B Streptococcus) is the **leading cause of neonatal meningitis**, typically transmitted during childbirth from the maternal vaginal flora. ### **NEET-PG High-Yield Pearls** * **Most common cause of meningitis overall:** *Streptococcus pneumoniae*. * **Most common cause in neonates (0–28 days):** Group B Streptococcus (GBS), followed by *E. coli* and *Listeria*. * **Most common cause in adolescents/young adults:** *Neisseria meningitidis* (often associated with outbreaks in dormitories). * **Listeria hint:** If the question mentions "tumbling motility" at 25°C or meningitis in a renal transplant patient, think *Listeria*. * **CSF Findings in Bacterial Meningitis:** Increased neutrophils, markedly low glucose, and high protein.

Question 1480: Vincent's angina is caused by Borrelia vincentii along with which other microorganism?

A. Lactobacillus
B. Lactobacillus
C. Fusobacterium (Correct Answer)
D. Bacteroides

Explanation: **Explanation:** **Vincent’s Angina** (also known as Trench Mouth or Acute Necrotizing Ulcerative Gingivitis - ANUG) is a classic example of a **synergistic infection**. It is characterized by painful, bleeding gums and the formation of a "pseudomembrane" on the tonsils and pharynx. The condition is caused by the symbiotic proliferation of two specific organisms: 1. **Borrelia vincentii:** A large, Gram-negative motile spirochete. 2. **Fusobacterium fusiforme:** A Gram-negative, anaerobic, spindle-shaped (fusiform) bacillus. This combination is often referred to as the **"fusospirochetal complex."** These organisms are part of the normal oral flora but become pathogenic under conditions of poor oral hygiene, malnutrition, or extreme physical stress. **Analysis of Options:** * **C. Fusobacterium (Correct):** As stated above, it works synergistically with *Borrelia vincentii* to cause tissue necrosis and ulceration. * **A & B. Lactobacillus:** These are Gram-positive rods that are part of the normal flora of the mouth, gut, and vagina. They are primarily associated with dental caries and maintaining vaginal pH, not necrotizing angina. * **D. Bacteroides:** While *Bacteroides* are common oral anaerobes involved in periodontal disease and abscesses, they are not the specific diagnostic partner in the classic description of Vincent’s Angina. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** Usually made via a Gram stain of the ulcerated lesion showing the characteristic "fusospirochetal" morphology. * **Clinical Presentation:** Punched-out interdental papillae, halitosis (fetid breath), and a greyish pseudomembrane. * **Treatment:** Penicillin is the drug of choice, along with metronidazole and surgical debridement. * **Related Condition:** If the infection spreads to the cheeks and face, it is called **Cancrum Oris (Noma)**, typically seen in severely malnourished children.

Practice by Chapter

Staphylococci

Practice Questions

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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