Bacteriology Practice Questions

Q: Which bacterium is primarily implicated in dental caries?

Streptococcus mutans. **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*.

Q: The Group A carbohydrate antigen of Streptococcus pyogenes shares antigenic similarity with which human tissue component?

Cardiac valves. **Explanation:** The correct answer is **Cardiac valves**. This phenomenon is a classic example of **molecular mimicry**, where exogenous antigens share structural similarities with host self-antigens, leading to an autoimmune response. 1. **Why Cardiac Valves?** The cell wall of *Streptococcus pyogenes* (Group A Strep) contains a specific **Group A carbohydrate antigen** (N-acetylglucosamine). This antigen shares structural homology with **glycoproteins** found in human **cardiac valves**. Following a pharyngeal infection, the immune system produces antibodies against the bacterial carbohydrate; these antibodies cross-react with the valvular tissue, leading to the inflammatory damage seen in **Acute Rheumatic Fever (ARF)**. 2. **Analysis of Incorrect Options:** * **Myocardium:** While the myocardium is involved in Rheumatic Fever, it is the **M protein** of *S. pyogenes* that mimics myocardial **sarcolemma and myosin**, not the Group A carbohydrate. * **Synovial fluid:** Joint involvement (migratory polyarthritis) in ARF is due to immune complex deposition rather than direct molecular mimicry with the Group A carbohydrate. * **Vascular intima:** Though vasculitis can occur in various streptococcal sequelae, it is not the primary target of the Group A carbohydrate cross-reactivity. **High-Yield Clinical Pearls for NEET-PG:** * **M Protein:** The most important virulence factor of *S. pyogenes*; it mimics **myocardial sarcolemma**. * **Hyaluronic Acid Capsule:** Mimics human **synovial fluid/connective tissue** (making the bacteria non-immunogenic). * **Cytoplasmic Membrane:** Shares antigens with the **subthalamic and caudate nuclei**, leading to Sydenham’s Chorea. * **Jones Criteria:** Used for the diagnosis of ARF (Major: Joint, Carditis, Nodules, Erythema marginatum, Sydenham chorea).

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

Streptococcus pyogenes. **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.

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

M. fortuitum. **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.

Q: Which of the following does NOT cause meningitis?

Streptococcus type A. 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.

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

Fusobacterium. **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.

Q: Which organism commonly causes early-onset neonatal septicemia?

Streptococcus agalactiae. **Explanation:** **Early-Onset Neonatal Sepsis (EONS)** occurs within the first 72 hours of life and is typically caused by the vertical transmission of pathogens from the maternal genital tract during labor or delivery. **Correct Option (A): Streptococcus agalactiae** Also known as **Group B Streptococcus (GBS)**, this is the most common cause of EONS worldwide. It colonizes the maternal vagina and rectum; during birth, the neonate can aspirate infected amniotic fluid or vaginal secretions, leading to pneumonia, septicemia, or meningitis. In developing countries like India, while GBS remains significant, Gram-negative organisms like *Klebsiella* and *E. coli* are also frequently isolated. **Incorrect Options:** * **B. Streptococcus bovis:** Now reclassified as *S. gallolyticus*, it is a Group D Strep associated with endocarditis and colorectal cancer in adults, not neonatal sepsis. * **C. Streptococcus pyogenes:** Known as Group A Strep (GAS), it causes pharyngitis, skin infections, and rheumatic fever. While it can cause puerperal sepsis in mothers, it is a rare cause of neonatal sepsis. * **D. Streptococcus viridans:** These are normal commensals of the oropharynx. They are low-virulence organisms usually associated with subacute bacterial endocarditis or dental caries. **High-Yield Clinical Pearls for NEET-PG:** * **Definition of EONS:** Onset 18 hours), and maternal fever. * **Drug of Choice:** Empirical treatment is a combination of **Ampicillin and Gentamicin**.

Q: Where does Neisseria gonorrhoeae initially infect the female reproductive tract?

Cervix. **Explanation:** The correct answer is **B. Cervix**. *Neisseria gonorrhoeae* is a Gram-negative diplococcus that specifically targets **columnar and cuboidal epithelium**. In the female reproductive tract, the **endocervix** is the primary site of initial infection because it is lined by simple columnar epithelium, which provides the necessary receptors (pili and Opa proteins) for bacterial attachment and invasion. **Why other options are incorrect:** * **A. Vagina:** In adult females, the vagina is lined by **stratified squamous epithelium**. This thick, multi-layered surface is resistant to gonococcal invasion. (Note: In pre-pubertal girls, the vaginal mucosa is thin and can be infected, leading to vulvovaginitis). * **C. Uterus & D. Fallopian tubes:** While the infection can ascend to the endometrium and fallopian tubes causing Pelvic Inflammatory Disease (PID), these are **secondary sites** of infection. The bacteria must first establish a colony in the cervix before ascending. **High-Yield Clinical Pearls for NEET-PG:** * **Most common site of asymptomatic infection:** The cervix (up to 50-70% of women are asymptomatic). * **Gold Standard Diagnosis:** Nucleic Acid Amplification Test (NAAT). Culture on **Thayer-Martin medium** is used for sensitivity testing. * **Fitz-Hugh-Curtis Syndrome:** A complication where the infection spreads to the liver capsule, causing "string-bread" adhesions. * **Treatment:** Due to increasing resistance, the current recommendation is often a combination therapy (e.g., Ceftriaxone IM + Azithromycin/Doxycycline) to cover co-infection with *Chlamydia trachomatis*.

Question 1

Which bacterium is primarily implicated in dental caries?

Accepted Answer

Streptococcus mutans

Answer

Pneumococci

Answer

Streptococcus pyogenes

Answer

Staphylococcus aureus

Question 2

Which of the following microorganisms is least likely to cause infection in patients with sickle cell anemia?

Accepted Answer

Mycobacterium tuberculosis

Answer

Streptococcus pneumoniae

Answer

Haemophilus influenzae type b

Answer

All of the above

Question 3

The Group A carbohydrate antigen of Streptococcus pyogenes shares antigenic similarity with which human tissue component?

Accepted Answer

Cardiac valves

Answer

Synovial fluid

Answer

Myocardium

Answer

Vascular intima

Question 4

Which Streptococcus species is known to cause rheumatic heart disease?

Accepted Answer

Streptococcus pyogenes

Answer

Streptococcus milleri

Answer

Streptococcus mutans

Answer

Streptococcus equisimilis

Question 5

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

Accepted Answer

M. fortuitum

Answer

M. scrofulaceus

Answer

M. hordinae

Answer

M. avium

Question 6

Which of the following does NOT cause meningitis?

Accepted Answer

Streptococcus type A

Answer

Listeria

Answer

Pneumococcus

Answer

Beta-Streptococci

Question 7

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

Accepted Answer

Fusobacterium

Answer

Lactobacillus

Answer

Lactobacillus

Answer

Bacteroides

Question 8

Which of the following most accurately describes the therapy available for the prevention and treatment of C. diphtheriae?

Accepted Answer

Antimicrobial therapy and prophylaxis, antitoxin, and toxoid (DPT)

Answer

Antimicrobial therapy for prophylaxis only

Answer

Diphtheria toxoid (DPT) booster vaccination only

Answer

Antitoxin only

Question 9

Which organism commonly causes early-onset neonatal septicemia?

Accepted Answer

Streptococcus agalactiae

Answer

Streptococcus bovis

Answer

Streptococcus pyogenes

Answer

Streptococcus viridans

Question 10

Where does Neisseria gonorrhoeae initially infect the female reproductive tract?

Accepted Answer

Cervix

Answer

Vagina

Answer

Uterus

Answer

Fallopian tubes

Bacteriology — MCQs

Bacteriology — MCQs

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