Which of the following statements regarding resistance of penicillin in Staphylococcus aureus is false?
Which of the following is called Preisz-Nocard bacillus?
Which anticoagulant is used when blood is sent for blood culture?
Patient came from Nagaland and shows positive test with OXK antigen. Diagnosis is?
Which of the following stimulate adenylate cyclase with G-protein coupled action ?
Which MRSA strain type is most commonly associated with hospital-acquired infections?
Which is the most common bacterial organism causing bacterial upper respiratory tract infections (including sinusitis, otitis media, and pharyngitis) in adults?
Which organism is responsible for producing Draughtsman (Concentric Rings) on culture?
What component is included in the vaccine against N-meningitidis?
Which of the following bacteria is not capsulated?
NEET-PG 2013 - Microbiology NEET-PG Practice Questions and MCQs
Question 21: Which of the following statements regarding resistance of penicillin in Staphylococcus aureus is false?
- A. Methicillin resistance is due to alterations in penicillin-binding proteins (PBPs).
- B. Penicillinase production is mediated by plasmids.
- C. Hospital strains predominantly produce a unique type of penicillinase. (Correct Answer)
- D. Penicillinase production can be transmitted by transduction.
Explanation: ***Hospital strains predominantly produce a unique type of penicillinase*** - This statement is **false** because hospital strains do not produce a truly "unique type" of **penicillinase** compared to community strains. - **Penicillinase (beta-lactamase)** is a common resistance mechanism found across various *S. aureus* strains, not exclusive to hospital environments. *Methicillin resistance is due to alterations in penicillin-binding proteins (PBPs)* - This statement is **true** as **MRSA** resistance involves the **mecA gene** encoding **PBP2a**. - **PBP2a** has low affinity for **beta-lactam antibiotics**, allowing cell wall synthesis despite antibiotic presence. *Penicillinase production is mediated by plasmids* - This statement is **true** because **penicillinase genes** are typically located on **plasmids**. - **Plasmids** facilitate horizontal transfer of resistance genes between bacterial populations. *Penicillinase production can be transmitted by transduction* - This statement is **true** as **transduction** via **bacteriophages** can transfer resistance genes. - **Plasmid-borne penicillinase genes** can spread through this horizontal gene transfer mechanism.
Question 22: Which of the following is called Preisz-Nocard bacillus?
- A. C. diphtheriae
- B. C. pseudotuberculosis (Correct Answer)
- C. M. tuberculosis
- D. Mycoplasma
Explanation: ***C. pseudotuberculosis*** - This bacterium is historically known as the **Preisz-Nocard bacillus**, named after its discoverers. - It is an important pathogen in animals, causing conditions like **caseous lymphadenitis** in sheep and goats. *C. diphtheriae* - This bacterium causes **diphtheria** and is known for producing a potent **exotoxin** that can lead to systemic complications. - It forms a characteristic **pseudomembrane** in the throat and is not associated with the Preisz-Nocard designation. *M. tuberculosis* - This is the causative agent of **tuberculosis** in humans, primarily affecting the lungs. - It is known for its **acid-fast staining** property and a complex pathogenesis involving granuloma formation. *Mycoplasma* - This genus includes bacteria that lack a **cell wall**, making them resistant to many common antibiotics. - They are known for causing various infections, including **atypical pneumonia** and genitourinary tract infections, but are not referred to as the Preisz-Nocard bacillus.
Question 23: Which anticoagulant is used when blood is sent for blood culture?
- A. Sodium citrate
- B. EDTA
- C. Oxalate
- D. SPS (Correct Answer)
Explanation: ***SPS*** - **SPS (Sodium Polyanethol Sulfonate)** is the preferred anticoagulant for blood cultures because it inhibits phagocytosis and complement activation, allowing microorganisms to survive and grow. - It also neutralizes the bactericidal effect of aminoglycoside antibiotics, which might be present in the patient's blood. *Sodium citrate* - **Sodium citrate** works by binding to calcium ions, preventing blood coagulation, and is typically used for coagulation studies. - It is not suitable for blood cultures as it does not have the anti-phagocytic or antibiotic-neutralizing properties of SPS. *EDTA* - **EDTA (Ethylenediaminetetraacetic acid)** is a strong chelator of calcium and is primarily used for hematology tests like complete blood counts (CBC) as it preserves cell morphology. - It can be toxic to some bacteria and would inhibit microbial growth, making it unsuitable for blood cultures. *Oxalate* - Various **oxalates** (e.g., potassium oxalate) also act as anticoagulants by precipitating calcium, and are commonly found in tubes for glucose testing when combined with a glycolytic inhibitor. - Oxalates are generally unsuitable for blood cultures because they can inhibit the growth of certain microorganisms.
Question 24: Patient came from Nagaland and shows positive test with OXK antigen. Diagnosis is?
- A. Trench fever
- B. Scrub typhus (Correct Answer)
- C. Endemic typhus
- D. Epidemic typhus
Explanation: ***Scrub typhus*** - A **positive Weil-Felix test** for the **OXK antigen** is highly suggestive of scrub typhus, particularly if the patient is from an endemic region like Nagaland. - Scrub typhus is caused by **_Orientia tsutsugamushi_**, transmitted by **chiggers** (larval mites), often presenting with fever and an **eschar**. *Trench fever* - Caused by **_Bartonella quintana_** and transmitted by the **human body louse**. - It does not typically show a positive Weil-Felix test for the OXK antigen. *Endemic typhus* - Also known as **murine typhus**, it is caused by **_Rickettsia typhi_** and transmitted by **fleas** (e.g., rat flea). - While it causes a positive Weil-Felix test, it usually involves the **OX19 antigen**, not OXK. *Epidemic typhus* - Caused by **_Rickettsia prowazekii_** and transmitted by the **human body louse**. - It yields a positive Weil-Felix test for the **OX19 and OX2 antigens**, but typically not for OXK.
Question 25: Which of the following stimulate adenylate cyclase with G-protein coupled action ?
- A. Shiga toxin
- B. Cholera toxin (Correct Answer)
- C. Diphtheria toxin
- D. Pseudomonas toxin
Explanation: ***Cholera toxin*** - Cholera toxin is a **G-protein-activating toxin** that irreversibly activates **adenylate cyclase**. - This leads to increased intracellular levels of **cAMP**, causing excessive fluid secretion into the intestinal lumen and severe diarrhea. *Shiga toxin* - Shiga toxin acts by inactivating the **60S ribosomal subunit**, thereby inhibiting protein synthesis in eukaryotic cells. - Its primary effect is **cytotoxicity**, not direct stimulation of adenylate cyclase. *Diphtheria toxin* - Diphtheria toxin inhibits **protein synthesis** by inactivating **elongation factor-2 (EF-2)** through ADP-ribosylation. - This toxin specifically targets host cells, leading to cellular death and tissue damage. *Pseudomonas toxin* - **Exotoxin A** produced by *Pseudomonas aeruginosa* also inhibits **protein synthesis** by ADP-ribosylating and inactivating **EF-2**, similar to diphtheria toxin. - It does not directly affect adenylate cyclase activity.
Question 26: Which MRSA strain type is most commonly associated with hospital-acquired infections?
- A. Phage type 83A
- B. Phage type 85
- C. Phage type 84
- D. Phage type 80/81 (Correct Answer)
Explanation: ***Phage type 80/81*** - This **phage type**, particularly **epidemic methicillin-resistant *Staphylococcus aureus* (EMRSA) 15 and 16**, is historically and currently the most common cause of **hospital-acquired MRSA infections** worldwide. - It rapidly diversified and spread globally, becoming a significant nosocomial pathogen. *Phage type 83A* - While this phage type has been associated with **MRSA outbreaks**, it is not the most common strain type globally for hospital-acquired infections. - Its prevalence is more regional and sporadic compared to the widespread dominance of EMRSA 15/16. *Phage type 84* - **Phage type 84** is not widely recognized as a predominant strain associated with the majority of **hospital-acquired MRSA infections**. - Other, more virulent and successful clonal types have outcompeted it to become the most prevalent. *Phage type 85* - Similar to other less common types, **phage type 85** does not represent the major lineage responsible for the bulk of **hospital-acquired MRSA cases**. - The landscape of MRSA epidemiology is dominated by a few highly successful clonal complexes, of which 80/81 is a prime example.
Question 27: Which is the most common bacterial organism causing bacterial upper respiratory tract infections (including sinusitis, otitis media, and pharyngitis) in adults?
- A. Staphylococcus aureus
- B. Haemophilus influenzae
- C. Streptococcus pyogenes
- D. Streptococcus pneumoniae (Correct Answer)
Explanation: ***Streptococcus pneumoniae*** - *Streptococcus pneumoniae* is the **most common bacterial pathogen** causing upper respiratory tract infections overall, including **bacterial sinusitis**, **otitis media**, and **community-acquired pneumonia**. - It is a frequent colonizer of the nasopharynx and leads to infection when host immunity is compromised. - Accounts for the highest burden of bacterial URTIs when considering all anatomical sites. *Haemophilus influenzae* - *Haemophilus influenzae* (particularly non-typeable strains) is the **second most common** cause of bacterial sinusitis and otitis media in adults. - While significant, it is less prevalent overall than *S. pneumoniae* across all URTI types. *Staphylococcus aureus* - *Staphylococcus aureus* primarily causes **skin and soft tissue infections** and device-related infections. - It is **not a common primary pathogen** in typical acute bacterial URTIs, though it may cause secondary infections or colonize the anterior nares. *Streptococcus pyogenes* - *Streptococcus pyogenes* (Group A Streptococcus) is the **most common cause of bacterial pharyngitis** (strep throat) in adults. - However, when considering the **full spectrum of bacterial URTIs** (pharyngitis, sinusitis, otitis media), *S. pneumoniae* has a broader overall impact and higher prevalence across multiple sites.
Question 28: Which organism is responsible for producing Draughtsman (Concentric Rings) on culture?
- A. Bacillus anthracis (Correct Answer)
- B. B. pertussis
- C. H. ducreyi
- D. Pneumococci
Explanation: ***Bacillus anthracis*** - *Bacillus anthracis* is the organism responsible for producing the characteristic **"Draughtsman" or "Concentric Rings"** pattern on culture media - On blood agar, colonies show a **"Medusa head"** appearance with **concentric rings** and **comma-shaped projections** radiating outward from the center - This distinctive morphology results from chains of bacilli growing in parallel alignment, creating the characteristic pattern - The colonies are typically **non-hemolytic, grey-white, and have irregular edges** *H. ducreyi* - *Haemophilus ducreyi* causes **chancroid** and is difficult to culture - Colonies appear as **small, grey-yellow, translucent** with a **cohesive ("school of fish")** appearance when pushed across the agar - Does not produce concentric rings *B. pertussis* - *Bordetella pertussis* grows on **Bordet-Gengou agar** or **Regan-Lowe medium** - Colonies have a characteristic **"mercury droplet"** or **"bisected pearl"** appearance due to their smooth, shiny, convex morphology - This is completely different from the Draughtsman pattern *Pneumococci* - *Streptococcus pneumoniae* produces **alpha-hemolytic colonies** on blood agar - Colonies are **small, dome-shaped, mucoid** with a **central umbilication** (draughtsman or checker-piece appearance refers to the depression, not concentric rings) - The "draughtsman" term when applied to pneumococci refers to the flattened checker-piece shape, not the concentric ring pattern of *B. anthracis*
Question 29: What component is included in the vaccine against N-meningitidis?
- A. Killed whole-cell vaccine
- B. Capsular polysaccharide (polysaccharide vaccine) (Correct Answer)
- C. Outer membrane protein (OMP)
- D. Live attenuated vaccine
Explanation: ***Capsular polysaccharide (polysaccharide vaccine)*** - *Neisseria meningitidis* vaccines commonly include **capsular polysaccharides** from different serogroups (A, C, Y, W-135) to elicit a protective immune response. - These polysaccharides act as antigens, stimulating the production of **antibodies** that can neutralize the bacteria. - **Conjugate vaccines** combine polysaccharides with carrier proteins to improve immunogenicity, especially in infants and young children. *Killed whole-cell vaccine* - **Killed whole-cell vaccines** are not used for *N. meningitidis* due to potential reactogenicity and the availability of more effective targeted antigens. - **Polysaccharide and conjugate vaccines** are the established formulations for meningococcal disease prevention. *Outer membrane protein (OMP)* - **Outer membrane proteins (OMPs)** are specifically used in **serogroup B vaccines** (e.g., Bexsero, Trumenba) due to the poor immunogenicity of the serogroup B polysaccharide capsule. - However, for serogroups A, C, Y, and W-135, **capsular polysaccharide** remains the primary vaccine component. *Live attenuated vaccine* - **Live attenuated vaccines** are not used for *N. meningitidis* as they pose risks of reversion to virulence and are unnecessary given the effectiveness of polysaccharide-based vaccines. - Meningococcal vaccines rely on **subunit approaches** (polysaccharides, conjugates, OMPs) rather than live organisms.
Question 30: Which of the following bacteria is not capsulated?
- A. Pneumococcus
- B. Klebsiella
- C. Meningococcus
- D. Proteus (Correct Answer)
Explanation: ***Proteus*** - *Proteus* species are **non-capsulated bacteria**; their primary virulence factor is urease production and motility via flagella. - While they can cause infections, the lack of a capsule means they do not evade phagocytosis in the same manner as the other listed options. *Pneumococcus* - **_Streptococcus pneumoniae_**, commonly known as pneumococcus, is a highly **capsulated bacterium**. - Its polysaccharide capsule is a major **virulence factor** that inhibits phagocytosis, contributing to its ability to cause pneumonia, meningitis, and other invasive diseases. *Klebsiella* - **_Klebsiella pneumoniae_** is a **heavily capsulated Gram-negative bacterium** with a prominent polysaccharide capsule. - The thick capsule gives colonies a **mucoid appearance** and serves as a major **virulence factor** that protects against phagocytosis, contributing to pneumonia, UTIs, and other infections. *Meningococcus* - **_Neisseria meningitidis_**, or meningococcus, is a **capsulated bacterium**. - Its **polysaccharide capsule** is vital for immune evasion and is the basis for serogrouping and vaccine development against meningococcal disease.