What is the most common genetic factor associated with increased susceptibility to Neisseria infections?
Pneumococcal vaccine is prepared from ?
Which anticoagulant is used when blood is sent for blood culture?
Inclusion body containing glycogen is seen in which of the following organisms?
In blood culture the ratio of blood to reagent is?
Which of the following bacteria is not capsulated?
Which of the following Chlamydia species is primarily associated with respiratory infections?
Boutonneuse fever is caused by which of the following?
'String of pearl' colonies on penicillin-containing nutrient agar are produced by:
Which of the following is a saccharolytic species of Clostridium?
NEET-PG 2013 - Microbiology NEET-PG Practice Questions and MCQs
Question 31: What is the most common genetic factor associated with increased susceptibility to Neisseria infections?
- A. HLA-B27
- B. Complement deficiency (Correct Answer)
- C. IgA deficiency
- D. Factor H deficiency
Explanation: ***Complement deficiency*** - Deficiencies in the **terminal complement pathway (C5-C9)**, particularly C5b-C9 (membrane attack complex, MAC), significantly increase susceptibility to disseminated *Neisseria* infections. - The MAC is crucial for lysing Gram-negative bacteria like *Neisseria meningitidis* and *Neisseria gonorrhoeae*, and its absence allows for uncontrolled bacterial proliferation. *Factor H deficiency* - **Factor H** is a regulatory protein of the alternative complement pathway, preventing its overactivation on host cells. - Its deficiency typically leads to conditions like **atypical hemolytic uremic syndrome (aHUS)** and **dense deposit disease**, not primarily increased susceptibility to *Neisseria* infections. *HLA B27* - **HLA-B27** is a human leukocyte antigen strongly associated with a group of autoimmune inflammatory diseases called **spondyloarthropathies**, such as ankylosing spondylitis. - It does not directly impact the immune response to *Neisseria* infections or increase susceptibility to them. *IgA deficiency* - **Selective IgA deficiency** is the most common primary immunodeficiency, characterized by low or absent IgA levels. - Individuals with IgA deficiency are more prone to **recurrent respiratory and gastrointestinal infections**, but not specifically disseminated *Neisseria* infections.
Question 32: Pneumococcal vaccine is prepared from ?
- A. Cell surface antigen
- B. Capsular polysaccharide (Correct Answer)
- C. Exotoxin
- D. M protein
Explanation: ***Capsular polysaccharide*** - The **polysaccharide capsule** of *Streptococcus pneumoniae* is the primary virulence factor, protecting the bacteria from phagocytosis. - Vaccines, such as **PCV13** (pneumococcal conjugate vaccine) and **PPSV23** (pneumococcal polysaccharide vaccine), are designed targeting these capsular polysaccharides to elicit a protective immune response. *Cell surface antigen* - While bacteria possess various **cell surface antigens**, not all are immunogenic or provide protective immunity as effectively as the capsular polysaccharides for *S. pneumoniae*. - For pneumococcus, the **capsule** is the most critical surface component for vaccine development due to its role in virulence and serotype specificity. *From exotoxin* - *S. pneumoniae* primarily causes disease through its **polysaccharide capsule** and other bacterial components, not through the production of an **exotoxin** that would be targeted by a vaccine. - Vaccines developed from exotoxins (e.g., diphtheria or tetanus toxoids) target specific toxins, which is not the mechanism for pneumococcal vaccines. *From M protein* - **M protein** is a major virulence factor for *Streptococcus pyogenes* (Group A Strep), not *Streptococcus pneumoniae*. - Vaccines targeting M protein are associated with **Group A Streptococcus** infections, for protection against diseases like rheumatic fever.
Question 33: 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 34: Inclusion body containing glycogen is seen in which of the following organisms?
- A. Chlamydia trachomatis (Correct Answer)
- B. Chlamydia pneumoniae
- C. Chlamydia psittaci
- D. None of the options
Explanation: ***Chlamydia trachomatis*** - This species is known to form **intracytoplasmic inclusion bodies** that contain **glycogen**. - The presence of this glycogen allows these inclusions to be stained by **iodine**, aiding in laboratory identification. *Chlamydia pneumoniae* - Inclusion bodies of *Chlamydia pneumoniae* are typically **round or pear-shaped** and **do not contain glycogen**. - They are often **vacuolated** and stain poorly with iodine, unlike those of *C. trachomatis*. *Chlamydia psittaci* - The inclusion bodies of *Chlamydia psittaci* are usually **dense and pleiomorphic**, but they **do not contain glycogen**. - They tend to be **larger** and are often found near the host cell nucleus. *None of the options* - This option is incorrect because *Chlamydia trachomatis* specifically forms **glycogen-containing inclusion bodies**. - The presence of glycogen within inclusions is a key distinguishing feature of this species.
Question 35: In blood culture the ratio of blood to reagent is?
- A. 1:5
- B. 1:20
- C. 1:10 (Correct Answer)
- D. 1:100
Explanation: ***1:10*** - A 1:10 ratio of **blood to reagent (culture media)** is recommended for optimal growth of microorganisms in blood cultures. - This dilution minimizes the inhibitory effect of **human antimicrobial factors** present in the blood, while still providing sufficient blood volume for pathogen detection. *1:5* - A 1:5 ratio would mean a relatively **higher concentration of blood** in the culture. - This higher concentration could lead to increased inhibition of bacterial growth by **host defense mechanisms** or **antibiotics** present in the patient's blood, potentially causing false-negative results. *1:20* - A 1:20 ratio implies a **lower concentration of blood** relative to the culture media. - While it further dilutes inhibitory factors, it also reduces the total number of microorganisms sampled, possibly leading to **false-negative results** if the bacterial load is low. *1:100* - A 1:100 ratio would result in an **extremely low concentration of blood** in the culture. - This significantly reduces the likelihood of detecting microorganisms, especially when the **bacteremia is sparse**, due to insufficient bacterial inoculum, leading to a high rate of false negatives.
Question 36: 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.
Question 37: Which of the following Chlamydia species is primarily associated with respiratory infections?
- A. C psittaci
- B. C pneumoniae (Correct Answer)
- C. C trachomatis
- D. None of the options
Explanation: ***C pneumoniae*** - *C. pneumoniae* is the **primary Chlamydia species** associated with **respiratory infections** in humans - It is a common cause of **community-acquired pneumonia** (5-10% of cases), **atypical pneumonia**, **bronchitis**, **pharyngitis**, and **sinusitis** - Transmission occurs via **respiratory droplets** from person to person - Causes both **acute and chronic respiratory infections** and has been linked to exacerbations of asthma and COPD - It is the most frequently encountered Chlamydia species in respiratory tract infections *C psittaci* - Causes **psittacosis** (ornithosis), a specific type of **atypical pneumonia** transmitted from **birds** (parrots, pigeons, poultry) - While it does cause respiratory infection, it is much **less common** than C. pneumoniae and is associated with **occupational or recreational bird exposure** - Not the primary Chlamydia species for general respiratory infections *C trachomatis* - Primarily causes **genital tract infections** (urethritis, cervicitis, PID) and **ocular infections** (trachoma, inclusion conjunctivitis) - Can cause **neonatal pneumonia** in infants exposed during birth, presenting at 1-3 months of age with a staccato cough - NOT a primary respiratory pathogen in adults *None of the options* - This is incorrect because **C. pneumoniae** is definitively the Chlamydia species primarily associated with respiratory infections
Question 38: Boutonneuse fever is caused by which of the following?
- A. A. Rickettsia japonica
- B. B. Rickettsia conorii (Correct Answer)
- C. C. Rickettsia sibirica
- D. D. Rickettsia australis
Explanation: ***Rickettsia conorii*** - **Boutonneuse fever**, also known as Mediterranean spotted fever, is specifically caused by the bacterium **Rickettsia conorii**. - This zoonotic disease is transmitted to humans through the bite of infected ticks, primarily the **dog tick** (Rhipicephalus sanguineus). *Rickettsia japonica* - **Rickettsia japonica** is the causative agent of **oriental spotted fever**, a tick-borne illness primarily found in Japan. - This Rickettsial disease is distinct from Boutonneuse fever, having a different geographical distribution and primary vector. *Rickettsia sibirica* - **Rickettsia sibirica** is responsible for **Siberian tick typhus** (North Asian tick typhus), which is prevalent in Siberia, Mongolia, and Northern China. - While also a tick-borne Rickettsial infection, its clinical presentation and geographic range differ from Boutonneuse fever. *Rickettsia australis* - **Rickettsia australis** causes **Queensland tick typhus**, an indigenous Rickettsial disease found in Australia. - This pathogen is distinct from Rickettsia conorii and causes a localized form of tick typhus.
Question 39: 'String of pearl' colonies on penicillin-containing nutrient agar are produced by:
- A. Salmonella
- B. Bacillus (Correct Answer)
- C. Proteus
- D. Klebsiella
Explanation: ***Bacillus*** - The "string of pearls" morphology is a classic characteristic of *Bacillus anthracis* when grown on a nutrient agar containing penicillin. - This appearance is due to the bacterium's cell wall being weakened by penicillin, leading to swollen, elongated cells that remain attached in a chain. *Proteus* - *Proteus* species are known for their characteristic **swarming motility** on agar, not a "string of pearls" appearance. - They are typically **resistant to penicillin** and would not show this inhibited growth pattern. *Salmonella* - *Salmonella* typically forms **smooth, round colonies** on agar and does not exhibit a "string of pearls" morphology. - While some species can be inhibited by penicillin, they do not develop this specific appearance. *Klebsiella* - *Klebsiella* species are typically **mucoid** due to their polysaccharide capsule, forming large, glistening colonies. - They are also often **resistant to penicillin** and would not show the "string of pearls" effect.
Question 40: Which of the following is a saccharolytic species of Clostridium?
- A. Clostridium tetani
- B. Clostridium septicum (Correct Answer)
- C. Clostridium cochlearium
- D. None of the options
Explanation: ***Clostridium septicum*** - *Clostridium septicum* is a **saccharolytic** species, meaning it ferments carbohydrates to produce gas, which contributes to the rapid tissue destruction seen in **gas gangrene**. - Its ability to rapidly consume sugars in tissues fuels its invasive growth and toxin production, which is characteristic of its pathogenic mechanism. *Clostridium tetani* - *Clostridium tetani* is a **proteolytic** species, primarily deriving its energy from **protein degradation**. - It does not ferment carbohydrates, and its pathogenicity is mainly due to the production of **tetanospasmin**, a neurotoxin. *Clostridium cochlearium* - *Clostridium cochlearium* is a **proteolytic** species that metabolizes proteins and amino acids. - It is not known for significant saccharolytic activity and is typically found in environments rich in protein. *None of the options* - This option is incorrect because *Clostridium septicum* is indeed a saccharolytic species, as detailed above.