Oil paint appearance on nutrient agar is seen in -
What is the most common age group affected by Streptococcus pyogenes?
Naegler's reaction is due to:
Disruption of which of the following oropharyngeal commensals predisposes to candidiasis?
Primary complex of M bovis involves:
Brill-Zinsser disease is a recrudescent form of which infection?
Which bacteria can be isolated using crystal violet blood agar?
Which bacteria can grow even in the presence of antiseptics?
A chronic alcoholic is presenting with clinical features of meningitis. Most likely organism to grow on CSF culture:
Which of the following statements about anthrax toxin is false?
NEET-PG 2015 - Microbiology NEET-PG Practice Questions and MCQs
Question 21: Oil paint appearance on nutrient agar is seen in -
- A. Staphylococcus aureus (Correct Answer)
- B. Streptococcus pyogenes
- C. Bordetella pertussis
- D. H. influenzae
Explanation: ***Staphylococcus aureus*** - *Staphylococcus aureus* forms characteristic **golden-yellow, smooth, opaque colonies** on nutrient agar with a **buttery or creamy consistency** - Some texts describe this appearance as **"oil paint-like"** due to the pigmented, smooth, and glistening surface that can resemble brushed paint - Colonies are typically **2-4 mm in diameter**, round, and show **golden pigmentation** (due to carotenoid pigments) - On **blood agar**, *S. aureus* shows **beta-hemolysis** with golden colonies *Streptococcus pyogenes* - *Streptococcus pyogenes* grows poorly on plain nutrient agar and requires **enriched media** like blood agar - On blood agar, it forms **small, translucent, grey-white colonies** surrounded by a wide zone of **beta-hemolysis** - Colonies are typically **pinpoint** in size and do not show pigmentation *Bordetella pertussis* - *Bordetella pertussis* is a **fastidious organism** that does **not grow on plain nutrient agar** - Requires specialized enriched media like **Bordet-Gengou agar** (with potato-glycerol-blood) or **Regan-Lowe agar** - On Bordet-Gengou agar, colonies appear as **small, smooth, pearl-like** or **"mercury droplet"** colonies after 3-7 days *H. influenzae* - *Haemophilus influenzae* is also fastidious and requires **X factor (hemin)** and **V factor (NAD)** for growth - Does **not grow on plain nutrient agar** - On **chocolate agar**, forms **small, smooth, translucent, greyish colonies** with a characteristic musty odor - Colonies are typically **1-2 mm** in diameter
Question 22: What is the most common age group affected by Streptococcus pyogenes?
- A. 30-40 years
- B. <5 years
- C. 5-15 years (Correct Answer)
- D. 20-25 years
Explanation: ***5-15 years*** - **Streptococcus pyogenes** (Group A Strep) commonly causes **pharyngitis** (strep throat), which primarily affects school-aged children. - This age group is more susceptible due to increased exposure in school and daycare settings. *<5 years* - While younger children can get strep infections, it is less common in those under **3 years of age**, as viral etiologies are more prevalent for pharyngitis in this group. - They are more prone to **non-streptococcal bacterial infections** and certain viral infections. *20-25 years* - Although adults can get **Streptococcus pyogenes** infections, the incidence significantly decreases after childhood. - Pharyngitis in this age group is more often **viral** in origin. *30-40 years* - The prevalence of **Streptococcus pyogenes** infections, particularly pharyngitis, is typically low in this age group compared to children. - Infections, if they occur, may stem from exposure to infected children or close contact environments.
Question 23: Naegler's reaction is due to:
- A. Lecithinase (Correct Answer)
- B. Coagulase
- C. Hyaluronidase
- D. None of the options
Explanation: ***Lecithinase*** - The **Naegler reaction** is a bacterial identification test used to detect the production of **lecithinase** (also known as alpha-toxin) by certain bacteria, particularly *Clostridium perfringens*. - This enzyme hydrolyzes **lecithin** (a lipid found in egg yolk), resulting in a visible opaque precipitate around the bacterial colonies on egg yolk agar. *Coagulase* - **Coagulase** is an enzyme produced by some bacteria (e.g., *Staphylococcus aureus*) that causes the coagulation of blood plasma. - While it is an important virulence factor, it is not involved in the **Naegler reaction**. *Hyaluronidase* - **Hyaluronidase** is an enzyme that breaks down **hyaluronic acid**, a component of connective tissue, facilitating the spread of bacteria. - It is often referred to as a **spreading factor** but is not detected by the **Naegler reaction**. *None of the options* - This option is incorrect because **Lecithinase** is directly responsible for the **Naegler reaction**.
Question 24: Disruption of which of the following oropharyngeal commensals predisposes to candidiasis?
- A. Staphylococcus
- B. Streptococcus (Correct Answer)
- C. Lactobacillus
- D. Haemophilus influenzae
Explanation: ***Streptococcus*** - **Streptococcus** species, particularly *S. sanguinis* and *S. mitis*, are major commensals in the oral cavity that **inhibit the growth of *Candida albicans*** through competition for nutrients and production of antimicrobial substances. - Disruption of this normal **streptococcal flora**, often by broad-spectrum antibiotics, creates an environment where *Candida albicans* can proliferate, leading to candidiasis. *Staphylococcus* - **Staphylococcus** species are primarily skin and nasal commensals; while *S. aureus* can be found in the oral cavity, it is not a primary competitor against *Candida* in the same way as streptococci. - Their presence or absence is not typically a direct predisposing factor for oral candidiasis compared to the dominant streptococcal flora. *Lactobacillus* - **Lactobacillus** species are common in the gastrointestinal tract and vagina, where they maintain an acidic environment that inhibits pathogen growth; however, they are less dominant in the oropharynx as a defense against *Candida*. - While beneficial for overall host health, their disruption in the oral cavity does not typically precipitate candidiasis as directly as that of the **streptococcal flora**. *Hemophilus influenzae* - *Haemophilus influenzae* is a common inhabitant of the **upper respiratory tract** and can be an opportunistic pathogen, but it is not known to have a significant role in directly inhibiting *Candida* growth in the oropharynx. - Its presence or absence in the commensal flora in the oral cavity does not typically influence the development of candidiasis.
Question 25: Primary complex of M bovis involves:
- A. Tonsil and skin
- B. Tonsil and intestine (Correct Answer)
- C. Tonsil and lung
- D. Skin and Intestine
Explanation: ***Tonsil and intestine*** - *Mycobacterium bovis* is primarily transmitted through **consumption of contaminated milk and dairy products**, making the **alimentary tract** the main route of infection - The primary complex (Ghon complex) involves the **initial site of infection plus regional lymph nodes** - In alimentary tuberculosis, the organisms enter through the **intestinal mucosa** (Peyer's patches) or **tonsillar tissue**, creating foci with associated mesenteric or cervical lymphadenopathy - Both tonsils and intestines are part of the **alimentary system**, representing the typical primary complex for M. bovis in humans *Tonsil and lung* - This incorrectly combines **two different routes of entry** (alimentary and respiratory) - A primary complex involves a **single portal of entry**, not multiple unrelated organ systems - While M. bovis can rarely cause pulmonary TB through inhalation, this would create a separate lung + hilar node complex, not a combined tonsil-lung complex *Tonsil and skin* - **Skin involvement** requires direct inoculation through cuts or abrasions and does not form a primary complex with tonsillar infection - These represent different portals of entry and would not occur together as a primary complex *Skin and Intestine* - **Skin infection** by M. bovis is rare and requires occupational exposure with direct inoculation (e.g., veterinarians, butchers) - This incorrectly pairs two different routes of infection that would not form a single primary complex
Question 26: Brill-Zinsser disease is a recrudescent form of which infection?
- A. Recrudescence of R prowazekii infection (Correct Answer)
- B. Recrudescence of R typhi infection
- C. None of the options
- D. Recrudescence of Coxiella burnetii infection
Explanation: **Recrudescence of R prowazekii infection** - Brill-Zinsser disease is a **late-onset complication** of **epidemic typhus**, caused by *Rickettsia prowazekii*. - It occurs years after the initial infection, due to **reactivation of dormant bacteria** in the body. *Recrudescence of R typhi infection* - *Rickettsia typhi* causes **murine typhus**, but its recrudescent form is not referred to as Brill-Zinsser disease. - Murine typhus is typically a **milder disease** compared to epidemic typhus. *Recrudescence of Coxiella burnetii infection* - *Coxiella burnetii* causes **Q fever**, which can have a chronic form but is not a recrudescence of a typhus infection. - Q fever presents with different clinical manifestations, such as **endocarditis** or **hepatitis**, not typically a rash or neurological symptoms seen in typhus. *None of the options* - This option is incorrect because the specific recrudescent form described in the question clearly points to *Rickettsia prowazekii*.
Question 27: Which bacteria can be isolated using crystal violet blood agar?
- A. Corynebacterium diphtheriae
- B. Staph aureus
- C. Meningococcus
- D. β-hemolytic streptococci (Correct Answer)
Explanation: ***β-hemolytic streptococci*** - **Crystal violet blood agar** is a selective medium that inhibits the growth of most Gram-positive bacteria, except for **beta-hemolytic streptococci**. - The crystal violet dye suppresses the growth of competing flora, allowing for better isolation and identification of these bacteria, which exhibit **complete hemolysis (beta-hemolysis)** on blood agar. *Corynebacterium diphtheriae* - This bacterium requires more specialized media, such as **Tinsdale agar** or **Loeffler's serum agar**, for optimal growth and identification due to specific nutritional requirements and colony morphology. - Crystal violet blood agar is not the primary medium used for its isolation. *Staph aureus* - **Staphylococcus aureus** is a common contaminant that is typically inhibited by the crystal violet in the medium. - It grows well on routine blood agar but is not selectively grown or isolated using crystal violet blood agar. *Meningococcus* - **Neisseria meningitidis** (Meningococcus) requires enriched media like **chocolate agar** or **Thayer-Martin agar** for successful isolation, as it is a fastidious organism. - Crystal violet blood agar is not suitable for its growth due to its inhibitory properties and lack of necessary nutrients.
Question 28: Which bacteria can grow even in the presence of antiseptics?
- A. Staphylococcus
- B. Streptococcus
- C. E. coli
- D. Pseudomonas (Correct Answer)
Explanation: ***Pseudomonas*** - **Pseudomonas aeruginosa** is unique among common bacteria in its ability to not just survive but actually **grow in the presence of antiseptics**. - It can multiply in **quaternary ammonium compounds**, **chlorhexidine solutions**, and even **distilled water** due to minimal nutritional requirements. - Resistance mechanisms include **efflux pumps**, **biofilm formation**, and **low outer membrane permeability** that exclude many antiseptic agents. - This characteristic makes it a notorious cause of **hospital-acquired infections** and contaminant of disinfectant solutions. *Staphylococcus* - While some strains like **MRSA (methicillin-resistant Staphylococcus aureus)** are resistant to many antibiotics, they are generally **susceptible to common antiseptics**. - Standard antiseptics like alcohols, iodophors, and chlorhexidine effectively kill Staphylococcus species. *Streptococcus* - **Streptococcus species** are generally **susceptible to most common antiseptics and disinfectants**. - They are known for causing infections like strep throat and cellulitis but do not exhibit antiseptic resistance. *E. coli* - **Escherichia coli** is typically **susceptible to standard antiseptic agents**. - While some strains can be antibiotic-resistant, their resistance mechanisms do not generally extend to antiseptics, unlike **Pseudomonas**.
Question 29: A chronic alcoholic is presenting with clinical features of meningitis. Most likely organism to grow on CSF culture:
- A. Streptococcus pneumoniae (Correct Answer)
- B. Neisseria meningitidis
- C. Escherichia coli
- D. Listeria monocytogenes
Explanation: ***Streptococcus pneumoniae*** - **Chronic alcoholism** is a significant risk factor for **pneumococcal infections**, including meningitis, due to impaired immune function. - **S. pneumoniae** is the most common cause of **bacterial meningitis** in adults, especially in those with underlying conditions like alcoholism. *Neisseria meningitidis* - While a common cause of meningitis, especially in **young adults** and crowded settings, it is not specifically associated with chronic alcoholism as a primary risk factor like *S. pneumoniae*. - **Meningococcal meningitis** often presents with a **petechial rash**, which is not mentioned in the clinical scenario. *Escherichia coli* - **E. coli meningitis** primarily affects **neonates** and **elderly** or immunocompromised individuals, usually associated with healthcare-acquired infections. - It is not a typical cause of community-acquired meningitis in an otherwise healthy adult alcoholic. *Listeria monocytogenes* - **Listeria monocytogenes** is a known cause of meningitis in **immunocompromised individuals**, **neonates**, and **elderly** people, often linked to contaminated food. - While chronic alcoholism does impair immunity, *S. pneumoniae* is a more frequently encountered pathogen in this specific population for meningitis.
Question 30: Which of the following statements about anthrax toxin is false?
- A. Increase cAMP
- B. Has three components
- C. Coded by plasmid
- D. Inhibits protein synthesis (Correct Answer)
Explanation: ***Inhibits protein synthesis*** - Anthrax toxin, specifically the **lethal factor (LF)**, is a **zinc-dependent metalloprotease** that cleaves and inactivates **mitogen-activated protein kinase kinase (MAPKKs)**, leading to cell death, not directly inhibiting protein synthesis. - The **edema factor (EF)** component of the toxin is an **adenylate cyclase** that increases **intracellular cyclic AMP (cAMP)**, which also does not directly inhibit protein synthesis. *Has three components* - Anthrax toxin is indeed composed of three distinct proteins: **protective antigen (PA)**, **edema factor (EF)**, and **lethal factor (LF)**. - PA is necessary for EF and LF to enter host cells, while EF causes edema and LF is responsible for cytotoxicity. *Increase cAMP* - The **edema factor (EF)** component of anthrax toxin is a **calmodulin-dependent adenylate cyclase**. - Once inside the cell, EF converts **ATP to cyclic AMP (cAMP)**, leading to increased intracellular cAMP levels, which disrupts water homeostasis and causes edema. *Coded by plasmid* - The genes encoding the anthrax toxin components (PA, EF, and LF) are located on a large plasmid known as **pXO1**. - This plasmid, along with another plasmid (pXO2) carrying genes for the capsule, is crucial for the full virulence of *Bacillus anthracis*.